Advances in Geophysics, Volume 5

A D V A N C E S IN GEOPHYSICS VOLUME 5 This Page Intentionally Left Blank Advances in GEOPHYSICS Edited by H. E...

Author: Landsberg | Helmut Erich; Mieghem | Jacques van

41 downloads 1223 Views 18MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!

Report copyright / DMCA form

DOWNLOAD PDF

A D V A N C E S IN

GEOPHYSICS

VOLUME 5

This Page Intentionally Left Blank

Advances in

GEOPHYSICS Edited by

H. E. LANDSBERG U. S.

Weather Bureau Warhingfon, D. C.

J. VAN MIEGHEM Royal Belgian Meteorological lnstifufe k l e , Belgium

Editorial Advisory Committee BERNHARD HAURWITZ WALTER D. LAMBERT

ROGER REVELLE R. STONELEY

VOLUME 5

ACADEMIC PRESS INC. NEW YORK, 1958

PUBLISHERS

Copyright@,1958, by

ACADEMIC PRESS INC. 111 FJFTH AVENUE NEWYORK3, N. Y.

ACADEMIC PRESS INC. (London) LTD.,PUBLISHERS 40 PALL MALL,LONDON, S. W. 1 ALL RIGHTS RESERVED NO PART O F THIS BOOK MAY B E REPRODUCED I N ANY FORM, BY PHOTOSTAT, MICROFILM, OR ANY OTHERS MEANS, WITHOUT WRITTEN PERMISSION FROM T H E PUBLISHERS.

Library of Congress Catalog Card Number: 52-12266

PRINTED I N T H E UNITED STATES OF AMERICA

LIST OF CONTRIBUTORS A. T. DOODSON, Liverpool Observatory and Tidal Institute, Liverpool, England N . C. GERSON, Secretary, U. S. National Committee for the IGY

B. GUTENBERG, California Institute of Technology, Pasadena, California R. A. HIRVONEN, Institute of Technology, Helsinki, Finland, and The Ohio State University, Columbus, Ohio JAMES E. MCDONALD, Institute of Atmospheric Physics, University of Arizona, Tucson, Arizona

K. WATANABE, Hawaii Institute of Geophysics and Department of Physics, University of Hawaii, Honolulu, Hawaii

V

This Page Intentionally Left Blank

FOREWORD This volume presents, as have the preceding ones, a wide variety of topics. We are particularly fortunate in having among our authors experts whose contributions take stock of the progress made during their decades of experience in the field of geophysics. These reviews show that for the most part science grows gradually. The steps forward are often agonizingly small and the spectacular advances, when appraised, have in most instances been based on years of hard and painstaking labors of a multitude of workers. In geophysics the effort has multiplied and research has accelerated. The International Geophysical Year (IGY) is drawing to a close. Many spectacular feats were accomplished, including the launching of the first artificial satellites. The foresight of the planners and organizers has been matched-all over the earth-by designers, observers, and bold explorers. A once obscure science has jumped into the limelight. But the real task is just about to begin. All the data collected in 18 months of intensive observing schedules have to be reduced, analyzed, and, if possible, fitted into theories. In this connection the lead article of this volume will serve as a memento. It points out that many of the hopes of the First and Second Polar Years remained unfilled because of incomplete analysis of the observations. There is a real challenge to the geophysicists of the world not to "drown" in data. These should be evaluated, assessed, and digested as soon as they become available. Otherwise, because of incomplete understanding, the pressure for more and more new data without essential guidance from the message contained in material already available will result in enormous waste. The direction which new collections of data should take ought to be dictated by theories evolved from the older data. Indiscriminate amassing of observations will retard rather than advance science. Our pages in the future will therefore be wide open to analyses and new hypotheses stemming from IGY data. The time lag is too short yet to have any substantial IGY material for the next volume. Plans for these volumes have usually to be laid two years in advance. Although the emphasis is on review articles we do not shy away from presenting information as the current milestones are passed in the steady progress of our science even though some material may still be in the controversial stages. Our present plans include articles on tracer techniques, model experiments, balloon exploration, high pressure geochemistry, and atmospheric ozone. J u l y , 1958 H. E. LANDSBERG vii

This Page Intentionally Left Blank

CONTENTS LIST OF CONTRIBUTORS. ..............................................

v

FOREWORD.. .........................................................

vii

From Polar Years to IGY

N. C. GERSON ............................ ......... History. . The First olar Year.. . . . . ..................... The Second International Polar Year. . . . . . . . . . . . . . . . . . The International Geophysical Year ........................ The International Years in Retrospect.. . . . . . . . . . . . . . . . Future of the International Years. . . . . . . . . .................... References . . . . . . . . . . . . . . . . . . . . . . ...........................

1. 2. 3. 4. 5. 6.

2 10 24

47 48

Microseisms

B. GUTENBERG

5 . Regular Microseisms with Periods of One to Three Seconds.. 6. Microseisms with Periods of About Four Seconds 9. Theory.. . . . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . .

......................

85

The Size and Shape of the Earth R. A. HIRVONEN

. . . . . . . . . 93 ...........

ix

Oceanic Tides

A. T. DOODSON 1. Introduction and General Remarks. . . . . . . . . . . . . . . . . . .

6. Future Research.

.

References.. . . . Ultraviolet Absorption Process in the Upper Atmosphere

K. WATANABE 1. Introduction. . . . . . . . . .

. . . . . . . . . . . . . . . . . . . 154

2. Density, Temp

4. Absorption Cross-Sections of A 5. Some Atmospheric Absorption Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............................ List of Symbols. . References ...........................................

210

The Physics of Cloud Modification

JAMES E. MCDONALD 1. Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 2. Clouds and the Atmospheric Water Vapor Cycl 3. Present Status of Cloud and Precipitation Phy 4. Recent Developments in Cloud-Modification Techniques 5. The Evaluation of Modification Experiments . . . . . . . . . . 6. Concluding Remark? . . . . . . ............................ 297

Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . ......................

298

AUTHORINDEX .......................................................

305

SUBJECTINDEX.. ....................................................

314

CUMULATIVE TITLEINDEX, VOLUMES I-IV.. ............................ CUMULATIVE SUBJECTINDEX, VOLUMES I-IV. . . . . . . . . . . . . . . . . . . . . . . . . . .

320 321

FROM POLAR YEARS TO IGY

N. C.

Gerson

Secretary, U. S. Notional Committee for the IGY*

Page

1. History.. .......................

..................

2.1. Observations.. 3. The Second Inter 3.1. Background. 3.2. The Internat

2

............................................ 10 . . . . . . . . . . . . . . . 10

....................

14

Commission. .................................

16 19

.......................................

..........

....................

........................

34

4.4.7. Latitude and Longitude.. . . . . . . . . . . . . . . . . . .

............................

36

4.4.10. Seismology. . . 4.4.11. Solar Activity. ...................... 4.4.12. World Days and Communication. . . . . 4.5. World Data Centers.. ...................... . . . . . . . . . . . . . 37 4.6. Preliminary Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. The International Years in Retrospect.. . . . . . .................................................. 43 . . . . . . . . . . . . . . . . . . 45

Bibliography. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* Now Consultant, Lincoln Laboratories, MIT. 1

2

N. C. GERSON

1. HISTORY 1.I. Introduction

The development of the International Polar Years, now culminat,ing in the IGY of 1957-1958, began as an inspiration to a young Austro-Hungarian naval officer, Karl Weyprecht. At a time when Arctic exploration subsisted on the hope of discovering new lands and the emotional drive of reaching the North Geographic Pole, Weyprecht espoused a fundamentally scientific approach. He realized that haphazard navigation of the “Sea of Ancient Ice” provided few results of basic importance. Weyprecht proposed a replacement of the born marauder by teams of careful scientific workers. The very soundness, depth of vision, and comprehensiveness of Weyprecht’s approach leads directly to the IGY (International Geophysical Year) of today. Indeed, his original thoughts come so close to fulfillment among the IGY objectives that it seems highly desirable to list some of his fundamental recommendations. 1.2. Karl Weyprecht

First, however, we might recall briefly the history of the man who originated the concept of international cooperation in geophysics. Karl Weyprecht was born in Hesse-Darmstadt in 1838 and entered the Austro-Hungarian Navy in 1856. Ten years later he volunteered to command a small vessel, manned by only four seamen, to sail from Hammerfest to the Arctic Ocean. His offer to explore the Arctic Basin formed the basis of the First German North Polar Expedition. Permission to participate in the expedition arrived as Weyprecht was aboard the frigate Elizabeth, in the Austrian squadron returning the body of Maximilian from Mexico. Because of an illness acquired in New Orleans, Weyprecht was in poor health until 1871. In the meantime, the First German North Polar Expedition sailed in 1868 and the Second in 1869. Weyprecht and Julius Payer were in command of the Tegetthof expedition, (June, 1872September, 1874) which departed for the Arctic and discovered Kaiser li’ranz Josef Land. In the early 1870’s it had been authoritatively announced that there was no open polar sea. Captain Nares, on his safe return from the Arctic Ocean with the Alert and Discovery, had telegraphed “Pole impracticable. No land to northward.” Nevertheless, public enthusiasm still sought a conquest of the geographic pole itself. Amid this popular climate, Weyprecht raised a voice of caution and reason. At an address before the German Natural Philosophers and Physicians Association (Graz, Austria, September 17, 1875), Weyprecht deprecated past Arctic expeditions as adventurous and of little value. He

FROM POLAR YEARS TO IGY

3

stated that they constituted an “international steeplechase to the North Pole, a system opposed to true scientific discoveries. . . its ulterior aim must lie higher than the mere sketching and christening in different languages of islands, bays, and promontories buried in ice. . . .” Immense sums had been spent and much hardship endured for the mere purpose of extending geographical and topographical knowledge, while strictly scientific observations had been given secondary stature. (This view, so clear today for the Arctic, was not to be appreciated for the Antarctic for another 75 years.) He expounded a new objective which would make Arctic expeditions more fruitful for natural science, and which would also enable smaller nations to participate. He maintained that the polar regions offered greater advantages than any other part of the globe for observations of such phenomena as magnetism, meteorology, aurorae, geology, zoology, and botany. He summarized his paper by defining six principles: a. Arctic exploration is of the highest importance to the laws of nature. b. Geographical discovery in these regions is of superior importance only insofar as it extends the field of scientific investigation in its strict sense. c. Minute Arctic topography is of secondary importance. d. The geographical pole has no greater significance for science than any other point in high latitudes. e. Observation stations should be selected without reference to the latitude, but for the advantages they offer for the investigation of the phenomena to be studied. f. Interrupted series of observations have only a relative value. He indicated certain Arctic circumferential stations which might be occupied and noted the desirability of standardized observations. Observers should utilize similar instruments according to similar instructions for recording simultaneous observations as long as possible throughout the year. Were it possible to establish stations for simultaneous observations in the ,4ntarctic, results of much greater value could be expected. His philosophy, program, and, except for details, his original objectives form the very basis of the present IGY. Weyprecht had launched the thought of studying the earth as a planet; his idea was to gain increasing momentum over the years. Weyprecht contributed materially in the Polar Conferences which followed his suggesbion. However, the fruition of his labors was not realized during his lifetime; he died of consumption a t the age of 43 years. Known foremost for his zeal in fostering the FPY (First Polar Year), he was a respected scientist in his own right. He strongly discounted a connection between aurorae and clouds. His observations confirmed the existence of a zone of maximum auroral frequency some distance south of the

4

N. C. GERSON

pole. He concluded that this zone moves northward during winter and southward during the equinoxes. He wrote many of the instructions for the auroral observations during the FPY, and devised a scale to indicate the auroral intensity. Although Weyprecht’s guiding hand to further a polar year was absent, his friend and colleague, Count Hans Wilczek continued the efforts. (Indeed, Count Wilcaek was the Maecenas of the Austrian contribution to the

FPY .) 1 3. German Commission on Arctic Exploration Shortly after delivery of Weyprecht’s paper, a German Commission on Arctic Exploration was appointed by Bismarck. The Commission, in its report to the Bundesrath, concluded that: “a. The exploration of the Arctic regions is of great importance for all branches of science. The Commission recommends for such exploration the establishment of fixed observing stations. From the principal station, and supported by it, exploring parties are to be made by sea and land. “b. The Commission is of the opinion that the region which should be explored by organized German Arctic explorers is the great inlet to the higher Arctic regions situated between the eastern shore of Greenland and the western shore of Spitzbergen. “c. It appears very desirable and, as far as scientific preparations are concerned, possible, to commence these Arctic explorations in 1877. “d. The Commission is convinced that an exploration of the Arctic regions based on such principles, will furnish valuable results, even if limited to the region between Greenland and Spitzbergen; but it is also of the opinion that an exhaustive solution of the problems to be solved can only be expected when the exploration is extended over the whole Arctic zone, and when other countries take their share in the undertaking. “The Commission recommends, therefore, that the principles adopted for the German undertaking should be communicated to the governments of the States which take interest in Arctic inquiry, in order to establish, if possible, a complete circle of observing stations in the Arctic zones.” Weyprecht’s proposal also came to the attention of the Permanent Committee of the Vienna Meteorological Congress. The Committee a t its third meeting (London, April 18-22, 1876) was convinced of the soundness of the proposition. The Committee ‘‘recommends most warmly to all countries that they should, in the interest of science, support and take part in any undertaking.’’ Locations were suggested for the new observing sites. Later, the Second International Meteorological Congress (meeting at Rome

FROM POLAR YEARS TO IGY

5

in April, 1879) instructed the International Committee to convene a special Conference at Hamburg to settle the details and implementation of Weyprecht’s scheme. Thus, the International Polar Conference convened a t the Deutsche Seewarte (in Hamburg, October 1-5, 1879) to deliberate on Weyprecht’s plan for an orderly and scientific examination of the polar regions. 1.4, Objectives of the International Polar Conference

The International Polar Conference, after its initial meeting at Hamburg (1879), met subsequently in Berne (1880) and St. Petersburg (1881). At

the latter conference, delegates were present from the United States and all leading European states except the United Kingdom. Because of their value, even in retrospect, the recommendations of the Hamburg Conference are given rather literally below. It will be seen that many of these objectives continue to be, almost without change, goals of the IGY. It, also, should be very carefully noted that the report of the Hamburg Conference established most of the precedents followed during the Second Polar Year and now during the IGY. These precedents, mainly stated explicitly in the Hamburg Conference documents, are : a. The principle of international collaboration and cooperation in geophysics. b. An emphasis on geophysical studies in both polar regions combined with strong support in temperate zones (especially from the permanent observatories). c. The hope for a continued existence of some of the stations expressly established for the International Year. d. The adoption of the principle of strong support by military and naval forces of participating countries. e. The use by cooperating nations of both governmental and private funds to finance International Year operations. f. A study of the planet in as many geophysical fields as technology and interest permit. g. The principle of standardized instruments, common techniques, and synoptic observations where necessary. h. The definition of intensified periods of observations; i.e., “term days,” international days, (world days), etc. i. The extension of observations to mobile craft; e.g., “merchant and naval vessels,” (aircraft). j. The principle of common interchange of data. k. The principle of publication of results within reasonable time periods after conclusion of the International Year. 1.4.1. Recommendations of the Hamburg Conference. “a. The object of the

6

N. C. GERSON

undertaking which occupies the Conference is in the first place the investigation of the meteorological and magnetical, and generally the physical conditions of the polar regions, and the immediately contiguous zones of the earth, according to a uniform plan to be determined by international agreement. “b. These investigations are to be carried out especially a t certain stations, and a t fixed observatories to be established a t the same, and commencing operations at the same time. “c. The expenses of establishment and maintenance of such station or stations are to be borne by the country (or persons participating) undertaking their establishment. “d. As a justification of the importance of the undertaking it should be set forth :(1) From a meteorological point of view, that without an exact knowledge of the occurrences and phenomena which take place in the polar regions, we cannot think of the possibility of establishing general principles and theories relating to the pressure of the air, the distribution and variations of temperature and air currents, the development and change of weather, or general climatological laws. For the northern hemisphere, and especially for the meteorological phenomena in North America, Northern Europe, and Northern Asia, this proposal is evident a priori, and may be immediately demonstrated by a study of synoptic charts and of the results of simultaneous observations. Owing to the close connection of the Antarctic regions-undisturbed by continental influence-with the contiguous districts of higher latitudes within which the international traffic by sea is carried on, the establishment of general laws, on the one hand, can be essentially promoted, in consequence of the homogeneity of the earth’s surface, whilst, on the other hand, the extension of meteorological investigation towards the south, by the further development of science cannot be dispensed with. Especially important is the extension of the domain of synoptic, that is, of simultaneous meteorological work, towards the Arctic regions, for the development of the prognostication of weather and storms for the whole of Europe and North America. (2) From the standpoint of the science of terrestrial magnetism, that simultaneous observation at stations chosen according to certain points of view to be determined upon, in both polar regions, for the development of the theory of the disturbances in the magnetic elements, whose relations to aurora and sunspots is a condition without the fulfillment of which no decided advance in our knowledge can be hoped for.

FROM POLAR YEARS T O IGY

7

(3) That for the knowledge of the distribution of the magnetic force of the earth, and its secular and other changes over the earth, a thorough investigation a t a definite epoch of the present time is evidently indispensable. (4) That the hydrography of the oceans-the theory of the distribution of heat and currents-is wanting in those fundamental factors by the help of which alone a scientific foundation, sufficing for all requirements, can be obtained, as long as a thorough examination of the polar regions, by means of accurate instruments, does not exist. ( 5 ) That the knowledge of the figure of the earth is incomplete and rests partly upon hypothesis, as long as no certain measurements exist, according to the most recent methods, in the polar regions and especially in the northern hemisphere. “e. It is shown sufficiently from these concise arguments, given to justify the importance of systematic scientific researches in the polar regions, that in almost all branches of science, and other reasons drawn from the natural sciences could be added to the list, the progress of human knowledge will be limited and retarded without the extension of scientific facts by means of observations in those parts of the earth. “f. With respect to the natural sciences generally, the Conference has to impose upon itself certain limits, because it represents especially the interests of meteorology and terrestrial magnetism; but it is also its duty to make further limitations with regard to these branches of science. “g. I n order to ensure the attainment to the end proposed, the Conference deems it expedient, with reference to the physical sciences, to distinguish more minutely between compulsory and special observations. “h. Compulsory observations are those which must absolutely be taken, if the system of observation is not to remain incomplete, and omissions to arise which would materially compromise the deduction of general results, and presumably even make them impossible. These comprise all meteorological and magnetical observations, observations of the aurora borealis, and hydrographical surveys, in which simultaneity must be held as a first condition. “i. The domain of special observations cannot be here precisely determined, because this would be synonymous with a catalogue of all the sciences interested. It will suffice here to draw attention to some individual observations, such as: pendulum observations to determine the figure of the earth; hydrographical observations made a t the fixed stations attached to the expeditions; astronomical determinations, referring to atmospheric refraction; the radiant points of meteors, etc. “j. With respect to the choice of places of observation (stations) the Conference unanimously arrived a t the following decisions :

8

N. C. GERSON

Considering the importance of the western and northern regions of Europe for the meteorological conditions of the Northern Hemisphere, and considering the importance that the investigation of the influence of the zone of the greatest intensity and frequency of the aurora borealis must have upon the character of the variations in the magnetic elements, the Conference proposes the establishment of fixed stations a t the following points of the Northern Hemisphere : Spitzbergen, Finmark (North Cape), Nova Zembla, Mouth of the Lena, Point Barrow, the archipelago of North America, Upernavik (West Greenland), and Jan Mayen, or the east coast of Greenland. “k. The Conference further declared it to be its well-considered opinion that the occupation at least of the stations named was absolutely necessary for the complete solution of the problems in the domain of meteorology and terrestrial magnetism. “1. I n consideration of the importance of synchronous magnetic observations in the Arctic and Antarctic regions for the extension of our knowledge of the character of magnetic disturbances, and of systematic meteorological observations in high southern latitudes, the Conference is of opinion observing stations should, if practicable, be established and maintained for a definite period a t the following places: South Georgia, Kerguelen Island, Auckland or Campbell Island, and Balleny Island, if a landing is possible. “m. The Conference is of the opinion that negotiations with the governments and the International Meteorological Committee, and the preliminary arrangements for carrying out the scheme of polar exploration will be expedited by taking the observations in 1881-1882, and that, accordingly, endeavors should be made that these observations should be begun in the Northern Hemisphere in the summer of 1881, and that they should be continued for a t least a year. “n. With respect to the publication of the observations taken during the period in question, the Conference expresses its opinion in the following points: (1) All the observations are to be published in eztenso as soon as possible. (2) As it will be of importance that, for the synoptical investigations of the meteorological conditions during the period of observation, ail abstract of the meteorological observations should be forthcoming as soon as possible, it should be urged, with the concurrence of the International Meteorological Committee, that a t the latest one year after the termination of the observations, as far as this may be possible, the publication of such an abstract should be made, according to a uniform plan to be settled by the above-named Committee. (3) It is very desirable that the whole of the observations, wherever

FROM POLAR YEARS TO IGY

9

measures come into question, should be published in metric measures, and the temperature in degrees centigrade. “0. The Conference is of the opinion that international participation in addition to the governments, societies, etc., already represented at this Conference is indispensable to the success of the enterprise, and hopes that the proximate convening of a second and more generally attended Conference, after this preliminary one, will meet with success, and setting out from this point of view, it adopts the following resolutions : (1) The promotion of the matter with the different governments, so. cieties, etc., must be taken up energetically and without delay. (2) The Conference should declare itself to be a permanent International Polar Commission, until the solution of the problems for which it has been assembled, and should constitute itself as such immediately, by the election of ti President, in order to have a central point for the direction of business and the promotion of the scheme. (3) The Conference desires that the International Polar Commission should be increased by the addition of delegates from other countries which have not been represented at this meeting. (4)The report and protocols of the proceedings of this Conference should be brought to the knowledge of the International Meteorological Committee without delay, with a request for their energetic support in the accomplishment of the resolutions adopted. “p. In order to obtain, by the carrying out of a common plan . . . as complete contributions as possible for the solution of the meteorological and magnetical questions over the whole globe, the International Meteorological Committee is requested: (1) To have the comparison of instruments, recommended by the Congress of Rome, especially that of barometers, carried out as soon as possible. (2) To take care, in good time, that not only the activity of the meteorological and magnetical stations already existing should be as comprehensive as possible, and extended in individual details during the periods of observation, but also that temporary stations be established wherever a good connection between the proposed Arctic and Antarctic observing stations, and the actually existing stations in the temperate zones does not exist. (3) To exert its influence that the magnetic observatories upon the continents also should take more frequent observations upon the term days fixed by the program of actual work, and at least at the special hours defined (as per protocols not printed here). (4)To exert its influence, that all over the globe, and especially in higher latitudes, as many self-recording instruments as possible be employed

10

N. C. GERSON

during this period, and that a certain selection of stations of the second order take observations several times a day, a t equidistant and synchronous epochs. (5) To exert its influence that the navies and mercantile marines of the different maritime nations should take part during the same period in the observations named under (4).” (Met. Council, 1881.) The conclusions of the Hamburg International Polar Conference were followed, although the time period was advanced one year to 1882-1883. Practically every statement and objective was reiterated in the succeeding International Years, the SPY (Second Polar Year) and the IGY.

2. THE FIRSTINTERNATIONAL POLAR YEAR 2.1. Observatirms

The FPY observations were initiated by all polar expeditions and temperate latitude observatories as soon as possible after August 1, 1882, and concluded as closely as possible to September 1, 1883. Hourly readings were attempted of all meteorological, magnetic, auroral, and optical phenomena. In addition, on the first and fifteenth of each month, readings of the magnetic elements were made every five minutes of the 24-hr period. During one hour of a predetermined day of the month, magnetic readings were taken every 20 sec. The standard of time chosen for the FPY was the mean time of Gottingen, Germany. 2.2. Stations

Expeditions dispatched specifically to establish FPY stations are listed in Table I. Several attempts were made also to undertake geophysical measurements in Antarctica. France indicated its intention of making observations a t Cape Horn. Germany instituted recordings at South Georgia. An Italian Antarctic Expedition dispatched under Lt. Bove was unfortunately wrecked in the Southern Hemisphere. Argentina had planned to send the ship Cape Horn to Antarctic waters. I n all, the FPY inspired 15 expeditions (12 to the Arctic and three to the Antarctic) by 11 nations. The Italian expedition did not establish a station. The Netherlands group, starting for Dikson in the Varna, became icebound and wintered in the Kara Sea where observations were taken. Although the ship later was crushed, all aboard were saved. The U.S. expedition to Ft. Conger successfully established its site and undertook the desired measurements. The party arrived in August, 1881. As relief ships failed to arrive with vitally needed food and supplies, the party marched out two years later. Trekking southward with scanty provisions and in indescribable misery, many of the party died. Seven finally

TABLEI. Arctic FPY stations. Station

Ft. Conger, Lady Franklin Bay Cap Thordsen Sagastyr Island Moller Bay, Karmakule Bay Point Barrow Jan Mayen Bossekop Alten Fjord Kara Sea (Dikson) Sodankylii Kultala Kingua Fjord, Cumberland Sound Six Moravian Missions Godthaab Fort Rae

Location Ellesmere Island, Canada Spitzbergen Mouth of Lena, Siberia Novaya Zemlya Alaska Jan Mayen Norway

Occupying Country

Latitude

Longitude

Station Leader

u. s.

81'45"

64"58'W

Lt. A. W. Greely

Sweden Russia

78"U)'N 73'23"

15"E 126"35'E

N. G. Ekholm Lt. N. D. Jurgens

Russia

72"23'N

52'45'E

Lt. K. P. Andreief

u. s. Austria Norway

71'18" 70'60'N 69'58"

156'24'W 828'W 23"15'E

Lt. P. H. Ray E. E. Wohlgemuth A. S. Steen Dr. Smaller

v

Kara Sea, Russia Finland Finland Davis Strait, Canada Labrador

Netherlands

69"42'N

64"45 'E

Finland Finland Germany

67"26" 68"30 'N 66'30"

26"34'E

Germany

Greenland Great Slave Lake, Canada

U. K.

(55"25'N58"48'N) 64'11" 62'39"

Denmark

26'46'E 66"W

c X Fc

? c3

0

W. Giese K. R. Koch

51'44'W 115"44'W

0

A. F. W. Paulsen Lt. H. P. Dawson

5Fc

12

N. C. GERSON

were found, just alive. A special court of inquiry investigated the disaster (U.S. War Department, 1884). 2.3. Results.

Preliminary results were announced at the Fourth International Polar Conference (Vienna, 1884), where the chiefs of most expeditions were present. The conferees were personally received by the Emperor. The Conference agreed to publish the data independently in each country according to a uniform plan, with hourly observations being given in detail. Although it had been hoped that results would appear before Christmas 1885, the first report was published in early 1886. Within a few years, reports for Pt. Barrow, Ft. Rae, Cape Horn, Sagastyr, Jan Mayen, Cumberland Sound, and South Georgia also appeared. The final meeting of the Commission convened in Munich (1891). The published material is indicative of the vigorous activity of all observational teams. Observations of air temperature, barometric pressure, and clouds were taken. Magnetic readings, auroral observations and, a t some sites, earth current intensities were recorded. Latitude or longitude determinations, geological studies, and geographic and topographic reconnaissances were conducted at several locations. Glacial, sea-ice, and limited oceanographic observations (including river water temperatures) were undertaken. Oceanographic measurements also were made en route by several ships while bringing or returning the expeditions. A listing of the research fields is given in Table 11. Some extensive reports were prepared in the field of the botanical sciences-a field not given pronounced prominence during the FPY. Practically all expeditions published some reports on the fauna and flora of the region. Such topics as mammals, birds, fishes, insects, marine invertebrates, lichens, crabs, fungi, tunicates, echinoderms, mollusks, crustaceans, insects, etc., were included. In addition, there were ethnological sketches and studies of the Eskimos. The group at Ft. Conger examined a petrified forest, while a t Sagastyr, the party investigated the preservation of mammoths in (and eventually caused their excavation from) the permafrost. The published observations were valuable many years later in comparing events with the SPY. Thus, FPY data were employed to study Arctic barometric pressures (Vincent, 1910); magnetic changes (Krakau, 1925) ; cold waves (Henry, 1928); magnetic storms (Bastamov, 1929); continental drift (Tollner, 1932); and auroral current systems (Rolf and Olsen, 1937). General references to the FPY include works by Borgen (1882); Breitfus (1930); Hoffmeyer (1880); Regele (1954); Wilczek (1933); and Wild (18821891). References to specific expeditions are given by Abbes (1884a); Bunge

TABLE 11. Fields Studied at FPY stations.*

X X X X X X X X X X

X X X X X X X X X X X X

X

X

X

x

x

x x

xt

x

x

X X X X X

x X

X

X X X X X X X X X X X X

x

x

x

x

x

x x

x x

x x

X

X

X

T

FROM POLAR YEARS TO IGY

Ft. Conger Cap Thordsen Sagastyr Island Karmakule Bay Point Barrow Jan Mayen Bossekop Kara Sea Sodankylii Kingua Fjord Godthaab Fort Rae

cd 0

5 a J m' +

I?

Tt, Gt

Tt

X

X

* S, sporadically; T, temperature; G , specific gravity; and C, chemical analyses. t On voyages to or from station.

13

14

N. C. GERSON

(1895) ; Dawson (1886); Deutsche Polar Kommission (1886) ; Ekholm (1886-1891, 1887); Greely (1884; 1886a, b); Grinevetsky (1884); James ( 1940); Jiirgens (1885) ; Lanman (1885) ; Lemstrom (1887); Lemstrom and Biese (1886);Lenz (1886); Neumayer (189G1891);Paulsen (1886-1894) ; Ray et al. (1885); Rust (1883); Schley (1887); Steen (1887-1888); Swedish Royal Academy of Science (1886) ; Tillo (1886-1895) ; Tromholt (1882) ; Wichmann (1884) ; and Wohlgemuth (1886). References to some published analyses include Abbes (188413); Andr6e (1883a, b, c); Carpenter (1887); Eschenhagen (1887); Johansson (1903, 1917); Kerbert (1887); Koch (1884); Kuznetsov (1886); Lamar and Ellis (1884); Lemstrom (1898) ; Liznar (1888) ; Lutken (1887) ; Murdock (1885, 1892); Pfeffer (1886); Ruijs (1887); and Scharizer (1884).

INTERNATIONAL POLAR YEAR 3. THESECOND 3.1. Background

Considerable advances organizationally, scientifically, and technologically occurred in the interval between the FPY and SPY. In the United States, meteorological activities were transferred from the Chief Signal Office of the Army to the Weather Bureau in July, 1891. In the same year, the International Meteorological Conference (Munich) adopted new cloud classifications, and agreed that after 1901: (a) all temperatures would be referred to readings of the air thermometer, and (b) the value of gravity a t 45" latitude would be adopted as standard. Jan Mayen was revisited by Professor Pouchet of France in 1892, ten years after the FPY. In 1893, the U.S. Weather Bureau invited various European governments to Washington for an international meteorological conference to consider: (a) uniformity in storm warnings and weather signals, (b) cooperation in preparing daily weather charts, (c) more equitable station distributions, etc., and to determine means for encouraging special scientific investigations leading to advances in meteorology. Other geophysical sciences realized the importance of coordinated measurements. The International Geodetic Congress (Berlin, 1895) proposed that observations be organized in a permanent manner at sites equally distributed around the earth (at about the same latitude), in order to determine small possible movements of the terrestrial axes. Marconi spanned the Atlantic with low-frequency radio waves in 1901. (Many physicists and mathematicians of Marconi's time vocally predicted dismal failure for these costly experiments.) The International Research Council was inaugurated (1918-1919) to promote scientific research and coordinate the research activities of adhering

FROM POLAR YEARS TO IGY

15

nations. The International Union of Geodesy and Geophysics (UGGI) was formed in 1919 to promote world-wide geodetic and geophysical studies and to stimulate international cooperation in these areas, The existence of an ionosphere was proposed by A. E. Kennelly and 0.Heaviside; L. G. Breit and M. A. Tuve (in the U.S.) and E. V. Appleton (of Great Britain) independently demonstrated its presence in 1925. British, German, and other expeditions studied meteorological and glacial conditions in Greenland. The Arctic slowly awakened; the advent of aircraft opened it to commercial exploitation, more permanent settlements, and rudimentary, but routine, geophysical studies. Since the FPY many geophysical fields (e.g., meteorology, terrestrial magnetism, auroral physics, etc.) had progressed considerably. Several new sciences (cosmic rays, ionospheric physics) had evolved. However, as knowledge expanded, a host of new problems arrived. A feeling was developing that some of these problems could be better attacked if data from ,more global sites were available. In essence, the time was approaching when a suggestion for a Second International Polar Year would be warmly received. Equipment improvements and increased knowledge in many fields were leading to the thought that the time was ripe to repeat the FPY enterprise but on the basis of the more advanced technology of the day. In this atmosphere, Johannes Georgi of the Deutsche Seewarte, Hamburg, proposed (in 1927) the implementation of another International Polar Year, I n geophysical circles, this proposal could only prosper. Admiral H. Dominik, President of the Seewarte, submitted the proposal to the International Meteorological Committee where it was directed to the attention of RBseau Mondial and Polar Meteorology. The International Meteorological Committee considered the design in London (1928). A subcommission was created to formulate a comprehensive plan for a Polar Year. The detailed report which was prepared was submitted to the International Meteorological Conference of Directors a t its Copenhagen (1929) meeting. By adopting the report as a basis for further research, the Copenhagen Conference sponsored the SPY. The Conference felt that the observations should be undertaken for one calendar year and proposed the jubilee anniversary of the FPY. Justification for the enterprise is given in one of the resolutions of the Conference: “. . . magnetic, auroral and meteorological observations a t a network of stations in the Arctic and Antarctic would materially advance present knowledge and understanding of (these phenomena) not only within polar regions but in general. . . This increased knowledge will be of practical application to problems connected with terrestrial magnetism, marine and aerial navigation, wireless telegraphy and weather forecasting.”

16

N. C. GERSON

3.2. The International Polar Commission

An International Commission for the Polar Year 1932-1933, under the presidency of D. LaCour of Denmark, was established to organize and integrate the total effort. The Commission was also charged with detailed planning of the observations to be made and the methods for making them. The SPY was brought to the attention of governments. The UGGI was invited to support the venture and cooperate with the International Commission. Special attention was given the importance of work which might be done in the Southern Hemisphere. Before delving into its activities, the Commission's philosophy may bc mentioned. It fully recognized the gains in knowledge and techniques for the different sciences, and also appreciated the emergent new zones of ignorance. In its records the Commission noted many unresolved problems. The interplay of forces and energy exchange which affect the atmosphere, hydrosphere, lithosphere, and deep interior of the earth were mainly unknown. The Commission deliberately proposed some SPY stations, not for, the purpose of solely observing or studying polar phenomena, but primarily to allow an integrated examination of polar and global processes. This attitude was a considerable step from the FPY where the terrestrial atmosphere had been studied mainly as an isolated entity. Advancing physical theories had made it increasingly apparent that the atmosphere was part of a complex system involving not only other areas of geophysics but the sun itself. The existence of ions, charged condensation nuclei, the ionic layers, and the atmospheric current systems were now known, but their behavior and perturbations with time were far from clear. The formation of luminous aurorae by extraterrestrial bombarding particles seemed qualitatively evident, but a complete understanding of the complicated physical processes contained many gaps. Large current systems in the high atmosphere were accepted, but knowledge of their effect on wireless telegraphy rested on uncertain premises. On a practical basis, the polar charts of magnetic variation currently in use were based upon data obtained during the FPY. New observations were necessary before the charts could be modernized to include the appreciable changes which had occurred in the interval, especially in the Arctic. In the field of weather forecasting, knowledge on the behavior of the polar atmosphere was considered essential before marked improvements in middle latitude prognostications could arise. Simultaneous determinations of the altitude of the polar and equatorial tropopauses were deemed necessary. Studies regarding the effect of the tropopause on atmospheric circul:1 t '1011 over large latitude differences, on the interaction of air masses of different properties, etc., were desired.

FROM POLAR YEARS TO IGY

17

The SPY, when compared to the FPY, differed in a very important respect. Its base, broadened by the advance in knowledge over the intervening half-century, was more critical and more scientific. Apart from an interest in collecting data for future archives, it was also more concerned with describing the physics of the events. In response to the plea of the Polar Commission, national committees were formed in nations adhering to the SPY. The committees organized their individual contributions and carried out recommendations in collaboration with the Polar Commission. The pattern established during the FPY repeated itself: Cascades of committees, both formal and informal, were established, and scores of meetings on both the national and international levels were organized to plan, promote, and coordinate this large effort. The International Union of Geodesy and Geophysics considered the SPY at its Stockholm (1930) meeting. The General Assembly of UGGI warmly supported the proposal and accepted the invitation to cooperate. UGGI appointed a special commission composed of members of its Meteorological and Magnetic Associations. This commission collaborated closely with the Polar Commission not only in evolving the final program but also in furnishing financial and technical assistance. UGGI, for example, supplied copies of Stormer’s Photographic Atlas of Auroral Forms and furnished auroral cameras and spectroscopes. In meteorology, no profound advances were expected, but results of importance for practical meteorology were anticipated. It was hoped that a better understanding of the global circulation could be obtained and, in particular, a clearer picture of the function of the enormous heat engine that represents the atmosphere. Better determinations of solar radiation, movements of the atmosphere, spatial gradients of temperature and relative humidity, etc., were desired. Aurorae were studied to determine their location in space, their global extension, their excited constituents, and their relation to the ionic layers. The dependence of auroral changes on solar events, magnetic variations, cosmic radiation, and radio-wave propagation disturbances were sought. The relationship of magnetic variations to solar phenomena and electrical characteristics in the high atmosphere was desired. A better insight into secular variations of the magnetic field was needed. More precise charting of the existing field of the globe would permit the construction of new magnetic maps for navigational purposes. The distribution and propagation of periodic and aperiodic magnetic changes, and their relationship to solar events were to be scrutinized. The morphology and structure of magnetic storms on a global basis were to be studied. Ionospheric investigations would present information on the oc-

18

N. C. GERSON

currence and variation of the ionic layers: their height, ion production, ion density, effect on radio-wave propagation, etc. At the stimulation of D. LaCour, the International Council for the Exploration of the Sea (1930-1931) recommended “that every effort should be made by all countries adhering to the Council to extend their hydrographical work in the (SPY).” The Council noted that some countries were planning extensive hydrographic observations in the Polar Sea, and implied the hope that more research be done in this area. The International Cloud Commission in several meetings between 19291931 extended the SPY into an international cloud year. The Radiation Commission of UGGI formulated a program of radiation observations for incorporation in the SPY. The International Commission on the Investigations of the Upper Air in Madrid (1931) resolved that upper air observation during the SPY would be of fundamental significance. At Copenhagen in the same year, the International Scientific Radio Union (URSI) strongly recommended that scientific radio observations be made during the SPY. The number of distinct scientific groups which affiliated themselves with the SPY was greater than anticipated. Most established commissions to prepare programs and issue minute instructions necessary for their fulfillment. Some (UGGI, URSI) provided funds or the means of collecting the records and undertaking initial reductions of the data. In view of the very broad geographic coverage of the SPY, it is no wonder that the president of the Polar Commission termed it “L’Ann6e Mondiale.” The Polar Year Commission held meetings a t Leningrad (1930), Innsbruck (1931), and Copenhagen (1933), and its publications subcommittee met in London (1930). In addition to reporting progress, these meetings noted program deficiencies and passed recommendations for their removal. Insofar as the time period of the SPY was concerned, it was agreed that stations in the Arctic would carry out their programs from August, 1932 to August, 1933, and in the Antarctic, from January, 1932 to January, 1934. During the planning stages, a severe economic crisis engulfed the world and almost wrecked the SPY. It became extremely difficult for a number of countries to provide the funds needed to implement their previously prepared programs. The Commission was strongly and repeatedly urged t o delay the Polar Year indefinitely. The effect of the unfortunate coincidence was overcome largely through the enthusiasm and energy of D. LaCour, president of the Polar Year Commission. He withstood reiterated attempts to defer the project. At the October, 1931 meeting of the International Meteorological Committee, he stated: “I wish to point out how the position is. . . Now the Commission presents its program and recommends unanimously the carrying out of the work. Now I ask: Will the Committee on this basis take

FROM POLAR YEARS TO IGY

19

the responsibility of stopping very useful work and the responsibility of an eventual failure of participation in a new Polar Year?. . . '' It was a personal triumph for D. LaCour (because of his untiring and persistent efforts) when the International Meteorological Committee unanimously agreed to undertake the venture as planned. Arguments against a delay of the SPY were considerably strengthened by Great Britain through its action in placing $10,000 at the disposal of the British National Committee. The Commission for the Polar Year had weathered a severe crisis. Later the Rockefeller Foundation granted $40,000 to the Polar Year Commission for the purchase of magnetic and electric equipment, and radiosondes, with perhaps $5000 devoted to operator training if necessary. This relatively immense and unexpected sum allowed an increase in the polar observational network, and to some extent off set the program curtailment caused by the financial depression. Nevertheless, some nations (e.g., Germany) found it impossible to formally join the SPY. The labors of the International Polar Commission had been arduous and diligent and they had succeeded. 3.3. Scope of the SPY The SPY extended from August 1, 1932 to August 31, 1933, essentially during a period of sunspot minimum. At its opening, national committees for the SPY had been established in 16 countries. During the SPY 22 nations dispatched expeditions or implemented stations beyond their borders. Other nations cooperated by following the SPY observational schedule a t permanent geophysical observatories. Data were collected in equatorial regions for use in studies of the general atmospheric circulation, and various Southern Hemisphere observations were instituted. A total of 44 countries participated. A list of these nations and the geophysical fields of inquiry is given in Table 111. Some curtailment of the original program resulted not only because of the global financial crisis, but also because of an epidemic on Kerguelen and political unrest in South America. In the Northern Hemisphere, the number of magnetic stations north of 60"N was increased from 7 to 30. Sixteen permanent geophysical observatories cooperated north of 55"N : Eskdalemuir, Lerwick, Godhavn, Meanook, Sitka, Sodankylii, Abisko, Lovo, Tromso, Rude Skov, Sloutsk, Koutchino, Kazan, Sverdlovsk, and Matotchkin Schar. In addition, 25 special stations were occupied : Julianehaab, Greenland (by Denmark) ; Thule, Greenland (by Denmark) ; Scoresbysund, Greenland (by France) ; Ivigtut, Greenland (by Germany) ; Angmagssalik, Greenland (by the Netherlands); College, Alaska (by the U.S.); Point Barrow, Alaska (by the U.S.); Fort Rae, Canada (by Great Britain); Chesterfield Inlet, Canada (by Canada); Kingua Fjord, Canada (by Canada) ;Bear Island (by Poland);

TABLE111. National programs during the Second Polar Year.

Algiers Argentina Australia Austria Azores Belgium Brazil Bulgaria Canada Chile China Columbia Czechoslovakia Denmark Egypt Finland France Germany Great Britain Haiti Hungary Iceland

x

X X X

X

x X

x

x x x

X

X X X

x

X

X

x x

x

x

X X

x

x

X

x

x

X X

X

x

x

x

x

x

X

x

x

x

x

X

X

X

X X X

x

X

x x

x

x x x X

x

x x

x x

To : Jan Mayen By: Portugal To: Elizabethville, Belgian Congo To: Tatuoca, Brazil

x x

X

X

x

X X

X

x X

x x x

x x

x

X

X

By: France

x

X

x

x

X

X

X X

X

X

X

x

x

To: Cape Hopes Advance, Chesterfield Inlet, Coppermine, Meanook, Saskatoon To: Magallanes

To: Godhavn, Julianehaab, Thule To: Petsamo, Kajaani X To: Scoresbysund, Tamanrasset, Bangui, Tananarive X To: Ivigtut (Many colonies included). To : Fort Rae, Tromso

x Reykjavik by Netherlands, Snaffellsjokull by Switzerland

?: ? 0

E 8

B

India Indonesia Italy Japan Latvia Mexico Morocco Netherlands New Zealand Norway Peru Philippines Poland Portugal South Africa Spain Sweden

X X

x x

X

X

x x

x

X X X X

X X

x

X

x x

x

Switzerland Syria Tunis Turkey X

U. S. S. R.

x

x x x

x

x

x

x x

x

x x x

x x

X X X

X

x

x x

x

x

x

X

x

x

X

x

x

X X X

X

x

x

To: Angmagssalik, Reykjavik

X

x x

x

X

Jonsbu, Storfjord, Finnsbu, Torgilsbu, E. Greenland; Bodo, Bossekop

x x x

To: Mogadiscio To: Toyohara, Sakhalin

X To:

X X X X

x

X X

X

X X X X

x x

x x x X X

X

u. s.

Yugoslavia

x

x

x

x

w

0

r

To: Bear Island To: Azores To: Fernando Poo,Montserrat,Montseny X To: Sveagruvan, Spitzbergen; Mt. Nordenskjold, Spitzbergen To: Snaffellsjokull

.-l

+4 M

+g +l 0

i;

rc

X

x

x

x

X

X

x

x

x

X

X

X

s

To: College, Pt. Barrow; Peary Lodge, Greenland To:Rudolf Land, Tikhaya Bay, Cape Chelyuskin, Dikson, Matotchkin Schar

E

22

N. C. GERSON

Cap Thordsen, Spitzbergen (by Sweden); Reykjavik, Iceland (by the Netherlands) ; Snaffellsjokull,Iceland (by Switzerland) ; Bodo, Norway (hy Norway); Bossekop, Norway (by Norway) ; Petsamo, Finland (by Finland) ; Kajaani, Finland (by Finland) ; Lycksele, Sweden (by Sweden); Yakutsk, U.S.S.R. (by U.S.S.R.); Bouloum, U.S.S.R. (by U.S.S.R.); Dikson, U.S.S.R. (by U.S.S.R.); Hooker Island, Franz Josef Land (by U.S.S.R.); and Kandalakscha, U.S.S.R. (by U.S.S.R.). The total expeditionary effort to the Antarctic suffered because of fund limitations, and was therefore not pursued in the manner originally desired. Chile established a station at Punta Arenas and Argentina in the South Orkneys. Norwegian whalers made meteorological and other observations when in the Southern Hemisphere and particularly when near Antarctica. LaCour magnetometers were installed at Christchurch, New Zealand ; Tananarive, Madagascar; Huancayo, Peru; and Watheroo, Australia. Because of the dearth of stations and the lateness in installing of some equipment, Southern Hemisphere observations later were continued until February 1, 1934. New magnetic observatories were established at Cape Town, Elizabethville, and Magallanes. In other geographic regions, all geophysical observatories were invited to participate, taking the same observations according to the same schedules as at the polar stations. A high degree of cooperation was obtained. Many new middle latitude stations were set up especially for taking upper air observations with radiosondes. Some of the special nonpolar stations established for the SPY include Tamanrasset, French Africa; Bangui, French Equatorial Africa; Elizabethville, Belgian Congo; Mogadiscio, Italian Somaliland; Montseny; Azores ; Fernando Po0 ; Toyahara, Sakhalin ;and Mt. Fuji, Japan. With the participation of the U.S.S.R. an extensive observational network (92 stations of which 32 were newly established) was added through Eurasia and the Arctic. A large sea-ice physics program, augmented by a comprehensive hydrographic and oceanographic survey in the Arctic Ocean was included. Glacial observations were made and an icebreaker dispatched to establish the most northerly station of the SPY-Tikhaya Bay, at 81’40’N. Icebreakers made meteorological and oceanographic studies near Spitzbergen, Franz Josef Land, in the Barents and Kara Seas, and during their voyages to supply the Siberian coastal stations. 3.4. The Scientific Program

The scientific program of the SPY was principally concerned with meteorology, terrestrial magnetism, and the aurora and, to a lesser extent, with atmospheric electricity. Many continuous records of the earth’s potential gradient, air conductivity, air-earth current, small ion content of air, and

FROM POLAR YEARS TO IGY

23

nuclei of the atmosphere were made. Radio-wave probings of the ionosphere were undertaken. In meteorology, complete surface and upper air observations were obtained. Pilot balloons were released twice daily. Kite flights were made a t some locations, weather permitting. Radiosonde measurements and aerometeorograph observations with aircraft were accomplished a t selected stations on a regular basis. The temperature difference between the ground and definite heights above ground were recorded. Detailed observations, including moving pictures, were made to obtain information on the height, speed, and evolution of clouds. Nacreous clouds were observed. Over two-hundred radiosondes were supplied to the different observing stations. Explosions were organized in Sweden to investigate the propagation of sound waves under polar conditions. Mountain stations were established for meteorological observations. Ozone determinations were made. Rapid-run magnetographs and other magnetic observations were organized at continental sites and at some island stations in temperate latitudes. Both normal and rapid-run earth current recordings were obtained. Magnetic stations cooperated in making meteorological observations. Auroral observations were made from the North Geomagnetic Pole to the south of Australia. More data than ever before recorded were obtained. The prior distribution of the auroral atlases had provided a unified, consistent approach which simplified the later analyses. Triangulation on specific aurorae from two stations separated by a known baseline allowed accurate altitude determinations. Radio probings of the ionosphere from a limited number of polar and middle latitude locations provided more information on the state of the ionosphere. Oceanographic observations in the Arctic and in the eastern South Atlantic Oceans were obtained. Cosmic-ray studies were included at some sites. Although the Polar Commission had hoped that similar instruments could have been utilized everywhere for similar observations, this objective proved unattainable. As during the FPY, special periods of intensified observations were designated. These international days of first and second order mainly applied to the fields of meteorology, aurora, magnetism, atmospheric electricity, etc. 3 5 . Results

The Polar Year Commission met in Warsaw (September, 1935) to receive reports and summaries of accomplishments, and to arrange for disseminating the observational material. It was recognized at this conference, and also in subsequent meetings, that some type of cataloguing was necessary if the overwhelming quantities of observational data were ever to become known and available to researchers. The International Meteorological Or-

24

N. C. GERSON

ganization, assisted by grants from the Rockefeller Foundation requested the International Polar Year Commission to oversee the indexing of data, distribution of reports, and reduction of the observations. The Commission continued its functions until 1946, when the International Meteoro1ogic:d Organization formally dissolved all permanent commissions. Iluriiig the 1940’8, the Commission suffered two major setbacks. The untimely death of its president (1942) removed the guiding spirit and vision of D. LaCour and produced a serious loss. Also, the advent of World War I1 severely hampered the Commission in its operations. Even after dissolution of the Commission, the thought persisted that some arrangement should be made to continue the cataloguings and reductions. Since, also, some funds were still available, the International Meteorological Committee (Paris, 1946) established a Temporary Commission on the Liquidation of the Polar Year 1932-1933 under the presidency of J. A. Fleming. Originally, the TCLPY was to exist only until December 31, 1950, but later its life was extended for another year. In general, the accomplishmentsof the SPY belied the many dire predictions regarding its fate. Over 800 scientific papers were published as well as innumerable official reports, periodic summaries, popular articles, unpublished memoranda, and data tabulations. Probably well over 90% of the reports have been listed in a readily available publication (Laursen, 1951), Perhaps the three outstanding results of the SPY were: (a) the construction of daily Northern Hemisphere meteorological maps for each day of the SPY, (b) the verification of the marked effect of some magnetic storms on the reflection of radio waves from the ionosphere, and (c) the transit within one season of the northeast passage from the Atlantic to the Pacific Oceans (from July 28, 1933 to October 1, 1933) by the icebreaker Sibiriakov. In addition, a host of other results were made available. Magnetic data from Cape Town, South Africa; Tatuoca, Brazil; and Magallanes, Chile, were reduced. Meteorological and aerological data from many isolated sites were printed. Reports on magnetic activity were prepared. Calculations of the three hourly magnetic K indices and correlation coefficients K / K , for the SPY were also completed.

4. THEINTERNATIONAL GEOPHYSICAL YEAR 4.1. Introduction

Within a score of years after the SPY, the world witnessed unparalleled changes on all scientific fronts. Perhaps the most outstanding practical accomplishments were the tremendous strides in electronic, rocket, and nuclear engineering. Each provided new tools for geophysics. The first

FROM POLAR YEARS TO IGY

25

two brought radio astronomy, radio meteorology, radar auroral studies, and direct rocket soundings of the upper atmosphere. The last allowed unprecedented potential for initiating crustal, seismic, deep oceanographic, and high atmospheric studies, and for causing marked and widespread changes on the face of the globe. In rocketry, man was a t the verge of the space era; a whetted interest could induce the attempt. If, as LaCour remarked, the SPY hastened the development of the radiosonde, then surely the IGY spurred the launching of satellite vehicles. A realization of the impact of these rapid advances, an appreciation of the value of the new instrumentation and improved measuring techniques, and apprehension over deteriorated international relations prompted Lloyd V. Berkner of the U.S. to suggest (1950) the implementation of a Third International Polar Year (TPY). The envisioned international collaboration would not only provide increased knowledge of the globe, but would permit nations to join a common cause-the battle against geophysical ignorance. The plan was formally sponsored (Brussels, 1950) by the Mixed Commission on the Ionosphere, which proposed to the International Council of Scientific Unions that a Third Polar Year be undertaken 25 years after the SPY. The Council (Washington, 1951) recommended that further consideration be given the proposal by the International Unions of Astronomy (IUA) , Geodesy and Geophysics, Geography, Pure and Applied Physics (IUPAP), and Scientific Radio, and the World Meteorological Organization (WMO). The Mixed Commission again deliberated on the TPY in Canberra (1952), where S. Chapman suggested that emphasis on polar aspects be modified and that tropical observations formally be included. The proposal was reindorsed with the thought that the name of the enterprise be changed to the International Geophysical Year. At its Amsterdam meeting (1952), the President of the Internatioiiul Council of Scientific Unions reported on the Mixed Commission’s request to organize a Third International Polar Year a quarter century (19571!358) after the SPY. The endeavor was supported by the Unions of Astronomy, Geodesy and Geophysics, and Scientific Radio. Further, there had been a suggestion that a fresh determination of the world longitude network be made. (For five years Japanese scientists already had been examining problems connected with variations in longitude and rate of the earth’s rotation.) Since interest seemed so much more widespread, geographically and geophysically, the Council accepted the Mixed Commission’s proposal and re-christened the endeavor “International Geophysical Year.” Previously, (January, 1952) the Council had distributed a circular letter

26

N. C. GERSON

to all adhering national bodies asking that national committees for the global geophysical enterprise be formed. These committees would sponsor, plan, and integrate the work within their respective countries. Japan was among the first to organize its committee. Later the Council formed a Comit6 Sp6cial Ann6e G6ophysique Int6rnationale (CSAGI) to act as the international guiding and policy making group for the IGY. CSAGI responsibilities included the preparation of a coordinated, integrated, and mutually acceptable global ICY scientific program in time to allow its initiation on July 1, 1957. The CSAGI was composed of representatives of the International Scientific Unions. Its president, vice president, and secretary general were S. Chapman, L. V. Berkner, and M. Nicolet, respectively. From among its members, reporters were designated to oversee the development and coordination of programs in the fields of auroral and airglow physics, cosmic rays, geomagnetism, glaciology, gravity, ionospheric physics, longitude and latitude, meteorology, oceanography, and seismology. Rockets and satellites were deemed of sufficient importance to warrant a special reporter to stimulate the total effort and to integrate global plans. Also, because of the need for prompt and widespread notification of nations, isolated stations, and permanent observatories whenever special periods of intensified observations were designated, a reporter was assigned for World Days and Communications. CSAGI presented plans for the IGY a t its first meeting in Brussels (1953). At that time, it considered proposals from 21 nations as well as from UGGI, URSI, IAU, WMO, and the Mixed Commission on the Ionosphere. CSAGI again invited all nations to participate. Additional meetings in Rome (1954), Brussels (1955), and Barcelona (1956) received scientific contributions from IUPAP and the Mixed Commission on Radio Meteorology. The meetings allowed a critical review of all national programs and permitted their integration as one coordinated inquiry into planetary physics. The degree of cordiality, friendship, and cooperativeness in evidence at these meetings (which occurred a t times of marked international differences) serves as a model for international collaboration.

4.9. Objectives The objectives of the IGY were fourfold. In order of importance, these were stated as: ‘(a. Geophysical problems requiring for their solution concurrent synoptic observations at many places on the globe and involving coordinated efforts by many nations. These problems include meteorological, ionospheric, auroral, cosmic ray, and geomagnetic observations.

FROM POLAR YEARS TO IGY

27

"b. Geophysical problems, the solution of which will be aided by the availability of the results of synoptic or other concentrated geophysical work undertaken during the ICY. Thus, disciplines such as meteorology, ionospheric physics, auroral physics, cosmic rays, and geomagnetism are studied in this manner. ('c. Observations of other major geophysical phenomena in which the main program involves the occupation of stations in regions of the earth to which comparatively little geophysical effort has been devoted in the past, but which will, as a result of the IGY be areas of accelerated interest. The Antarctic and certain equatorial regions are examples. "d. Observations of slowly varying geophysical phenomena, for comparison with similar future observations at later epochs." As during the previous occasions (FPY and SPY), national programs were based on the interests and capabilities of the scientists concerned. Whenever possible, special attempts were made to implement a t least several sites for the same type of observation. In one sense the areas of interest, general objectives, and planning for the IGY had already been delineated or undertaken previously (in the SPY if not in the FPY). However, the detailed development of the IGY was truly remarkable. In grandeur, magnitude, funds, scientists, and stations, it far surpassed its predecessors. It is true that much of the precedent already had been established implicitly or explicitly by the Commissions of the previous International Years. These precedents and the basic tenets of the FPY (see Section 1.4) were followed closely. However, the details involved in implementation, standardization, and coordination had increased almost exponentially with time and directly with the greater complexity of the endeavor. That the IGY will accomplish its missions successfully can only be attributed to the untiring devotion and labors of the host of geophysical workers in every participating country. During the IGY, special attention is being devoted to six geographic areas: the Arctic, Antarctic, and equatorial, and the pole-to-pole sectors centering on the meridians near 7Oo-80"W, lO'E, and 140"E. Establishment of these zones was considered to be of great importance in studying the magnetic field, the ionosphere, aurorae, and meteorology. One major st,udy, for example, concerns the mass movement of the atmosphere (or the oceans) across the latitude circles, including the equator, and around the globe.' The station chains will allow some inferences on the heat exchange between temperate and polar regions. The closely spaced networks, by measuring ionospheric drifts, will also provide some inkling on the planetary circulation pattern a t ionospheric altitudes. 1

This objective and others were common to both the IGY and SPY.

28

N. C. GERSON

4.3. General Program Implementation

Fifty-seven nations have endorsed the IGY, with most of them having adhered formally to the IGY and having formed National Committees. These nations include Argentina, Australia, Austria, Belgium, Bolivia, Brazil, Bulgaria, Burma, Canada, Ceylon, Chile, China (Peking),2 China (Taipei), Columbia, Cuba, Czechoslovakia, Denmark, Dominican Republic, East Africa, Ecuador, Egypt, Ethiopia, Finland, France, German Democratic Republic, German Federal Republic, Greece, Guatemala, Hungary, Iceland, India, Indonesia, Iran, Ireland, Israel, Italy, Japan, Korea (Democratic Republic), Malaya, Mexico, Mongolian Peoples Republic, Morocco, Netherlands, New Zealand, Norway, Pakistan, Panama, Peru, Philippines, Poland, Portugal, Southern Rhodesia, Spain, Sweden, Switzerland, Tunisia, Union of South Africa, U.S.S.R., United Kingdom, United States, Uruguay, Venezuela, Viet Nam Democratic Republic, Viet Nam Republic, and Yugoslavia. The degree of involvement of any nation in the global IGY program is a function of its interest, capabilities, and the funds available. In general, the larger and more scientifically advanced nations committed themselves to a greater number of disciplines and a greater number of projects. It is not possible in the limited space available to provide a complete listing of all projects included, stations occupied, or of the IGY geophysical studies in progress. Such a compilation would require a volume in itself. However, a very brief outline will be given of IGY objectives in several selected areas; viz., the Antarctic, Arctic, rocket explorations, and satellite studies. With the unprecedented effort presented by the IGY to explore the planet, some of the difficult aspects of the programs were initiated early. Antarctic surveys by several nations began with the cruise of the USS Atlca (October, 1954) to Little America and the Western coast of Antarctica. By the beginning of the IGY, practically all nations undertaking an Antarctic expedition had become established on the continent, although all planned bases were not fully occupied. The Antarctic operation proved to be the most extensive and coiicentrated drive ever made by mankind to one region for geophysical investigations. About 40 sites were manned on the continent itself and approximately 15 more on the surrounding sub-antarctic islands. Twelve nations were involved as shown in Table IV. The massive onslaught on Antarctica is in principle comparable to the coordinated establishment of 2 China (Peking) has withdrawn from the IGY in protest at the inclusion of China (Taipei). However, China (Peking) will pursue its planned ambitious program despite withdrawal. An oceanographic vessel will participate. The meteorological network will be expanded from 50 t o 1000 stations. Twelve satellite tracking stations will be implemented. Magnetic and seismic stations and a geophysical observatory a t Lhasa, Tibet have been established. USSR press reports indicate that China (Peking) may launch an earth satellite.

FROM POLAR YEARS TO IGY

20

Arctic bases during the FPY. The present more comprehensive, more complex, and more involved Antarctic undertaking (which incidentally is proceeding with far greater safety and comfort), may be considered an indication of 75 years of technological advances, Another measure of the magnitude of the Antarctic effort is provided by a comparison with the programs in progress in other regions. The number of new stations established in Antarctica exceeds that on any other Southern Hemisphere land mass, while the total costs involved in the Antarctic venture undoubtedly exceed the combined outlay for all other Southern Hemisphere stations established for the IGY. In addition to observations a t newly established bases, field and traverse parties are making extended trips to map, explore, and probe the continent. An IGY weather center was established a t Little America to provide the first clear concepts of Antarctic weather patterns and to allow preparation of complete Southern Hemisphere weather maps. The IGY Arctic effort continued the tradition of strong emphasis on the Arctic during the International Years. To complement a well-designed and integrated station network across the North American and Eurasian continents, four stations were established on the drifting ice of the Arctic Ocean. The two U.S. stations are known as drifting stations A and B. Shtion A was located initially near 80"N, 159"W on sea ice about 7 ft thick and 2 mi2. Station B, Fletcher's ice island, was a t 83"N, 1OO"W or1 ice 150 ft thick. The U.S.S.R. stations, SP6 and SP7 were originally near 79"N, 175"W and the north geographic pole, respectively. Stations A, B, and SP7 were established by early 1957,and station SP6 in late 1956. At the drifting stations, emphasis is placed on glaciology, meteorology, and oceanography, although observations in the other disciplines are also included. Heat budget studies are an integral and important portion of the program, not only for evaluations of the planetary and Arctic heat budget but also for comparison with similar investigation in Antarctica. Participating in the global ICY rocket program are Australia, Canada, France, Great Britain, Japan, U.S., and the U.S.S.R. The U.S. and the U.S.S.R. plan small rocket or rockoon flights from Antarctica and probably from shipboard in neighboring Antarctic waters. In the Arctic, the U.S.S.R. will launch rockets from Franz Josef Land. The U.S. will utilize ship launched rockoons in the Davis Strait from Thule to Frobisher Bay and probably thence southward. In a joint Canadian-U.S. program, a scientific rocket launching facility was developed a t Churchill. At this location, Canadian, US., and possibly British rocket borne experiments will probe the high polar atmosphere, particularly to gain an understanding of conditions and events in the auroral zone. Other sites from which rockets will be fired include the Woomera range, Australia; White Sands and Pt. Magu, U.S.; Guam, Japan, North Africa, Danger Island and central U.S.S.R.

w

0

NOSX33 ‘3 ‘N

Seismology

Oceanography

Meteorology

Ionospheric physics

Gravity

Glaciology

Geomagnetism

Geology

Cosmic rays

Aurora

X

15 20 10

x x

x x

x

x

X

X

x

x

X

x

x

X X

X

x

x

X X

x x

x x

X X X

0 M

z

8

x

X

X

x x x x x x x x x x

x x

xx

x x x x

xxxxxxxxxx

X

X X

x x x x x x x x x x

xx

x

X

xxxxx

x

X

x x x x

x

X

x X

xxxxxxxx

x

X

x

xxxxx

X X X X

X

x

xx

158'58'E 62'53'E 77" E 147'20'E 23 'E

x

xx

68

42'53' 70'30'

'

X

xx

67'36

6 12 15 8 15 7 20 12

x

54 "29'

67"06'W 38O48'W 59"57'W 62'59'W 56O49'W 62'52'W 60"43'W 44"43'W 68'19'W 67"47'W

xx

68"08' 77"58' 62'37' 64"20' 63'16' 64"53' 62'59' 60"45' 54"48' 53"48'

xxxxxxxxxx

P

x

Argentina San Martin Belgrano Tte. Camara Melchior Esperanza Alme. Brown Decepcion Orcadas Ushuaia Rio Grande Australia Macquarie Mawson Vestfold Hobart Belgium

Complement

Latitude

Longitude

TABLE IV. IGY Antarctic studies.*

Chile Yankee Bay O'Higgins Copper Mine P. A. Credo Evangelist Videla Prat Arenas Ramires France Geologie Dumont D'Urville Kerguelen Amsterdam Traverse Worse1 Japan Soya New Zealand Scott Campbell Island Invercargil Christchurch Norway Queen Maud Land Union of South Africa Marion Island U . S. S. R. Mirny Vostok Sovietskaya Pioneerskava

62'37' 63"19' 62"23' 62"56' 52"25' 64"49 ' 62%' 5390' 56'30'

59"50'W 57"54'W 59"52'W 60'36'W 74"55'W 62'51'W 59"37'W 70"55'W 68"45'W

66"40 71" 48'40' 37"50

140'01 'E 137" E 69'14'E 77'34'E

X

20

x

X

x

X X X

x

X

x

X

x

x

x

X

x

X

x

X

8

x x x

X X X

x x x

x x x

X

x x x

x x x

x x x

X

20 30

x

x

X

x

x

X

X

x x

x

X

x

X X

X

x

X

X 70' 77"52' 5232' 4625' 43"32 71"

E

40

x

167'30'E 169"59'E 168'19'E 172'37 'E

14

X

w

14

35"

1"

66'51'

37"52 'E

66"35' 7827' 78"24' 69'44'

93" E 106"52'E 8735'E 95"30'E

x x x

x

x

x

X

30 15

x X X

x

X X X X

x x x

X

x

x

-

X X

X X

X

x

X

x

X

x

x

x X X

x

X X

X X X

x

x X X

x

32

w

TABLE IV. IGY Antarctic Studies (Continued)

t G

E

X

N. C. GERSON

x

x

x x

X X X

x

~~

x

X

xx

_

X

x

xx

_

5 P

x

_

x

x

~

xx

~

x

x

x

x

~

x

x

x

xx X

x

X

x

X X

x

x

X X X X X

xxxxxxx

X

xxxxx

xx

x

x x x x

xxxx

xxxx

xx X X

x

x x x x x

X X X X X X

X X

X

x X

X

x x

xxxxxxxxxxxx

xx

X

x X

xx

121 18

x

~~~

14

xxxxx

~

X

X X X X X

x

X X X X

x

X

xxx

~

X

X

X X X

90'00'

X X

x

96 23 39 29

x

x

162"lO'W 12O"Ol'W 41'08'W 110'31'E 170'20 'E 166'44 'E

x

x

78"ll' 77 "53' 77 "43' 66'16' 72"29' 77"53 '

w

x

X X

X

20 5 30 12 10 10 10 8 10

X

W"30'W 63'31'W 60"34 56"59'W 64"16'W 58"25'W 45'36'W MOO5 W 67"17'W 67 "00'W 57"52'W 36'31'W

x

75"40' 64"50' 62"59' 63'24' 65'15' 62'05' 60"43' 64"45' 67 "49' m a l l' 51'42' 5476'

xx

United Kingdom Hallet Bay Base A Base B Base D Base F Base G Base H Base N Base Y Stonington Pt. Stanley S. Georgia U.S. Little America Byrd Ellsworth Wilkes Hallet McMurdo Amundsen -Scott

h

X

~~

* Note: Special studies (e.g., in physiology, psychology, meteor observations, telluric absorption, etc.) have been included a t a number of stations but are not listed above.

FROM POLAR YEARS TO IGY

33

Probably the most impressive development during the IGY was the program for earth satellites. While earth satellite programs undoubtedly were in various stages of development in the principal nations, the IGY accelerated the effort by providing a definite target date (as distinct from a “drift toward completion”). The U.S. and the U.S.S.R. both announced plans for launching earth satellites, and did so during the early IGY period. Press reports imply that China (Peking) also may launch earth satellites.

4.4. ScientiJic Program Some indication of the scope of the ICY may be obtained from a brief description of the planned programs in each discipline, as given below. 44.1. Aurora and Airglow. Observing stations have been established from the Arctic drifting stations to Antarctica and the South Geographic Pole. Visual observations are underway at many stations as well as from mobile craft. Horizon-to-horizon photographs with all-sky cameras have been implemented on all continents. However, a semblance of an adequate spacing for the all-sky camera networks is found only in the Northern Hemisphere. Better determinations of the distribution of aurorae over the globe during sunspot maximum, and of auroral altitudes will be sought. Longitudinal networks of spectrographic stations will provide an insight on the distribution, energy, and constitution of the incoming particles as a function of latitude and time after onset of an auroral display. Patrol spectrographs and scanning spectrophotometers in the western hemisphere will provide detailed information on the character of rapid changes in auroral excitation patterns. Correlations will be attempted of specific events in the aurora borealis and aurora australis as a function of both time and space. Radio probings of the aurora will provide information on the electron density of, the magnitude of radio-wave energy absorption by, and the degree of interference and modulation produced on signals returned by, the ionized aurora. Measurements on the absorption of cosmic noise by the aurora will assist in these determinations. Studies of the airglow are being made with spectrophotometers of various degrees of sophistication at many sites over the globe. The movement of excitation patterns (Leuchtstreifen) at specific wavelengths will be plotted to gain some insight on the meaning of such drifts and the speed of the apparent winds. 4.4.2. Cosmic Rays. Cosmic rays and associated neutron intensities will be measured by means of cloud chambers, ionization chambers, neutron monitors, neutron and cosmic-ray telescopes, photographic emulsions, etc. Observations will be made from a large network of ground stations, and from ships, aircraft, balloons, rockets, and earth satellites. Determinations will be made of the cosmic-ray mass and charge spectra, and the types of

34

N. C. GERSON

particle causing large intensity increases during solar flares. The relationships among cosmic-ray changes, solar activity, and magnetic variations will be examined closely. Balloon, rocket, and satellite experiments will allow studies of the cosmic-ray symmetry with respect to the earth as a dipole in space. The discrepancy between the geomagnetic equator and the “cosmic-ray equator” (i.e., the earth’s magnetic equator as “seen” by corpuscular cosmic radiation) will be investigated further. High altitude emulsion blocks and photographic emulsions will permit more definite studies on the nature of primary cosmic radiation, especially its isotopic constitution and fragmentation probabilities. Studies are underway on extremely large (if not gigantic) air showers, the movement of the knee of the latitude curve, and the association of cosmic rays to other geophysical phenomena. The intensity of cosmic rays deep within the ground also is being studied. 4.4.3. Geomagnetism. As during the SPY, rapid-run and standard magnetographs will provide details on short term magnetic changes. From closely spaced station networks, investigations on the strength and duration of the atmospheric current systems will be made. In this fashion, more detailed information on the current systems near the auroral zone and near the equatorial electrojet will be provided. The spatial gradient of the magnetic field and the rate of change of this gradient will be measured. Very low frequency magnetic variations (about 1 to 50 cy/sec) are being recorded a t several locations from Thule to Puerto Rico to determine their relation to lightning, ionospheric electric currents, and extraterrestrial currents. Earth current measurements are underway a t a number of sites in Canada, Europe, and Asia. Rocket and satellite borne magnetometers will determine the terrestrial magnetic field at distances of 100 to 1000 km above the planet. A magnetic survey of the globe by the nonmagnetic ship Zarya is underway. 4 4.4. Glaciology. The glaciological program of necessity is associated intimately with the programs of oceanography, meteorology, and climatology. The program is linked with studies of the net planetary heat budget and climatic trends of the earth. Objectives of the program include a determination of the ice volume now locked in Antarctica., Greenland, the Arctic Ocean Basin, and Arctic and middle latitude glaciers. Aerial surveys showing the extent of selected glaciers and glacial areas will be made. Additional frequent surveys in the Arctic Ocean Basin will indicate the’ short-term sea-ice changes as influenced by wind, ocean movement, and temperature changes. Detailed heat interchange studies in the Arctic Ocean will provide information on the heat distribution and flow among the atmosphere, the sea, and the ice. Glacial and Antarctic ice examinations will provide data on ice movements, ablation-accretion cycles, snow

FROM POLAR YEARS TO IGY

35

stratigraphy, ice topography, and topography of the underlying land mass. Atlases for comparison with glaciation in the past and future epochs will be prepared. 4.4.5. Gravity. As most gravity determinations have been made in populous or mineral-promising areas, much more information is needed from the Arctic, Antarctic, southern continents, and most of the oceanic surface. The present serious gaps in the international gravity network probably will not be eliminated, but attempts will be made to reduce the deficiencies. Pendulum and gravimeter base points in Antarctica will provide points of departure for continental surveys and will permit more accurate pole-to-pole gravity surveys along several longitude lines. Earthtide measurements will be made at many locations and the rigidity of the earth will be determined. From the many gravity determinations, the shape of the earth may be more precisely calculated. Extension of the measurements to both polar regions will improve the international formula for determining the variation of gravity at sea level. From the observations the true value for the flattening of the reference ellipsoid may be better defined. 4.4.6. Ionospheric Physics. Radio-wave probings of the ionic layers have been extended so that a global station network much greater than ever before has been obtained. Vertical incidence and backscatter stations will transmit and receive radio-wave energy through given frequency ranges in order to determine the state of the ionosphere overhead and at a distance from the station. Several pole-to-pole networks of vertical incidence stations have been implemented. Radio-wave absorption is under intensive study a t a number of locations, particularly western Europe; both the pulse comparison technique and the recording of cosmic noise are being utilized. Ionospheric drifts are being examined at a fair number of sites well distributed throughout the globe. Special experiments in forward scatter, meteor ionization, auroral ionization, and intense equatorial ionization are in progress. Radio amateurs in the 50 mc/sec and higher frequency bands are cooperating in sporadic E, auroral, and trans-equatorial propagation programs. Several studies are underway of radio waves in the frequency range 5 to 20 kc/sec (whistlers), their propagation through interplanetary space and their use in allowing deductions on interplanetary proton concentrations. The association of whistlers with lightning discharges, aurorae, and extraterrestrial events is under investigation. Correlation of ionospheric, magnetic, and solar activity will be intensified. Rocket and satellite experiments will attempt to determine the type and character of ions forming each layer, the true electron densities as a function of height, the character of solar radiation producing the layers, etc.

36

N. C. GERSON

4.4.7. Latitude and Longitude. Photographs of the moon with the specially designed Markowitz cameras will be made at about 20 astronomical observatories. From photographs of the moon and the surrounding stars, station locations relative to the earth’s center may be more accurately ascertained. These data will permit latitudes and longitudes to be more precisely known. 4.4.8. Meteorology. Because of the increase in number of polar ohserving stations, particularly in the Southern Hemisphere, more information will become available on the characteristics and planetary dynamics of the lower atmosphere. Special effort is being devoted to raising the maximum altitude of sounding balloons from 25 km to 30 km, and to increasing the number of aerological sounding stations. Pole-to-pole meridional cross sections will allow charting of air mass and moisture transport. Detailed polar studies, including measurements of solar radiation, and ice and snow reflection coefficients will provide data for hentbalance determination. Air radioactivity will be measured. Concentrations of ozone, carbon dioxide, and other gases will be studied. Instances of occurrence and duration of the Scherhag effect (explosive warming in the stratosphere) is being sought throughout the globe. The Antarctic weather central is undertaking detailed analyses of Antarctic and Southern Hemisphere weather patterns. 4.4 3.Oceanography. Investigations are being made from coastal and insular stations as well as from ships conducting extensive oceanic surveys. Over 200 sea-level measuring sites and more than 70 oceanographic vessels are participating in the ICY explorations. Many new stations have been established in the Southern Hemisphere and in equatorial regions. Topography of the ocean bottom and the concentration and types of life at different depths will be determined wherever possible. Multiple ship surveys of temperature, salinity, chemical composition, and ocean current systems are underway. Water currents to all depths will be studied where feasible. To determine the rate of ocean turnover, the age of sea water as a function of depth will be obtained through carbon 14 dating procedures. Ocean bottom samples and cores will be obtained. 4.4.10. Seismology. In general, seismic stations operated only for the 18-month interval of the ICY will not provide marked benefits to seismology; the time period being too short geologically. Probably the greatest value to seismology during the IGY will stem from those stations which, once established, may remain in existence for much longer periods of time. Seismic studies in Antarctica and Greenland will reveal the profile of the underlying land. Long-period wave observations are being collected over widespread geographic areas. Investigation of coastal structure is being made through simultaneous land and sea measurements. Crustal strain accumulation is being measured in certain earthquake-prone zones. The

FROM POLAR YEARS TO IGY

37

installation of new stations has been closely coordinated with all interested seismologists. The greatest number of new stations is located in Antarctica. 4.4.11. Solar Activity. The number and size of sunspots, the state of the solar corona, the intensity and location of flares, the unusual emission of solar radio noise or corpuscles, etc., will be recorded a t a fair number of observing stations. The magnetic field pattern over the solar disk is being mapped daily. Correlations between specific solar events and cosmic-ray, geomagnetic, and ionospheric changes are being made. 4.4.12. World Days and Communications. An extensive communications network has been established to provide for coordinated intensification of certain observations. Following the pattern of the FPY and SPY, certain days have been designated for making comprehensive observations in' selected scientific fields. These periods of intensified study, known as World Days, have been divided into three categories: Regular World Days, World Meteorological Intervals, and Special World Intervals as shown in Table V. The two former are confined to meteorological and similar observations. They include appropriate phases of the moon, the beginning of spring, summer, fall, and winter, and the occurrence of meteor showers. Special World Intervals are prescribed on a day-to-day basis when solar activity is (or is expected to be) unusually high, and when outstanding geomagnetic storms, with corresponding abnormal effects in aurora, cosmic rays, and ionospheric physics are probable. 4.5 World Data Centers The data avalanche implicit in the above observations necessitates a careful program for the storage and dissemination of the material to interested researchers. Toward this end three World Data Centers (A, B, and C) are being established to serve as primary repositories of the data. These centers will catalog, store, and make available IGY information. Either copies of the original material, or the original data themselves, will be deposited in these archives. The ICY material to be interchanged in each geophysical discipline includes those records, observations, and preliminary tabulations which have already been agreed upon internationally. World Data Centers will maintain central catalogs and indices (of all deposited information), which will be available to participating IGY National Committees, to national and international scientific groups, arid to their sponsored scientists. Further, the centers will permit scientific bodies arid their scientists access to all IGY material in their possession, arid will provide copies of the data at cost. The U.S. will establish Center A and the U.S.S.R., Center B. World Data Center C will consist of a number of distinct subcenters located, respectively, in various countries throughout western Europe and the

38

N. C. GERSON

TABLEV. International days of the IGY World Meteorological Interval

20-29 June 18-27 September 12-21 December 17-26 March 15-24 June 13-22 September 12-21 December Regular World Day (*at new moon) 27*, 28, 29t June July (twith unusual meteoric activity) 4t, 26, 27*t 12t, 25*, 26 August 1, 23*, 24, 30 September 22, 23*, 24 October 14, 21*, 22 November December 13t, 16, 21*, 22 January 3t, 4t, 19*, 20 February 10, 18*, 19, 26 20*, 21, 28 March April 18, 19*, 20 5t, 18*t, 1Qt May June Qt,17*, 18, 24 July 16*, 17, 27t August 7t, 127, 14t, 15* 6, 13*, 14, 20 September October lot, 11, 12*, 13 4, 10, 11*, 18 November lo*, 11, 13t, 17 December January 3t, 4t, 9, lo* Days of total eclipse 23 October April 19 October 12 Days of unusual meteoric activity 8,9,10 June 5 August 4 July Designated on a day-to-day basis Special World Intervals necessary.

1957

1958

1057

1958

1959 1957 1958 1957 1958 when

Pacific. In the U.S., Data Center A will be composed of one central coordinating headquarters or clearing house and eleven subsidiary centers. The latter will serve as archives for data in definite areas of geophysics. General references to the planning and program of the IGY are given by Berkner (1954), Chapman (1953, 1955), International Union of Geodesy and Geophysics (1957), and Kaplan (1954, 1956). 4.6. Preliminary Results

Although the IGY is still in progress as this article is published, several noteworthy results have been announced. Perhaps the most strik-

FROM POLAR YEARS TO IGY

39

ing was the launching of the earth's first satellite vehicle, Sputnik I, on October 4, 1957. Sputnik I was followed by Sputnik 11, Explorer I, Vanguard, Explorer I11 and IV, and Sputnik 111. Launching dates and some of the geophysical measurements being made from the satellites are listed in Table VI. (U.S. National Academy of Sciences, 1957; U.S.S.R. Embassy, 1958; Smithsonian Astrophysical Observatory, 1958; Whipple et al, 1958; Yatoukin, 1958). The information received from all satellite vehicles is now under analysis; however, from the preliminary statements already issued, no startling departures from anticipated values have yet been found below 1000 km. A band of intense radiation has been discovered near 1000 km; its cause and nature is still not entirely clear. Several interesting results were reported from Antarctica. A minimum temperature of -102.4'F was observed at the South Pole on September 17, 1957 and -108"F, the lowest ever recorded on earth, at Vostok on May 2-3, 1958. At some Antarctic stations a bifurcated annual temperature variation was found with a weak secondary maximum occurring in winter (June-August). The cause for such a double maximum is unknown at the present time. (This phenomenon does not occur in the Northern Hemisphere). Also found in the aerological soundings was an explosive warming that traveled as a warm pulse from the Knox coast to the Ross Sea. An ice island 54 mi. by 18 mi. and 120 ft high was sighted near Antarctica. Seismic soundings through the Antarctic ice cap, although very spotty, imply that the level of the ground underlying the ice mantle may be underwater in many locations. Thus, Byrd Station, for example, a t the foot of Ford Mountains was found to rest on about 10,000 ft of ice. Since Byrd Station is only about 5000 ft above sea level, it has been tentatively concluded that at its location there exists either a large inland sea or a deep fjord. At the South Pole, the ice mantle seems to be nearly 8300 ft in thickness supported by solid rock about 900 f t above sea level. Other Antarctic developments include the completion, by a British Commonwealth team, of the first transcontinental traverse in history (from McMurdo Sound to the Weddel Sea by way of the South Pole). The IGY weather center at Little America, staffed principally by the U.S., but with meteorologists from the U.S.S.R. and Argentina, fulfills both scientific and operational requirements; e.g., the preparation of daily synoptic weather maps, issuance of daily forecasts, etc. Ozone measurements at Little America indicate a surface ozone concentration about 25% greater than that existing in New Mexico. The percentage of carbon dioxide in Antarctica was found to be about the same as that in other nonindustrial areas. In the Arctic Ocean Basin, Drifting Station A reported that 12 in. of

TABLE VI. Comparison of IGY satellite vehicles. Name Launching datet Mass (lb) Payload (lb) Shape Diameter (in) Length (in) Initial orbit: Period (min) Apogee (mi) Perigee (mi) Equatorial angle (") Lifetime Transmission frequency (mc/sec) Experiments*

Sputnik I Oct. 4, 1957 184 ... Spherical 23.2

Sputnik I1

Explorer I

Vanguard I

Explorer I1 March 5, 1958 78.1 30.8 Cylindrical-conic 6.02 78.7

Feb. 1, 1958 78.1 30.9 Cylindrical-conic 6.02 78.7

Mar. 17, 1958 3.24 1-2 Spherical 6.46

...

Nov. 3, 1957 7055 1121 Cylindrical-conic 48 (?) 22.9 (?)

96.2 590 140 65.3 92 days

103.7 1038 149 65.4 161 days

114.95 1578 229 33.14 3-5 years

134.29 2464 405 34.30 years

Failed to orbit 33

20.005 40.002

20.005 40.002 CR, M, Ph, SR, Ts, T

108.03 108.00 CR, M, Ts, T

108.03 108.00

108.03 108.00

...

F 9

T

...

SB

* CR, cosmic rays; H, magnetic field; I , ionic number density; P, pressure; Ph, physiological (on dog); SB, solar batteries; SR, solar radiat)ion (ultraviolet, x-ray or corpuscular) ; Ts, satellite internal temperature; T, satellite surface temperature. t Universal time.

41

t?

2

41

FROM POLAR YEAR8 TO IGY

TABLEVI. Comparison of IGY satellite vehicles (Continued) Name Launching datet Mass (lb) Payload (lb) Shape Diameter (in) Length (in) Initial orbit: Period (min) Apogee (mi) Perigee (mi) Equatorial angle (") Lifetime Transmission frequency (mc/sec) Experiments*

Sputnik 111

Explorer I11 March 26, 1958 78.1 31.0 Cylindrical-conic 6.02 78.7

May 15, 1958

Explorer IV July 26,1958

...

...

2913 Cylindrical-conic 68 137

38.4 Cylindrical-conic 6 80

115.91 1740 117 33.5 94 d= 2 days

105.80 1167 135 64.8 5-6 mos.

110.29 1373 163 50.13 1 year

108.03 108.00 CR,M,Ts, T

20.008 40.009 CR,H,I,M,P,SB, SR, T

108.03 107.995

CR, I

t Universal time.

* CR, cosmic rays; H, magnetic field; i, Ionic number density; P, pressure; Ph, physiological (on dog) ; SB, solar batteries; SR, solar radiation (ultraviolet, x-ray or corpuscular) ; Ts, satellite internal temperature; T, satellite surface temperature. NOTE:The U. S. program of satellite launching8 has been as follows: Test Vehicle zero (TVO), December 8, 1956 (successful); TV1, May 1, 1957 (successful); TV2, October 23, 1957 (successful); TV3, December 6, 1957 (failure); Explorer I, February 1, 1958 (successful); TV3 backup, February 4, 1958 (failure); Explorer 11, March 5, 1958 (failure); Vanguard, March 17, 1958 (successful); Explorer 111, March 26, 1958 (successful); Vanguard, April 28, 1958 (failure); Vanguard, May 27, 1958 (failure) ; Vanguard, June 26,1958 (failure). surface ice melted, and 18 to 24 in. of new ice accreted on the bottom of the ice floe during the summer of 1957. Glacial pits have been dug and ice cores obtained for ice-age analysis in Greenland and Antarctica. A U.S. group found a new Arctic Ocean submarine ridge, probably parallel to the Lomonosov Range, and rising about 5000 ft above the ocean floor. The new range intersects the meridian through Barrow about 850 mi to the north. Two U.S. atomic submarines made underwater passages beneath the Arctic ice via the North Pole. Other IGY accomplishments include the development of the first successful sea-surface gravimeter by the Lamont Geological Laboratory. Oceanographic studies in the Gulf Stream show the presence of a counter Gulf Stream current (with a velocity of about 13 km/day to the southwest) a t a depth 9000 ft. Pronounced changes in the dissolved oxygen concentration have been found in the Atlantic Ocean near 15"s. Rocket probings of the atmosphere during a period of increased solar

42

N. C. GERSON

activity demonstrated the presence of an additional ionic layer about 19 km below the normal D layer, and indicated the usual electron distribution in the higher ionic layers. This sub-D layer presumably was created by increased solar X-emissions. Rocket and balloon flights a t high latitudes have led to the positive identification of X-radiation in the energy range lo4 to 106ev; these X-rays are tentatively believed to arise in the atmosphere from the bombardment of atmospheric molecules by auroral primaries (U.S. National Academy Sciences, 1957). Rocket firings at Churchill indicated the following preliminary results : (a) The temperature maximum (occurring below 50 km in middle latitude) occurs a t about 60 km. (b) The atmospheric pressure and temperature distribution with altitude is different from that found a t lower latitudes. (c) Diffusive separation of gases, determined with a mass spectrometer, seems to appear somewhat above 100 km. At the South Pole station the electron concentration of the ionosphere seems to remain high throughout the polar night despite the absence of solar radiation. Radar auroral studies were made on the austral and boreal aurorae simultaneously. Sputnik observations indicated that cosmic ray intensities increased approximately 40 % from 225 km to 700 km. The increase was related to a decrease in the earth’s cosmic ray albedo (i.e., a reduction in the magnetic screening effect). Studies were made on the propagation of radio waves, particularly at 40 mc/s. From observations on radio dawn and radio dusk deductions were possible on the electron concentration. The electron density appeared to decrease 5-6 times as slow above the maximum as it increased below the maximum; i.e., during October, the electron density decreased by a factor of 0.5 from 300 km to 500 km. These results confirm previous USSR rocket studies which indicated electron number densities of 106/cm3at 473 km. There are good grounds to believe that the temperature and density at 225 km is higher than that determined from rockets. Possible sources of the higher temperature are heating associated with (a) injection into the atmosphere of minute particles; (b) infrasound waves propagating outwards from the troposphere; and (c) electric current systems in the ionosphere. The behavior and condition of the dog during Sputnik 11’s ascent to orbit revealed that the animal withstood the acceleration. It continued to move its head and body freely only up to a certain point. Beyond this acceleration, the dog was pressed to the floor and no obvious movements were detected. Immediately after launching the pulse trebled from its original rate but no morbid symptoms were apparent. During ascent breathing was three-four times as rapid as previously. Once in orbit and in a weightless state, the dog pushed itself off the floor. The rate of breath-

FROM POLAR YEARS TO IGY

43

ing (respiration) declined. After a brief increase in pulse rate, the systole frequency gradually approached normalcy. Movement was moderate in the state of weightliness. A hermetically sealed chamber provided normal atmospheric pressure with an oxygen content of 20-40 % and a carbon dioxide concentration not exceeding 1%. Active chemical compounds (not specified) absorbed water vapor and carbon dioxide, and emitted oxygen. The compounds also absorbed such noxious gases as ammonia. 5. THE INTERNATIONAL YEARSIN RETROSPECT 5.1. General

As a promotional venture, the International Years (FPY, SPY, and IGY) have been tremendously successful. They have been popular with laymen, scientists, and governments. They have been favorably received by heads of state. Indeed, during the planning for the IGY, Pope Pius XI1 commented encouragingly upon the enterprise. It must be admitted that the International Years have ameliorated international tensions and spread good will. They have attempted to provide a common meeting ground and a common goal for nations that disagreed on about all else. But above all, they have inspired in men of all levels of education, training, and fields of pursuit an increased curiosity about the planet whose confines man now plans to leave. The results of the First Polar Year may not have been fundamental, but they nevertheless were valuable. The data allowed verification of the map by H. Fritz (1874) showing the distribution of aurorae over the Northern Hemisphere. The meteorologist confirmed his suspicions regarding the influence of polar meteorology on middle latitude weather and has been left with an insatiable appetite for more polar data. The magnetician obtained material from which to compile more accurate magnetic charts over the earth (particularly in polar regions) and to evaluate dipole and quadrapole moments of the earth’s field. Some analyses of the data were undertaken and a good number of accounts of the expeditions published. However, most of the tangible results of the years of planning, hardship, and expense for the FPY were a series of volumes containing data. These data, incidentally, covered many fields: aurora, geology, geomagnetism, earth currents, meteorology, glaciology, oceanography, and latitude and longitude. In addition, extensive biological surveys were accomplished. About 80 papers were published, including popular accounts, military proceedings, and narratives. Of the scientific material, most articles were devoted to data compilations, with perhaps 10 to 20 papers being scientific studies.

44

N. C. GERSON

The SPY provided a greater return. A set of Northern Hemisphere weather charts was prepared for most days of the Polar Year. A series of magnetic observations was obtained that even today provides the basis for theoretical investigations of atmospheric electric current systems. Correlation studies were made between auroral displays and telluric currents. The association of many magnetic storms and ionospheric disturbances was verified. I n addition, the approach and advent of the SPY inspired several reexaminations of FPY data (Bastamov, 1929; Henry, 1928; Krakau, 1925; Rolf and Olsen, 1937; Tollner, 1932). Observations and analyses were published in the fields of meteorology, radiation, ozone, aerology, geomagnetism, earth currents, atmospheric electricity, ionospheric physics, auroral physics, cosmic rays, hydrography, glaciology, noctilucent clouds, nacreous clouds, biology, and astronomy. Nevertheless, considering the miles of recordings, thousands of photographs, and millions of readings taken, the analytical return from the SPY was by no means commensurate with the observational input. It should be noted that in research, the SPY differed significantly from the FPY. Superficially, it seemed to be a repetition of the FPY but on a much broader scale. Its magnitude was greater, but, more important, its objectives were much more scientific. It occurred a t a sufficient time interval after its predecessor for material advances to have been made in the different sciences. This general advance in knowledge sharpened curiosity and demanded the very awareness that entered into the planning. Although its stated objectives were “to observe. . . ”, there was implicit in the entire venture the hope for deep, continued, and critical analyses of a great portion of the data. The difference between the IGY and the SPY (as compared to the SPY-FPY) is perhaps less striking in some respects but much more startling in others. For example, the physical entirety of the IGY is almost majestic. The number of participating nations is impressive. A veritable army of scientists are deployed. The number of participating stations exceeds all expectations. Its costs are astronomical; easily two billion dollars. Further, the truly remarkable feature of the IGY is, perhaps, its boldness of execution in the space-age era. By his suggestion for the inclusion of satellite launchings and the large international invasion of Antarctica, L. V. Berkner correctly assessed the capabilities of the times. Undoubtedly the space age would have been launched by 1960-1965, in any event, and Antarctica sporadically studied by nations either singly or in consort for many years to come. However, the inclusion of satellite vehicles and Antarctic studies in the IGY hastened their responsible fruition. Thus, the IGY implemented Weyprecht’s original suggestion (whose implementation was impossible with the technology of his day), by ac-

FROM POLAR YEARS TO IGY

45

complishing coordinated station networks in the Arctic, Antarctic, and middle latitudes. It retained major emphasis on geophysics and especially those aspects requiring synoptic ohservations for their study. It retained a minor emphasis in biology and geographic exploration. However, the IGY also expanded Weyprecht’s concepts by including longitudinal sectors, emphasizing equatorial regions, and embracing interplanetary space. 5.2. Dejciencies

In retrospect, the International Years have stimulated the geophysical sciences and contributed somewhat to their general advance. Unfortunately, the FPY and the SPY accepted as a measure of accomplishment the standards of the production factory: number of men employed, number of plants (stations) operating, monetary cost of plant (station) expansion, and number of units (hourly observations of meteorological, ionospheric, oceanographic, etc., phenomena) produced. But advancement in science can scarcely be measured in terms of the data amassed. At this time, it seems as if the IGY also were adopting the production factory measure of success. In retrospect, it is equally clear that the past International Years have produced no great break-throughs. Most of the effort was channeled into data obtainment. The observational program has been tremendously successful, but the follow-up in terms of research on the accumulated data has been noticeably small. While the International Years have provided reservoirs of potentially valuable data, these pools have remained mainly untapped. Most investigators have preferred to utilize their own data, gathered in furtherance of their own theories, surveys, or experiments. Other reasons also contribute to the utilization of but a fraction of the International Year data: general reluctance to examine “cold” data obtained by unknown observers having different personal bias; lack of a station reliability factor; and obtainment of some data because “field parties were in the area,” with little regard to whether the data were needed or not, or how they could be properly analyzed. As one example, we may cite Birkeland’s curiosity and interest in auroral and magnetic storms, which had been further stimulated by results of the FPY. However, to investigate his theories more thoroughly, he found it prudent to instigate three additional expeditions to Bossekop (Birkeland, 1901, 1908, 1909). It is, of course, true that the major portion of the data analysis which took place occurred after conclusion of the International Years. But as a whole, the geophysical advances occurring as a result of the International Years have been modest. Probably the greatest advances in geophysics

46

E. C. GERSON

occurred in the quieter, more academic atmosphere found between the International Years. The SPY gave to the International Years the global concept envisioned by Weyprecht; this planetary outlook continued during the IGY. The SPY also injected a more scientific attitude into the International Years. But it accepted fully from the FPY-and passed to the IGY-a good measure of the principle of observation for observation’s sake. In this connection, the history of major contributions in physics and geophysics provides an effective reminder on a method of maximizing benefits from an experimental program. A careful delineation and planning of the experiments to be performed is a priori necessary, followed by effective and meticulous sifting of the data. Thus far the International Years have been deficient in the latter aspect; in the main, they have revolved about collections of data for climatological or time-averaging purposes. The weakness of the analytical programs of the First and Second International Polar Years has been well recognized in the past. In his address to the General Assembly of the International Council of Scientific Unions, D. LaCour (1935) remarked: “The SPY is designed to supply information for immediate application to problems now awaiting solution; it i s not designed simply to contribute to the existing, overwhelming large mass of undigested observational data from ordinary latitudes.” Even earlier, the Polar Year Commission recognized “ . . . (the) successful completion (of investigations) can alone justify all the time, energy and expense which has been devoted to the Polar Year.” Yet within a few years after the SPY and with but a tiny portion of SPY data analyzed, the opposite view again appeared. F. Debenham (1935) wrote “The conclusions which have been drawn from these (the FPY and SPY) results are, as yet, scarcely in full circulation, but few meteorologists do not sigh for more and more data from the polar regions.” (It may be seriously asked today: Will meteorologists with mountains of undigested IGY data molding in warehouses utter the same sighs several years hence?) Fifteen years after the conclusion of SPY, Laursen (1948) reported that “In terrestrial magnetism, the bibliography will reveal, similarly as in meteorology, the need for completion of various national programs.” These statements forcefully demonstrate that the previous International Years, consciously or unconsciously, concentrated on gathering hordes of records. Today, the ICY is collecting literally billions of observations in many geophysical fields. Further, many millions of observations taken since 1940 or even 1950 are also available, some unreduced, most unanalyzed. While the immediate availability of these records has some utility, by no means does the value of availability compare with the new concepts, ideas, or relationships which may arise through careful examina-

FROM POLAR YEARS TO IGY

47

tion of the data. It is to be hoped that the I C Y of 1957-1958 will be much more successful in analyzing its accumulated information. In geophysics, a shift in emphasis from data gathering to data analyses seems long overdue. Perhaps the answer lies in automatic sensor-computer systems which from continuous recordings a t many different sites feed into a continuously integrating and analyzing device located a t one central location. Such equipments would immediately indicate not only simple factors as timeaverages but could also be programmed to test theories on global patterns. Thus, the computers could indicate the calculated state of mass movements in the atmosphere, hydrosphere, or ionosphere; solutions of the wave equation in seismology, ionospheric physics, oceanography, or meteorology; prognostication of future events in the state of the weather, ionosphere, water supply, or floods; etc. It should be evident that the mere collection of data does not guarantee scientific advancement. Indeed, a patent example to the contrary is found in meteorology where the methods of Polar Front Analysis were developed by Norway primarily because of a dearth of reports from foreign countries during World War I.

6 . FUTURE OF

THE

INTERNATIONAL YEARS

I n view of the tremendous strides in technology, science, and geophysical knowledge since Weyprecht’s time, we may question whether the approach of the International Years for studying the pIanet is as effective now as when first proposed. The value of these simultaneous observations is obvious. Even today, we need but remind ourselves of James C. Maxwell’s remarks (about 100 years ago) on the then newly formed International Magnetic Union: “Bacon’s conception of ‘experiments in consort’ was thus realized, the scattered forces of science were connected into a regular army, and jealousy and emulation became out of place, for the results of one observer were of no value till they were combined with those of the others. ” Large areas of the globe which three-quarters of a century ago were completely uninhabited, today possess sizable and relatively permanent populations. Technology now permits man to establish permanent bases anywhere above sea level (if not well beneath it) on the globe, and with sufficient energy sources to live as comfortably as in his homeland. Thus, for example, permanent geophysical sites may be implemented in Arctica or Antarctica and even in space by any of the major powers without noticeable perturbations of their economy. With the diffusion of peoples into oiice scantily populated regions, with the possibility for rapid implementation of isolated stations anywhere on the planet, man is asymptotically approaching conditions of a constantly

48

E. C. GERSON

existing International Year in the sense proposed by Weyprecht. In short, simultaneous or sporadic observational data in most disciplines from almost any points on the planet can be obtained a t will. Under such circumstances, the thought arises that the time has already passed when synchronous global observations need rely upon International Years for their attainment. Many land regions of the globe may soon have in existence the station networks desired for simultaneous observations in any given geophysical disciplines. The oceanic regions are somewhat more difficult, but the answer may well be in the more widespread use of self-contained, miniaturized, geophysical observatories located in anchored buoys. Perhaps the greatest value of future International Years may lie in the epochal type of observation : to determine from undisturbed base stations the average trend or climatology of the planet as a whole in such fields as heat content, weather, geomagnetic field, water content, sea level, ice cover, fauna, flora, marine life, seismicity, advances or retreats of arid lands, polar environments, planetary (atmospheric, hydrospheric, and lithospheric) chemistry, etc. Such base stations might be 50-mi sq International Parks located in representative areas of the planet (at least one to a continent) and specifically set aside to insure a complete lack of man-made change of any type in the next millennium. Within this park, measurements of the desired type may be undertaken with a fairly secure knowledge that man’s presence has not materially altered the observational site. Further, the International Years could be expanded to include studies outside the earth. Thus, they could be broadened to include investigations made within the solar system; e.g. , observations periodically made from other planets and in interplanetary space. In this connection, measurements of interplanetary “weather,” interplanetary ion content, intensity and spatial gradient of solar corpuscular radiation and their variation with time, intensity of cosmic radiation (both corpuscular and radiative) , variations in the electric and magnetic fields of other planets, the sun, the galaxy, and interplanetary space, etc., could be included in a planetary International Year Program. Finally, the International Years could accelerate the trend away from solely the physical sciences, and embrace a more highly organized program in the biological and even sociological fields. I n all cases, however, the attack should be based upon a sound scientific footing with adequate plans and resources for data analysis. REFERENCES

Note: Exhaustive references to the Second Polar Year have been prepared by Laursen (1951) and the Department of Defense (1954). Rather than repeat the comprehensive listings contained in those volumes, it was deemed advisable to list but

FROM POLAR YEARS TO ICY

49

a few representative, general works. Additional material regarding the International Geophysical Year may be found in National Science Foundation (1957), Nicolet (1958) and Jones (1958). References dealing primarily with the FPY, SPY, and IGY have been indicated by (*), (t), and (I), respectively. * Abbes, H. (1884a). Die Eskimos des Cumberland Sundes. Globus 46, 198-201, 213218. * Abbes, H. (188410). Die deutsche Nordpolar-Expedition nach dem Cumberland Sunde. Globus 46, 294-298, 312-315, 328-331, 343-345, 365-368. * AndrBe, S. A. (1883a). Om Yrsnon i de Arktiska Trakterna. Kgl. Svensk. Vetakad. Handl. 40, 3341. * AndrBe, S . A. (188313). Nagra Anmarkningar om Luftelektricitetens Variationer. Kgl. Svensk. Vetakad. Handl. 40,43-50. * AndrBe, S. A. (1883~).Om sambandet Mellan Luftelektriciteten Och Jordmagnetismen. Kgl. Svensk. Vetakad. Handl. 40, 3-12. * Bastamov, S. L. (1921). Magnitnye Vozmushchenya PO Nablyudenym Mezhdunarodnykh Polyarnykh Ekspeditsy. Meteorol. Vestnik 30, 40-50. * Bastamov, S. L. (1929). Magnetic storms observed by the international polar expeditions. Terr. Mag. 34, 35-38. $ Berkner, L. V. (1954). International scientific action : The International Geophysical Year. Science 119, 569-575. Birkeland, K. (1901). “ExpBdition norvegienne pour 1’Btude des aurores bor6ales.” J. Dybwad, Christiania. Birkeland, K. (1908). “The Norwegian Aurora Polaris Expedition.” H. Aschehoug, Christiania. Birkeland, K. (1909). Sur les orages magnetiques polaires. Compt. rend. 148, 10061009. * BBrgen, C. N. J. (1882). Die internationalen Polarexpeditionen, Deut. Geograph. Blatter 6, 283-307. * Breitfus, L. L. (1930). Das Internationale Polarjahr einst und jetzt. Arktis 3 , 1 6 3 0 . * Bunge, A. A. (1895). “Die Lena Expedition 1883-1884.” Imperial Academy of Sciences, St. Petersburg. * Carpenter, P. H. (1887). Zoologische bijdragen tot de kennis der Karzee. Bijdr. dierk. 14, 3949. 1Chapman, S . (1953). The International Geophysical Year. Nature 172, 327-329. $ Chapman, S. (1955). The International Geophysical Year. Nature 176, 55. * Dawson, H. P. et al. (1886). “Observations of the International Polar Expedition Ft. Rae.” Eyre and Spottiswoode, London. Debenham, F. (1930). “The Polar Regions.” E. Benn, London. $ Department of Defense (1954). Arctic Bibliography, 1-6. U. S. Govt. Printing Office, Washington, D. C. * Deutsche Polar Kommission (1886). “Die Beobachtungsergebnisse der deutschen Stationen.” Deutsche Polar Komission, Berlin. * Ekholm, N . G. (1886-1891). “Observations faites au Cap Thordsen, Spitzbergen.” Stockholm. * Ekholm, N. G. (1887). “Observations faites au Cap Thordsen, Spitebergen.” Stockholm. * Eschenhagen, M. (1887). Die erdmagnetischen Beobachtungen in Systeme der internationalen Polarforschung 1882-83. Ann. Hydrog. 16, 129-234. t Fleming, J. A. (1930). International Polar Year Proceedings. Terr. Mag. 36, 245248.

50

t Fleming, J. A.

E. C . GERSON

(1950). Temporary Commission on the Liquidation of the Second International Polar Year. Polar Record 6, 621-623. Fritz, H. (1874). Die geographische Verbreitung des Polarlichtes, Peterrriann Geograph. Mitt. 20, 347-358. * Greely, A. W. (1884). Recent discoveries in Northern Greenland. IZoy. Geograph. SOC.PTOC. 6, 679-689. * Greely, A. W. (1886a). Arctic exploration. Roy. Geograph. SOC.PTOC.8, 156-176. * Greely, A. W. (1886b), “Three Years of Arctic Service.” Scribner’s, New York, New York. * Greely, A. W. (1910). “Handbook of Polar Discoveries.” Little, Brown, Boston, Massachusetts. * Grinevetsky, L. F. (1884). “Quer durch Nowaja Zemlja.” Ges. Erdk. Leipzig, Mitt. pp. 131-148. t Harradon, C. (1931-1934). International Polar Year Proceedings. Terr. Mag. 36, 324-332; 37, 185-186; 38, 241-245; 39, 119-120; 62, 531-533. * Henry, A. J. (1928). Whence Come Cold Waves. Monthly Weather Rev.66, 142-144. * Hoffmeyer, N. H . (1880). Internationale Polarstationen, Geograf. Tidskr. 4, 38-40, Copenhagen. t International Meteorological Organization (1930). Rapport de la Commission Internationale de L’AnnBe Polaire. Organisation MBtCorol. Intern. 6, 1-152; 10,59-241; 16, 1-128. t International Meteorological Commission (1949). Report of Chairman of TCLPY, Washington Session 1947. World Meteorol. Organization App. K-IV,pp. 882-S96. t International Meteorological Organization (1929-1946). Prochs verhaux des seances du Comite MBt6orologique International. Organisation MBtBorol. Intern. 3, Part I, pp. 27-30,86; Part 11,pp. 138-141,148-155,157-180; 10,23-36,4344, W-56; 17, 22-25, 31-33, 72-75; 29, Part I, pp. 94-95; Part 11, 9-10, 49-53, 97-100; 40, 28-29, 141-145; 46,72,153-155; 62,211-216; 66,222-223,279-284,433434; 71,564,882-896. $ International Union of Geodesy and Geophysics (1954-1958). International Geophysical Year. CSAGI Bull. Nos. 1-9. CSAGI Secretary General, Brussels. * James, B. R . (1940). “Six Came Back.” Bobbs-Merrill, Indianapolis, Indiana. * Johansson, 0. V. (1903). Om den dagliga gangen af temperaturen, Sodankyla under polararet, Finsk. Vet. Soc. 46, No. 14. * Johansson, 0. V. (1917). Meteorologiski ach geofysiska data for Sodankyla, Finsk. Vet. SOC.69A, No. 8. $ Jones, Sir H. S., ed. (1958). Ann. Intern. Geophys. Year. Pergammon Press, London. * Jurgens, N. P. (1885). Ekspeditsya k ustyu rieki, Leny. Vsesojznoe Geogra3cheskoe Obshchestuo. Izvest. 21,249-302. $ Kaplan, J. (1954). The International Geophysical Year. Science 119, 3A. $ Kaplan, J. (1956). U. S. program for the International Geophysical Year. Nature 178, 665467. * Kerbert, C. (1887). Zoologische bijdragen tot de kennis de Karazee. Bijdr. dierk. 14, 51-60. * Koch, K. R. (1884). Die Kuste Labradors und ihre Bewohner. Deut. Geograph. Blutter 7, 151-163. * Krakau, E. V. (1925). Issledovanie Amplitudy Sutochnogo Khoda Magnitnogo Sklonenya V Zavisimost ot Magnitnoy Shiroty Mesta. Zhur. Geofiz. 12 i met 9, 89-120. * Kuznetsov, N. I. (1886). 0 Lishainikakh Novoi Zemli. Trudy, Protokoly Zasiedony 17, 80-81.

FROM POLAR YEARS TO IGY

51

j La Cour, D. (1935). L’AnnBe polaire internationale, Proc. Second Gen. Assembly

Intern. Council Sci. Unions, London pp. 191-207.

t La Cour, D. and Staff, J. M. (1932). The International Polar Year. * Lamar, W.

H., and F. W. Ellis. (1884). “Physical Observations during the Lady Franklin Bay Expedition of 1883.” U. S. Govt. Printing Office, Washington, D. C. * Lanman, C. (1885). “Farthest North.” Appleton, New York, New York. j Laursen, V. (1951). “Bibliography for the Second International Polar Year.” International Meteorological Organization. Horsholm, Copenhagen. * Lemstrom, S. (1887). Jamfortse Emellan Kostnaderna for de finska, danska och norska Polarstationerna 1882-1883. Finsk. Vet. Soc. 29,170-172. * Lemstrom, S., and Biese, E. (1886-1898). “Observations faites aux stations de Sodankyla et de Kultala.” de Simelius, Helsingfors. * Lemstrom, S. (1898). On Earth Currents and Electrical Currents of the Atmosphere. Finsk. Vet. SOC.41, 60-104. * Lenz, R. E. (1886). “Beobachtungen russischer Polarstationen auf Nowaja Semlja.” Imperial Academy of Sciences, St. Petersburg. * Liznar, J. (1888). Die 26-tagige Periode des Nordlichtes. Kgl. Akad. Wiss. Wien 97, 1101-11 16. * Lutken, C. F. (1887). L‘Dijmphna-Tugtets Zoologisk Botaniske Udbyt.te.” H . Hagerup, Copenhagen. * Meteorological Council (1881). Rept. Intern. Meteorol. Comm. Meeting, Berne, 1880, (Nonofficial) No. 14. * Murdoch, J. (1885). Description of seven new species of Crustacea and one worm. Proc. U.S. Nat. Museum pp. 518-522. * Murdoch, J. (1892). Ethnological results of the Pt. Barrow expedition, U.S. Bur. Am. Ethnol. Ann. Rept, 9, 1887-1888. 3 National Science Foundation. (1957). “A Bibliography for the International Geophysical Year.” Publ. No. NSF 77-25. U. S. Govt. Printing Office, Washington,

c.

I).

* Neumayer, G. (1890-1891). “Die deutschen Expeditionen und ihre Ergebnisse. Berlin.

3 Nicolet, M. (1958). IGY Bibliography, Cornit6 SpCcial Annee

GCophysique Internationale, Uccle, Belgium. * Paulsen, A. F. W. (1886-1894). Observations faites A Godthaab. Meteorol. Institut Danske, Copenhagen. * Pfeffer, G. J. (1886). Mollusken, Krebse und Ekinodermen vom Cumberland Sunde. Jahresber. Hamburg. Wiss. Anstalten 3, 23-50. * Rae, R. W. (1951). Canadian meteorology. Encyclopedia Arctica I Part 2. * Ray, P. H. et al. (1885). “Report, International Polar Expedition.” U. S. Govt. Printing Office, Washington, D. C. * Regele, 0. (1954). Beitrage zur Geschichte des Internationalen Polarjahres. Polarforschung 3 , 22,188-192. * Rolf, B. and Olsen, J. (1937). Contributions to the study of overhead current systems in the Arctic. Geograjis. Ann. 19, 278-293. * Ruijs, J. M. (1887). Zoologische bijdragen tot de kennis der Karazee. Bijdr. tot de dierk. 14, 1-38. * Rust, F. (1883). The Dutch Polar Expedition of 1882-1883. J . A m . Geograph. SOC. 16, 375-380. * Scharizer, R. (1884). Uber Mineralien und Gesteine von Jan Mayen. Jahresber. Geol. Bundesanstalt 34, 707-728.

52

E. G. GERSON

* Schley, W. S.(1887). “Report of Commander of Greely Relief Expedition of 1884.” U. S. Govt. Printing Office, Washington, D. C. $ Scientific American. (1955). The planet earth. Sci. Am. 193 No.3. $ Smithsonian Astrophysical Observatory (1958). Special Reports, Nos 1-19, Cambridge, Mass. * Steen, A. S. (1887-1888). “Beobachtung-Ergebnisse der Norwegischen Polarstation Bossekop in Alten.” Grondahl, Christiania. * Swedish Royal Academy of Science. (1886-1891). “Observationes faites au Cap Thordsen.” P. A. Norstedt, Stockholm. * Tillo, A. A., ed. (1886-1895). “Beobachtungen russischer Polarstationen an der Lenamundung.” Imperial Academy of Sciences, St. Petersburg. * Tollner, H. (1932). Astronomische Ortsbestimmungen auf Jan Mayen. Akad. Wiss. Wien 143, 87-97. * Tromholt, S. (1882). Vorliiufige Mitteilungen uber swei Nordlichtwerke. Petermanns Geograph. Mitt. 38,201-214,236-240,259-262. 1U.S.S.R. Embassy. (1958). “Sputniks.” U.S.S.R. Information Agency, New Delhi. $ U. S. National Academy of Sciences. (1957-1958). IGY Bulletin. Trans. A m . Geophys. U n . 38; 39. * U. S. War Department. (1884). “Proceedings of the Proteus Court of Inquiry of the Greely Relief Expedition.” U. S. Govt. Printing Office, Washington, D. C. $ Vassy, E. (1953). L’AnnBe GBophysique Internationale. Mhtborologie Ser. .G 24,

50-60.

* Vincent, E. (1910). “Sur la marche des minima barometriques d a m la region polaire Arctique.” AcadBmie Royale Belgique, Brussels. $ Whipple, F. L., L. G. Boyd, J. A. Hynek and G. F. Schilling (1958). “Orbital Data and Preliminary Analyses of Satellites 1957 Alpha and 1957 Beta”, Smithsonian Contributions to Astrophysics, 2, No. 10. * Wichmann, H. (1884). Die amerikanische Polarexpedition nach Lady Franklin Bai. Petermanns Geograph. Mitt., 30, 339-348. * Wilczek, N. J. (1933). “Hans Wilczek Ersahlt.” Laykan, Gras, Austria. * Wild, H. (1882-1891). “Communications of the International Polar Commission,” Parts 1-7. Imperial Academy of Science, St. Petersburg. * Wohlgemuth, E. E. (1886). “Beobachtungen und Ergebnisse Osterreichische Polarstation Jan Mayen.” K. K. Hof-und Staatsdruckerei, Vienna. 1Yatoukin, R . (1958). Artificial Earth Satellites, Ukrainian Academy of Sciences.

MICROSEISMS B. Gutenberg Professor of Geophysics, California Institute of Technology, Pasadena, California

1. Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page 54

...............

. . . . . . . . . . . 59

4.

5.

6.

7.

3.2. Microseisms from Traffic and Industry.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Microseisms Produced by Water Flowing over a D a m . . . . . . . . . . . . . . . . . . Natural Microseisms with Periods of Less Than Two Seconds.. . . . . . . . . . . . . 4.1. Short-Period Microseisms Connected with Meteorological Phenomena. . . . 4.2. Short-Period Microseisms Produced by Local Surf. . . . . . . . . . . . . . . . . 4.3. Volcanic Tremors.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Regular Microseisms with Periods of One t o Three Seconds.. . . . . . . . . . . . 5.1. Microseisms Recorded in the Eastern United States.. . . . . . . . . . . . . . . 5.2. Microseisms with Periods of About Two Seconds Recorded in Southern California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Microseisms with Periods of One t o Three Seconds Recorded Elsewhere. . . Microseisms with Periods of About Four Seconds. ...................... 6.1. General Remarks.. . . . . . . . . . . . . . . . . . . . . . . . . . . . ...................... 6.2. Irregular Microseisms with Periods of About Four Seconds near Coasts. . 6.3. Microseisms Produced by Monsoons . . . . ........... “The” Regular Microseisms with Periods fro ......... 7.1. Appearance; Beats. . . . . . . . . . . . . . . . . . . . 7.2. Periods and Period-Amplitude Relation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. Change in Period with Increasing Distance the Source . . . . . . . . . . . 7.4. Direction of Approach of Microseisms . . . . . ...................... ...................... 7.5. Velocity of Microseisms. . . . . . . . . . . . . . . . . . . 7.6. Propagation across Continents and along Ocean Bottoms 7.7. “Barriers” t o Propagation of Microseisms. Effect ........ tudes ........................... 7.8. Change of Amplitudes with Depth.. .................................. 7.9. Wave Types. Ratio of Vertical to Horizontal Components. ............ 53

59 59 59 59

60 61 62 62 62 63

63 63 63 64 65

67 68

54

B . GUTENBERG

7.10. Cyclical Variations., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.11, Observed Correlation between Microseismic Activity and Meteorological Phenomena., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.11.1. Cyclones over the Ocean. . . . . . . . .......................... 7.11.2. Cyclones over Continents. . . . . . . .......................... 7.11.3. Regular Microseisms and Cold Fronts.. .......... 7.11.4. Strong Winds Blowing against Mountains.. . . . . . . . . . . . . . . . . . . . . 7.11.5. Effects of Pressure Changes. . . . . . . . . . . 7.12. Observed Correlation between Ocean Waves an 7.12.1. Correlation between Amplitudes of Ocean Waves and Those of

Microseisms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

69 51 71 71 i1 72

72

7.12.2. Relationship between Periods of Microseisms and of Ocean

Waves.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.13. Surf and Regular Microseisms. . . . . . . . . . . . 7.14. Use of Microseisms in Meteorology. . . . . . .

74 . . . . . . . . . . 75 . . . . . . . . . . 76

8 . Microseisms with Periods of Ten Seconds to Several Minutes.. . . 8.1. Microseisms from Surf. . . .............................. 8.2. Microseisms Connected wi ......................... 8.3. Microseisms with Periods of Several Minut ............................................................ 9.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . .............................. 9.1.1. The Problems. . . . . . . . . . . . . . . . . .................... 9.1.2. Types of Elastic Waves., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. Transmission of Energy from the Surface to the Bottom of the Ocean. . . . 9.3. Waves in the System Ocean-Earth’s Crust.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4. Waves in the System Air-Earth’s Crust.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5. The Energy Problem.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6. The Period Problem., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6.1. Hypotheses Assuming That the Periods Are Caused near the Source 9.6.2. Changes in Periods during the Propagation of Microseismic Waves 9.6.3. Effects of the Geological Structure near the Recording Station on ............................................ the Periods. . . 9.6.4. Relation betw e Mode of the Vibrations and the Periods of

76

78

80 81 81 81

82 83 83 83

the Microseisms., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 List of Symbols.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . . . . . 84 References. . . . . . . . . . ................................................ 85

1. INTRODUCTION 1 .l. DeJinition of Microseisms; Nomenclature

Microseisms are defined here as more or less regular motion of the ground which is not produced by earthquakes or explosions, has periods or pseudoperiods of not exceeding several minutes, and continues for many periods. Some authors exclude motions produced by artificial causes (industry, traffic, etc.). The term “microseisms” is not well chosen, since the word “microseismic” is often used to indicate motion recorded by sensitive instruments in contrast to “macroseismic,” observed by people. The term “pulsations”

MICROSEISMS

55

used about the turn of the century referred to microseisms which were recorded on slowly moving paper (often less than 1 mm/min) and looked like saw teeth with periods of several minutes; however, they were not as regular as the frequently reproduced drawing of l<ebeur-Paschwite [ 11 indicates. Microseisms with variable periods and amplitudes were called ‘(tremors” (see [2],pp. 266-285). When Omori I31 introduced a much higher speed of his recording drums he found regular microseisms with periods of 3 to 8 sec which others had observed on occasional high-speed runs of their instruments. For these waves the term “pulsatory oscillations” was used hy some. To avoid the incorrect conclusion that microseisms have a connectioii with earthquakes (“seisms”), the term ‘(ground unrest” has been suggcsted, but did not displace “microseisrns.” In seismic exploration, the term (‘noise” is widely used for the continuous background on records. 1.2. Historical Rcmarks

Microseisms have been observed early during the nineteenth century on sensitive instruments for nonseismic research, for example, during gravity investigations. Moreover, ground motion near machines was studied. About 1870, Bertelli [4] installed a pendulum and observed during many years that sometimes the pendulum moved continuously for hours or days. He noticed a correlation between these “microseisms” and disturbed air pressure. Summaries for investigations of microseisms during the nineteenth century have been given for example by Sieberg (see [5], pp. 184-194) Guenther [6], Milne [a], and Macelwane [7]. The literature on microseisms is so extensive that only selected references can be given here. A bibliography containing 600 references has been prepared by Gutenberg and Andrews [8].

1.3. Classification of Microseisms After the introduction of relatively fast moving recording drums, Hecker [9] differentiated among four kinds of microseisms, depending on whether the periods were less than 4 see (usually local motion), about 7 sec, about 46 min, or longer. Several additional types have been found since; in addition to microseisms from artificial causes, about a dozen types have now been established (Table I). Differences of opinions about microseisms may easily result from confusion of different types.

i .4. Units and Symbols As generally in seismology, amplitudes are measured from the rest line. Trace amplitudes A are given in millimeters, ground amplitudes a in microns, the period T from crest to crest in seconds [lo]. When special

56

B. GUTENBERG

TABLEI. Types of microseisms. Period (sec)

Type of movement

1-4 2-6 4-10

Regular Irregular Regular Irregular Regular Regular

4-10

Regular

4-10 4-10 10-20 20-100

Regular Regular Regular Irregular

40-200

Irregular

0.001-0.5 0.2-2 1-4

Hypothetical cause Traffic, industry, wind Surf Fronts, turbulent wind Effects of wind on trees, buildings Ocean waves in hurricanes, typhoons Ocean waves in extratropical disturbances Surf driven by wind against steep coasts Air-pressure pulsations? Monsoon and similar types of wind Water waves striking the coast Wind? Air currents in instrument vault? Freezing of ground? “Icing” of instruments?

Distaiirc of cause Nearby Nearby Nearby Local Distant Distant Distant Medium? Medium? Medium? Nearby Medium

continent-wide or world-wide observations are scheduled, details for hours, type of observation (e.g., maximum amplitude within 5 min) are published. Specific suggestions have been made a t the symposium in 1951 (see [ll], p. 401). 2. INSTRUMENTS FOR THE INVESTIGATION OF MICROSEISMS

2.1. Seismographs Constants of seismographs intended mainly for the investigations of microseisms have to be selected to give a maximum magnification for the respective wave periods; vice versa, to obtain a minimum disturbance of seismograms by microseisms, seismographs with such properties should he (but frequently are not) avoided for earthquake study. Graphs showing the response of seismographs with various constants to continuous waves of a given period have been published by Benioff [12]. Macelwane [13] has designed and Sprengnether [14] has built special instruments for recording of microseisms. Portable instruments for this purpose have been described by Krug [15]and by Trommsdorff [El. Resonant seismometers each tuned to cover a part of the spectrum of microseisms have heen used by Donn et al. [17]. 2.2. Use of Tripartite Instruments

Hecker [18] has made the first but unsuccessful attempt to determine, by use of two instruments, the direction from which microseisms arrive

57

MICROSEISMS

and their velocity. Shaw was the first to use three instruments [19]. However, the distances of about 15 km between them were too great to permit identification of waves. The “tripartite” instrument method was used successfully first by Krug [15] and later by Trommsdorff [16] and by Ramirez [20]. The first correct equations were given by Gilmore [21]. Schuyler [22] has designed nomograms to speed up their solution. If it is assumed that waves of one type only arrive from a single direction, and if the intervals between the times a t which corresponding points of a wave arrive a t the points A , B and C of a triangle (Fig. 1) are, respectively, t A B, tBc , t C A, the angle 4 between the wave front and the side AC of the triangle can be found from tan4 =

sin a

K -CO where a is the angle between AB and AC and

S ~

(2.2)

if b = AC, c = A B . Finally, the angle J, between the direction toward north and the direction from which the waves arrive is given by (2.3.)

J,

= 6

- 4 - 90”

where 6 is the angle between the direction toward north and AC (see Fig.

Direction of arriving waves

FIG.1. Sketch for calculation of the direction from which microseisms arrive a t a tripartite station ABC.

58

B. GUTENBEHG

1). The sign of $ depends on whether it is counted from N toward E or from N toward W. The velocity V of the waves is found from (2.4)

While horizontal instruments record both types of surface waves, vertical instruments record Rayleigh waves only and are therefore better suited for determinations of azimuth and velocity of microseisms. 9.3. Vectorial Recorders

Vectorial recorders are designed to record on a standing surface the combination of two elements. They can be used, for example, to record the horizontal ground motion (Fig. 2, inset), or the motion in any vertical plane, or one component of the ground motion combined vectorinlly with one of the strain. After a time interval, depending on the period of the waves, the film or photographic paper is moved to a new fixed position. Such a device [23] designed by H. Benioff and constructed by I?. Lehner (schematic in Fig. 2) consists of two galvanometers constructed on a single frame. The light beam deflection of one galvanometer is rotated by fixed mirrors. Time intervals (e.g., seconds) are marked, for example, by interruptions of the recording light or by momentary increases in its intensity. A photographic image of an illuminated clock can be used to give the time of the start of each exposure [23]. A different type of vectorial recorder has been constructed by Strobach [24]. Use of vectorial records in investiDUAL GALVANOMETER I

1

TRANSFORMING HORIZONTAL MOTION OF LIGHT BEAM INTO VERTICAL MOTION

IMAGE OF F I L A M E N T

FIG.2. Optical system of vectorial recorder (schematic) ;inset: vectorial record of horizontal motion during 6-sec microseisms a t Pasadena.

MICROSEISMS

59

gation of microseisms is limited frequently by simultaneous arrival of different wave types, so that the record is too confused for interpretation. 2.4. Observation of Sea Waves

Because several types of microseisms have been found to be correlated to sea waves, the recording of sea waves a t well-selected points a t and near the coast and of pressure changes a t the ocean bottom is very desirable for the investigations of microseisms. Methods to record sea waves have been summarized by Roll [25]. An instrument designed especially to record sea wave motion for correlation with microseisms has been described by Berilascoiii et al. [2G]. Results of visual observations of ocean waves are useful, if made by trained observers. 3. MICROSEISMS PRODUCED BY ARTIFICIAL CAUSES 3.1. “Noise” in Seismic Exploration

Records of waves produced by artificial explosions or other means in seismic exploration are frequently disturbed by microseisms in the frequency band between 100 and 1 cy/sec. This “noise” is discussed in most textbooks on seismic exploration. Wilson [27] found it everywhere within two miles of major roads or communities. Traffic prevailed as source, but heavy machinery and aircraft also contribute frequently. 3.2. Microseisms from Trafic and Industry

Microseisms with periods of a fraction of a second are frequently recorded by seismographs with short-period characteristics, especially on soft ground with a shallow ground water level. They have been traced to specific machinery [28-301 or to traffic [30, 311. Near Goettingen, periods had various frequency maxima from 0.02 to 0.G sec, amplitudes ranged up to 0.05 p [32]. Possibly, free oscillations of layers play a role. 3.3. Microseisms Produced by Water Flowing over a Dam

Grunmach [33] constructed the first seismograph to investigate shortperiod waves by use of electromagnetic recording. He found that microseisms produced by water flowing over a dam had prevailing periods of 0.03 and 0.005 sec. In the shortest waves the acceleration occasionally reached one tenth of gravity.

4. NATURALMICROSEISMS WITH PERIODS OF LESS THANTwo SECONDS 4.1. Short-Period Microseisms Connected with Meteorological Phenomena

About 1930, Macelwane noticed small regular oscillations with periods of 0.2 to 0.7 sec appearing a t frequent intervals on the records of torsion

60

B. GUTENBERG

seismographs a t St. Louis. Study of four years’ records by Walsh [34] indicates that the great majority of these microseisms is associated with passage of cold-front-type discontinuities or local nonfrontal convective activity. Short-period microseisms connected with wind and rain have been studied by Wilson [30]. Microseisms with periods of less than 2 sec have been observed by Chakrabarty and Sarkar [35] in Calcutta, when cyclones passed very close to the station. 4.2. Short-Period Microseisms Produced by Local Surf

Irregular microseisms of this type with periods of one or more seconds have been found on Helgoland [36]. They merge into motion with longer periods. On the Island of Sylt microseisms from surf with periods of a fraction of one second have been observed [37] a t distances up to 2 km from the coast. 4.3. Volcanic Tremors During periods of activity, volcanoes produce continuous trembling of the ground with periods of between 0.1 and several seconds. Usudly, periods of ?,ito $6 sec prevail [3841]. 5. REGULARMICROSEISMS WITH PERIODS OF ABOUTONE TO THREESECONDS 5.1. Microseisms Recorded in the Eastern United States

Regular microseisms with periods of 2 sec recorded a t Fordham University have been studied by Father Lynch [42] since 1947. They are associated with the passage of cold fronts. Records of several tripartite stations have indicated the Great Lakes as source, and progressive water waves recorded by special instruments in Lake Michigan gave “the right period to produce two-second ground oscillation^.^^ The velocity of these microseisms was slightly less than 1 km/sec. 5.2. Microseisms with Periods of About Two Seconds Recorded in Southern

Calijornia Microseisms with periods of about 2 sec (Fig. 3) have been recorded repeatedly by tripartite vertical instruments, two horizontal seismographs and two Benioff strain seismographs a t Palomar, California; these instruments had been installed for the study of microseisms under the auspices of the Air Force Cambridge Research Center [43]. At Palomar, these microseisms have a prevailing vertical component (Fig. 3b) ; however, similar microseisms a t Pasadena show frequently about equal amplitudes on all components. Their amplitudes decrease so rapidly with increasing

61

MICROSEISMS

distance from the source (Fig. 3c) that they may not be visibly recorded after having traveled only about 100 km. As their velocity is about 346 km/sec, this corresponds to about 10 or 20 wavelengths. Usually their amplitudes increase with increasing turbulence of the air, especially after passage of a cold front. Frequently, they start after the beginning of rain. In all instances investigated on Palomar tripartite vertical records, they came from the direction of the coast. The differences in velocity and nttenuation between these microseisms and those discussed in Section 5.1 suggest that they represent different types. 5.3. Mieroseisms with Periods of One to Three Seconds Recorded Elsewhere

Microseisms in New England with periods of 2 to 3 sec have been investigated repeatedly by Donn but his results seem to cover several types. In two instances [44], such short-period microseisms “correlate very well with times of high wave activity. . .and are suggested of being of true surf or shallow-water wave origin.” Microseisms with periods of 1 to 2 sec have been recorded near Manila. Their amplitudes increased as thunderstorms approached the station, and the vertical component prevailed [45]. Caloi [46] has found microseisms with periods of 2 to 3 sec a t Trieste and also a t Catania, Sicily, but not a t Messina, 85 km away. He assumes that microbarometric waves set up oscillations of the sea near the coast. b-m-30 v

a

b

SECONDS-----r(

v

(

HORIZONTAL TRIPARTITE VERTICAL

Barrett

2 Palomar

Pasadena

Riverside

C

430 SEC FIG. 3. Records of 2-sec microseisms; (a) recorded by tripartite vertical instruments a t Palomar, sides of triangle about 700 meters, February 1, 1956; (b) recorded by the same tripartite vertical instruments as in (a) and, in addition, by two horizontal instrumentn a t one corner of triangle, October 29,1956; (c) recorded simultmeously by short-period Benioff seismographs, sensitivit,y and periods roughly the same, a t southern California stations on February 1, 1956; successive lines correspond t o time intervals of 15 min.

62

B. GUTENBERG

Giorgi [47] has correlated microseisms a t Rome having periods of ahout 3 sec with the passage of cold fronts. Similar findings have been reported from other observatories, but apparently they refer to various types of microseisms.

6. MICROSEISMS WITH PERIODS OF ABOUTFOUR SECONDS 6.1 .General Remarks

Microseisms with periods of 3 to 5 sec are fieyuently observed. If their amplitudes are small, they may belong to the type discussed in Section 7, but microseisms of the types discussed in Section 5 are also involved. In addition, there are possibly one or more types of regular microseisms intermediate between these two. As will be pointed out later in Section 9.6.4, some of these types may differ in the mode of vibration; perhaps, the 6sec waves correspond to the first mode, the waves having shorter periods to higher modes. Prevalence of distinct periods has been found in Japan [48,491. These microseisms are frequently reported to be correlated with high wind over the ocean near the station. In addition, a t some stations near the coast irregular waves with periods of about 4 sec are recorded. Moreover, monsoons produce microseisms with prevailing periods of between 2 and 5 sec (see Section 6.3). Except for the source of the wind, severttl types of the 4-sec microseisms are possibly produced by the same process. 6.2. Irregular Microseims with Periods of About Four Seconds Near Coasts

Near coasts, irregular microseisms are frequent, during near-by storms [50], (Fig. 4).Discussions of regular microseisms with periods of 4 to 10 sec occasionally include instances of the irregular type, considered here, but usually it is difficult to separate the information for the two types. I

I MIN

J

FIG.4 . Irregular microseisms recorded by EW component of Galitzin seismograph a t Santa Clara, California, on December 3, 1951.

63

MICROSEISMS

6.3. Microseisms Produced by Monsoons

Microseisms related to monsoons have been described repeatedly; most of these were recorded a t stations in India [51, 521. Amplitudes of monsoon microseisms recorded a t Poona and a t Bombay show parallelism with the swell activity over the Arabian Sea [52]. Similar microseisms recorded a t Zi-Ka-Wei, China, have been reported by Gherzi [53].

7. "THE" REGULAR MICROSEISMS WITH PERIODS FROM FOURTO TENSECONDS 7.1. Appearance; Beats Microseisms of this type are recorded everywhere by sufficiently sensitive instruments. In discussions of microseisms, authors sometimes indicate that they studied only this type, although their investigations include or even are based only on records of types discussed under Section 6. How-

4 ONE p

w

----wwvvv7*-

MINUTE

-

+

m

-vvWJl&~------'----""""rn -

-

A

l

v

v

-

v

'

~

-

FIQ.5 . Regular microseisms with periods of 5 to 6 sec, recorded by long-period Benioff seismograph at Pasadena, California.

ever, the microseisms discussed here are much more regular (Fig. 5). The fact that they show beats (order of magnitude of period one minute) has been discussed as early as 1909 [54, 551. According to Jensen [56] the beat interval is logarithmic-normally distributed with one parameter, and the mean value of the beat interval is proportional to the period of the carrier wave and is greater in vertical than in horizontal records.

7.2. Periods and Period-Amplitude Relation Usually I571 the periods increase with increasing amplitudes (Table II), and show a similar yearly variation as the amplitudes, with a maximum during the winter [58-62], (Table 111). Periods usually increase or decrease simultaneously over large areas [58, 641. In some microseismic storms, appreciably higher periods prevail everywhere on a continent than in others (see [58], p. 279).

7.9. Change in Period with Increasing Distance from the Source For many types of waves, a gradual increase of periods with increasing distance from the source has been observed. This includes regular micro-

64

B. GUTENBERG

TABLE 11. Average amplitudes a of regular microseisms, NS component, in microns, for given periods T. [58, 60, 611. Period 2’ in seconds Station

Apia Kew Hamburg Potsdam Pulkovo Goettingen Vienna Uppsala

4

5

6

7

8

1.7 0.7 0.6 0.6 0.1 0.3 0.5

2.3 1.1 1.4 1.0 1.2 0.2 0.5 0.8

2.4 1.5 4.0 2.1 1.6 0.4 0.7 1.2

2.6 1.8 6.0 1.7 1.5 1.0 0.9 1.3

2.9 3.4 7.2 3.5 1.8 1.7 1.4 1.3

9

3.7 3.6

-

2.2 2.0 (1.9)

TABLE 111. Annual changes in periods of 6-sec microseisms for Eskdalemuir, ICew, De Bilt [63], Uppsala [61], and for Russian stations [59] during 1913: n = number of years.

Eskdalemuir Kew De Bilt Uppsala Pulkovo Tashkent Irkutsk

NS

14 6.1 6.0 5.7 5.3 4.7 4.6 4.3 4.5 5.0 5.2 5.6 5.9

NS Z NS all all all

9 7 18 1 1 1

6.5 6.1 5.9 5.4 6.4 6.2 5.5 4.9 5.5 5.4 5.1 5.0 5.8 5.8 5.3 5.0 5.7 6.5 5.9 5.2 6.1 6.2 5.8 5.3

4.9 4.1 4.8 4.2 4.9 5.3

4.7 4.0 4.5 4.3 5.2 5.2

4.4 3.6 4.3 4.0 5.2 4.9

4.6 3.9 4.4 4.4 4.8 5.0

5.0 4.4 4.7 4.3 5.4 4.9

5.4 4.8 5.0 4.6 5.2 4.9

6.0 5.3 5.2 5.8 6.3 5.7

6.4 5.8 5.4 5.1 5.5 5.6

seismic waves traveling across northern Eurasia [58], southeastern Europe [SS] and through India and Ceylon [66]. BeLth [61] finds that in Scandinavia the period increases roughly by sec/km as the microseisms are propagated. However, no, or only small, increase in periods has been observed for microseisms traveling across North America [67, 681. Either the wave periods actually increase [69] or the absorption coefficient is greater for short-period than for long-period waves (Section 9.6.2). A discussion of waves in a dispersive medium has been given by Munk [i’O].

7.4. Direction of Approach of Microseisrns To find the direction from which microseisms arrive a t a station, either tripartite stations (Section 2.2) are used, or Rayleigh waves are selected, and the direction of propagation is determined from the motion of the waves, which is retrograde in an ellipse with one axis vertical, the other

65

MICROSEISMS

horizontal in the direction of propagation. In 1935, the second method was used by Lee with some success. It was later improved by Darbyshire [71]. Since 1937, the first method has given many useful results. The accuracy of such azimuth determinations is rarely better than &20" [72] for the following reasons: waves frequently arrive simultaneously from different directions [73]; different wave types-at least shear and Rayleigh surface waves-are involved; moreover, the effect of refraction of the waves near coasts may be appreciable [74]. In most instances the regular microseismic waves with periods of 4 to 10 sec have been found to arrive from the direction of a near-by ocean.

7.5. Velocity of Microseisms Equation (2.4) permits the determination of the wave velocit,y from records a t tripartite stations on certain assumptions. If waves arrive from different directions, the true velocity is smaller than the apparent velocity [73]. Moreover, simultaneous arrival of waves from different directions or of different types make it frequently difficult to identify accurately corresponding points of a given wave on the three records, and effects of the ground may distort considerably the arriving waves [75]. Consequently, data on velocity of microseisms are frequently rather uncertain, Table IV contains some results. As the wavelength is of the order of 20 km, the velocity may show appreciable differences as a consequence of differences in local structure. Moreover, it will depend on the wave type. Shear waves should show a higher velocity than Rayleigh waves. Waves with periods of less than 4 sec have much smaller velocities [76]; see also Section 5.1. 7.6. Propagation across Continents and along Ocean B o t t m s

Microseisms with periods of 4 to 10 sec are frequently propagated to large distances across continents without much loss of energy but their amplitudes decrease rather rapidly in the bottom of deep oceans (Fig. 6). Microseismic storms originating near Norway are propagated to Central TABLEIV. Velocitv V in km/sec of microseisms with Deriods of 4 to 10 see. Location St. Louis Richmond, Florida Guantanamo Bay, Cuba Puerto Rico Guam Palomar, California

V 2.7 3.3 2.6 4 3.2 3.3

Author Ramirez [20] Gutenberg [76] Gutenberg [76] Gutenberg [76] Gilmore and Hubert 1771 Gutenberg [78]

66

B. GUTENBERG

HURRICANE

I

I0 MY, TRACE AYPLITVOE

it STORM NEAREST TO STATION

RICHMOND,FLA. R GUANTANAMO G SANJUAN S

REYKJAVIK

1945. OCT.

II~I~~II~II~II~II~II~

SITKA VICTORIA PASADENA

ESKDALEMUIR

TUCSON

UPSALA

CARTUJA

A

/ /"'*

SASKATOON MILWAUKEE CHICAGO FLORISSANT ST. LOUIS OTTAWA SEVEN FALLS

PULKOVO MAKEEVKA TlFLlS BAKU

JCOPENHAGEN PVLKOVO ABISKO

d

FIG.6. Change in amplitudes of microseisms; upper left: in the Caribbean during a hurricane traveling from north of Haiti and Cuba to and across Florida; after Gilmore [21]; lower left: in North America during a storm in eastern Canada [67]; center: in Eurasia during a storm approaching northern Norway [80];a t right: in Europe and Greenland during a storm north of Scotland, after Lee [MI. Arranged by Gutenberg [761.

Asia. Gutenberg [SO] observed that in Irkutsk their amplitudes may &ill be roughly one quarter of those in Uppsala, Sweden. Bernard [MI, Lee [81], Bonchkovsky [82], Rothe [82a], and B%th[83] found similar results. Microseisms have also been found to cross North America from east to west [69] and from Alaska to the East Coast [68, 721 without much loss of energy. Ewing and Donn (see [ll],p. 356) consider the possibility that a storm can generate microseisms effectively only where there is a strong gradient in crustal thickness. On the other hand, microseisms suffer appreciable loss of energy if they travel under deep oceans, for example, from the North American continent to Bermuda or Puerto Rico [68,72];between Iceland and Norway [83]; from the deep Pacific to Guam [84] or to Antarctica (see [ll],p. 355); across the Caribbean [20, 851.

MICROSEISMS

67

7.7. (‘Barriers’’ to Propagation of Microseisms. Eflects of Ground on Amplitudes In addition to the “barriers” to propagation of microseisms produced by the crust under deep oceans, tectonically disturbed areas under continents may act similarly. Gutenberg [80] observed already in 1914 that in Eurasia the microseisms are propagated best through areas which belong to the same tectonic unit. Boundaries between different blocks of the earth’s crust produce relatively rapid decrease in amplitudes [58, 861. In a detailed investigation of this problem B%th [87] found that the main barrier in Scandinavia is the fault along the west coast. Lacaze [SS] has discussed barriers in the western Mediterranean. Portions of the mountain systems near the west coast of North America act as strong barriers [89]. In addition to the effects of these barriers and of the distance from the source which is usually near coasts, the amplitudes of microseisms are affected by the ground at the station. Similar to earthquake waves [75] the microseismic waves show usually greater amplitudes under otherwise equal conditions a t stations on recent sediments, especially if these are water saturated, than on solid rock (see [ll], p. 20; [86, 90, 911) (compare Table V in Section 7.10). Whipple [92] has pointed out that the ground conditions seem to affect more the horizontal than the vertical components. 7.8. Change of Amplitudes with Depth

Only few data exist for the change of microseismic amplitudes with depth. A discussion remark by Bhth (see [ll], p. 711) did not concern microseisms but earthquake waves [93]. Schneider [54, 941 has referred to a report by Benndorf, that microseisms were about equal at two stations near Ptibram, one a t the surface, another a t a depth of 1100 meters; Rossi in Italy and Hecker at Potsdam found only about half the amplitudes observed at the surface in 18 and 25 meter depths, respectively. Records of microseismic storms written a t 700 meter depth in rock near Clausthal, Harz, gave roughly one-fifth of the amplitudes found about 25 mi away at Goettingen at the surface [go], but noticeably less on quiet days [36]. A portion of this difference may result from ground effects, but the results indicate that relatively large friction of the small instrument at Clausthal may have resulted in too small calculated ground amplitudes for Clausthal, especially, if the wave amplitudes were small. The reported results are inconclusive, but seem to indicate that there is some decrease in amplitudes of 6-sec microseisms with depth. For 20 km long surface waves in a homogeneous medium the decrease in amplitudes should be too small to be observable at a depth of 1000 meters.

68

B. GUTENBERG

7.9. Wave Types. Ratio of Vertical to Horizontal Components

As soon as reliable vertical seismographs had been installed, the problem of the types of waves in microseisms was discussed. Zoeppritz [95] found that in Goettingen the ratio of the horizontal component H to the vertical component Z varies between 3: 1 and 1. Hence he concluded that microseisms cannot be pure Rayleigh waves because for these H:Z theoretically should be about 0.7:l. Later, similar results have been found for other stations. The maximum of H: Z of 6: 1 in Hamburg during winter is possibly affected by water-saturated surface layers [96]. Poor ground seems to increase the amplitudes of the horizontal components more than those of the vertical [92]. Moreover, a t many stations too small ground amplitudes in the vertical component have been calculated when the effect of-frequently considerable-friction in relatively small vertical instruments was neglected. The investigation of the types of motion in microseisms was improved when Benioff and Gutenberg installed tripartite vertical seismographs, two perpendicular horizontal pendulums and two strain seismographs at Palomar [43, 781. No body waves have been found in microseismic records. In surface shear waves (compare Section 9.1.2) the two strain record amplitudes should be always equal and in opposite directions, the horizontal pendulums should show wave motion perpendicular to the direction of the wave propagation, and the vertical component should be absent. In Rayleigh waves the strain records should be in phase (but not necessarily equal) and about parallel to the vertical record (depending on instrumental conditions), and the particles should move in retrograde ellipses. A statistical investigation of the records (examples in Fig. 7) shows that at Palomar Rayleigh waves prevail by far, but that surface shear waves (Love waves) are frequently present and occasionally prevail. Geussenhainer [97] observed that microseisms in Goettingen had predominating

x:

VERTICAL

L

30acs

I

R A Y L E I G H WAVES P R E V A I L I N G

TRIPARTITE

L

30 S I C

I

SHEAR WAVES P R E V A I L I N G

FIG.7 . Microseisms recorded a t Palomar by various instruments, (a) from prevailing Rayleigh waves; (b) from prevailing surface shear waves (Love waves) from [78].

MICROSEISMS

69

periods of 6, 755, and 9 sec in the horizontal component, while the periods changed gradually in the vertical component. He concluded that the horizontal components indicate free vibrations of the earth's crust while the vertical component (Rayleigh waves?) records in addition forced vibrations. However, the problem is complicated by the possibility that the 6-sec microseisms correspond to the second mode of surface waves. Moreover, they may be channel waves; this is discussed in Section 9.1.2. 7.10. Cyclical Variations

Already during early investigations it was found that the 6-sec microseisms have a maximum in winter and a minimum in summer. Selected results are listed in Table V. Bernard [64] concluded that the annual variation of the amplitudes is largest near 55" North Latitude. Small semiannual cyclical variations with maxima toward the end of *June and of December were found by Bernard [64] for Kew, Strasbourg, La Plata and, with some irregularities, for Eskdalemuir. At most stations there is no diurnal cycle beyond the limits of error [58, 60, 61,811. Meissner applied Fourier analysis to data for Kew [98] and found a weak minimum a t noon, whereas he had observed a maximum a t noon for Potsdam during the winter when amplitudes are frequently large. Lacoste [loo] reported for Strasbourg a small maximum about 8 A.M., Zatopek [loll for Prague one about noon, and Kishinouye [lo21 observed a t Tokyo and other stations a well-developed maximum a t 10 A.M. local time for disturbed days, but found no diurnal variation on quiet days. Whipple [92] has pointed out that maxima during the day have been found mainly on records of stations equipped with mechanically recording seismographs, and that friction between pen and paper of these instruments may be overcome more easily in daytime when high-frequency oscillation from traffic keep the pen moving. The existence of an eleven year variation was suggested by Lacoste [99] and later by Bernard [64] and by BBth [6l, 1031. Maxima of microseismic activity seem to have been observed a t Uppsala, Pare St. Maur, and Hamburg near 1910; then, especially strong in 1920-1921 at the same stations and a t Strasbourg; about 1930-1931, less strong a t these stations (but without data for Hamburg) and a t La Plata; and in 1942 a t Uppsala (no data for the other stations). Sun-spot maxima preceded the microseismic activity maxima frequently by about one quarter cycle; a t Uppsala, the maximum of the microseisms occurred even close to the following sun-spot minimum. Bernard [64] concluded that the correlation between microseisms and sun spots confirms the influence of solar activity on the frequency and intensity of cyclones for which there is only doubtful direct evidence [104], and B i t h [lo31 arrived a t a similar conclusion.

-J

0

TABLE V. Annual variations of amplitudes (in microns) of microseisms (T Ref.

Station, Comp.

Eskdalemuir, NS De Bilt, EW De Bilt, Z Hamburg, H. Uppsala, NS Strasbourg, H Pulkovo, EW Tashkent, EW Irkutsk, EW Tokyo, ? Dakar, H Bombay, NS 64 La Plata, H 64 Apia, ?

81 81 81 96 61 99 58 58 58 91 64 64

n

40

13 3 3 3 2 7 3

9 2

to 10 sec); n = number of years of observation.

Jan.

Feb.

Mar.

Apr.

May

June

July

Aug.

Sept .

Oct .

Nov.

Dee.

2.5 6.1 2.4 8.2 0.78 5.2 1.2 0.5 0.22 10 1.1 ? 1.0 6

2.3 5.6 2.2 6.8 0.74 4.0 1.0 0.5 0.18 7 1.0

1.8 3.8 1.3 5.6

1.0 7

1.2

0.3 1.3 0.7 1.1 0.07 1.0 0.2 0.2 0.09 2 2.5 1.0 1.3 4

0.5 1.9 0.8 1.4 0.13 1.4 0.3 0.1 0.10 3 2.3 0.8 1.1 4

0.9 2.5 1.1 3.1 0.31 1.8 0.4 0.2 0.13 5 2.2 1.0 1.1 2

1.2 3.8 1.4 3.6 0.46 2.6 0.7 0.4 0.20 9 2.3 1.o 1.2 4

1.8 4.9 1.9 5.4 0.61 3.8 1.2 0.5 0.30 6 1.9 0.9 1.1

2.3 6.0 2.4 6.7 0.66 4.8 1.0

?

0.7 1.8 0.8 1.9 0.16 1.5 0.3 0.1 0.06 3 1.8 0.9 1.1 5

0.5 1.9 0.9 1.5 0.11 1.2 0.2 0.2 0.06

?

1.2 3.1 1.1 3.3 0.36 2.5 0.6 0.2 0.10 6 1.5 0.8 0.9 5

-~ 14 7 7 16

=4

0.54

3.4 0.7 0.3 0.14 9 1.4

8

4

2.2 0.9 1.3 4

5

0.4

0.23 9 1.5 ?

1.0 6

p1 0

2 M

3 9

*

MICROSEISMS

71

7.1I . Observed Correlation between Microseismic Activity and Meteorological

Phenomena Many observers have found good correlations between meteorological phenomena and microseisms. However, such correlations may be indirect and may result, for example, from the correlation between ocean waves and meteorological processes. 7.11.1 . Cyclones over the Ocean. Correlations between microseisms and cyclones over the ocean have been observed already during early investigations of microseisms although frequently there is doubt about the type of microseisms involved. Correlations between effects of storms, such as high ocean waves or surf, and microseisms will be discussed in following sections. Gherzi [105, 1061 attributed the regular 6-sec microseisms directly to cyclones. In many publications he pointed out that rapid pressure changes (pumping) in cyclones may directly produce microseisms. Bradford [lo71 reached similar conclusions. However, most of those who have found a good correlation between microseismic and cyclonic activity (see [ll], pp. 109 and 272; [20, 108-1101) are doubtful about the physical connection between the two phenomena, or give reasons why they do not believe that cyclones are a direct cause of microseisms [36, 72, 83, 89, 111-1131. Microseisms may increase an appreciable time after the cyclone has reached its maximum intensity or its closest point to the station, especially for tropical disturbances. Sometimes there are microseismic storms without welldeveloped cyclones. The correlation between microseismic and cyclonic activity is frequently rather poor for tropical, and occasionally for nontropical disturbances. 7.11.2. Cyclones over Continents. Microseismic activity usually decreases when a cyclone crosses the coast from ocean to land. The details vary with the location of the stations-whether they are near west coasts or east coasts of continents, and whether they are in a belt of prevailing high pressure or in one of relatively high cyclonic activity. Sometimes microseisms increase after a cyclone has moved inland [89, 1091, but there may be other reasons for the amplitude increase in these instances, for example, a trailing front with strong winds causing relatively high ocean waves near the coast [89]. 7.11.3. Regular Microseisms and Cold Fronts. It has been pointed out repeatedly that microseismic activity increases when cyclones develop a strong cold front; however, frequently this front is not believed to be the cause of the microseisms. Unfortunately, in many publications it is not made clear which type of microseisms is being discussed. However, some results on effects of cold fronts on microseisms definitely refer to the regular 6-sec microseisms. For example, BMh (see [ll], p. 273; [114, 1151) has

72

B . GUTENBERG

observed that their amplitudes are increased by cold fronts. Hardtwig [113] believes that cold air masses, regardless of any connection with cold fronts, increase the microseismic activity. Leet [116] and Donn [117, 1181 have found considerable effect of cold fronts on microseisms in New England, and Gilmore [21] of coId fronts for the Caribbean; however, some of the microseisms discussed by them had relatively short periods and may belong to types discussed in Sections 5 and 6. B%thhas pointed out that passage of a well-developed cold front results in stronger winds and in an increase in other meteorological and oceanographic elements, and Donn states that cold winds generate ocean waves more readily than warm winds. 7.11.4. Strong Winds Blouling against Mountains. The possibility that cyclonic winds blowing against mountains may produce microseisms was pointed out by Father Algu6 in 1895 [119]. Bonchkovsky [82] concluded that strong winds near cold fronts may affect the mountain ranges along western Scandinavia and that they are essential in the formation of microseismic storms observed in northwestern Eurasia. 7.11.6. E$ects of Pressure Changes. The idea of Gherzi [105, 1061 that “pumping” in the eye of a typhoon causes microseisms is a special case of the more general hypotheses that rapid changes in air pressure are their source. Leet [lo91 concluded that microseisms may radiate from the center of a storm area over water and proposed for purpose of discussion “that microseisms are generated when a pressure gradient of magnitude as yet undefined moves over the crust, and in effect, kneads the surface layer in such a way as to set up vibrations.” Macelwane [110] has suggested that the air vortex of a hurricane generates a water vortex which extends to the sea bottom and produces microseisms there by setting up ground vibrations.

7.12. Observed Correlation between Ocean Waves and Regular Microseisms At many stations microseisms show parallelism with high ocean waves. However, numerical data are fairly scanty since ocean waves have been recorded only in recent years, and only a t or close to coasts. There are two fundamentally different hypotheses: one, that the waves in the open ocean or over shelves produce vibrations of the ocean bottom with various types of mechanisms involved; the other, that surf hitting the coast furnishes the source of energy. Wiechert [120] who proposed this second hypothesis, was apparently the first to investigate a relationship between ocean waves and microseisms. 7.12.1. Correlation between Amplitudes of Ocean Waves and Those qf Microseisms. Following Wiechert’s research various methods were used to study the correlation between surf and microseisms. Since no observa-

73

MICROSEISMS

tions of the surf were available, Gutenberg [36] used data on the motion of the sea (Seegang) which were daily estimated at various coastal points and reported on an arbitrary scale 0 to 9 on weather maps. For the winter months 1906-1909 he found the following amplitudes in microns of the vertical component Z of microseisms in Goettingen for a given average motion N of the sea at three stations in Norway: N

Z

1 0.2

2 0.3

3 0.6

4 1.0

5 1.2

6 2.7

Mendel [96] found a similarly good correlation for microseisms in Hamburg and the estimated state of the sea a t the three Norwegian stations with a correlation coefficient of 0.96 f 0.01 for five years. During five years for which Banerji [121] studied microseisms at ColabaBombay several storms formed in the Arabian Sea and the Bay of Bengal, he concluded that “all of them gave rise to microseisms of this kind from the time of their formation until they passed inland and ceased to disturb the sea.” Bernard 1641 found good correlation between ocean waves and microseisms; he pointed out that it is often difficult to separate the effect of relatively high waves near the point of observation and those farther away. A close correlation between microseisms and recorded ocean waves

FIG.8. (A) Amplitudes of regular 6-sec microseisms recorded a t Santa Clara and highest ocean waves recorded at Ellwood, California. (B) Periods of microseisms recorded at Santa Clara and periods of ocean waves recorded at Ellwood and a t Camp Pendleton, California; from [89].

74

B . GUTENBERG

has been found for southern California [89] (Fig. 8A). Additional data are given in the following section. 7.12.2. Relationship between Periods of Microseims and of Ocean Waves. The approximate relationship between periods of ocean waves, elements of a cyclone and distance of the waves from the storm center has been discussed by Munk [122] and by Roll [25]. Table VI shows the order of magnitude of the quantities involved. Ocean waves from near-by sources have shorter periods than those which have traveled a greater distance. During January and February, 1904, Wiechert had measurements made of the period of the surf near a lighthouse in Norway (see [36], p. 345). While the surf period varied between 8 and 12 sec, the period of the microseisms in Goettingen was usually about 6 sec. Bernard [64] has pointed out that frequently the periods of the ocean waves are roughly twice those of the corresponding microseisms. Later Miche [123] found such a relationship theoretically. Deacon [124] reported that microseisms in England had periods about half the ocean wave periods which he had found from frequency analyses, and Darbyshire [125, 1261 confirmed this ratio. In three series of simultaneous wave and microseismic records he could identify each band of microseismic activity with a band of sea waves twice its period [125]. Longuet-Higgins [127] provided further theoretical background. Many others believe that Miche's theory is confirmed by observations, for example Geddes [128] who studied microseisms a t Aberdeen, or Dinger who compared periods of ocean waves recorded a t the east coast of Florida with periods of hurricane microseisms recorded about 50 mi inland. Dinger and Fisher [129] refer to similar experiments made in Guam. I n Japan the two-to-one relation between the periods was found by some [130], but Kishinouye [131] states that the period of the swells was not always twice that of microseisms. Similarly, Gutenberg [89] observed that during microseismic storms, when the amplitudes (Fig. 8A) of microseisms and ocean waves showed good correlation in southern California, TABLE VI. Wind velocity v in m/sec, height h in meters and period T in sec of ocean waves; the index gives the distance from the storm center in km. Interpolated from Munk [122]. V

ho

hiooo

10 15 20 25

2% 6 9 12

1 2% 4%

7

To

Tiooo

7

10% 11% 12 13

8%

9% 10

T3000

15

1535 16 17

MICROSEISMS

75

the corresponding periods (Fig. 8B) did not show the 1:2 ratio. In some instances, when a storm center was close to the coast, both periods were more nearly equal. Near the storm center the ocean wave periods are relatively small (Table VI), while the periods of large microseisms from a near-by storm are usually relatively large. Doubts about the 1:2 ratio of the periods were also expressed by Donn [132]. B%th [133] found that the frequency maximum of the ratio of microseismic to ocean wave periods at Uppsala, Bergen, Reykjavik, and Scoresby Sund (about 220 instances at each station) is significantly higher than 0.5. The following are average frequencies at the four stations: 0.4 0.5 0.6 0.7 0.8 0.9 1.0 >1.0 Ratio 16 14 34 15 7 Frequency 3 5 6%

In all such investigations it is difficult to determine on the one hand the prevailing ocean wave period in the area of the actual generation of the microaeisms and, on the other hand, the period of the microseisms there. 7.13. Surf and Regular Microseims

It is difficult to establish numerical relations between surf and microseisms since practically no measurements are made of the height of surf. Wiechert [120] and Zoeppritz [95] published the first data supporting the connection. Linke [134] found that microseisms in Samoa increased each time a storm approached one of the nearby islands. Gutenberg [36] pointed out that at times of strong winds towards the coast of Norway the microseisms a t Goettingen are noticeably larger than during strong winds towards the ocean. Mendel [96] found for five years a correlation of 0.96 f 0.01 between the observed state of the sea in Norway and the microseisms in Hamburg. Tams [135] calculated for 22 days a correlation coefficient of 0.86 f 0.04 between state of the sea in Norway (3 stations, distance considered) and the horizontal component of the microseisms in Hamburg, if the direction of the wind was considered. Jung [136] found for the microseisms at Potsdam an even closer correlation with the state of the sea in southern Norway than for those in Hamburg. In various recent investigations (for example [89, 111, 1311) surf is considered to be a t least partly the cause of the microseisms discussed here. On the other hand, Ewing and Donn (see 11 I], p. 353) have pointed out that microseisms were recorded in Palmer Land, Antarctica, when large portions of the ocean were covered by ice. However, T. Hatherton (oral communication) has observed that the large microseisms at Scott Base disappear when ice covers the ocean.

’

76

B. GUTENBERG

7.14. Use of Microseisms in Meteorology Linke [134] was the first to use microseisms in forecasting of storms after he had observed that the microseisms at Apia became more irregular and their periods shorter when storms hit islands closer and closer to Apia. For locating storms approaching Europe, Gutenberg [137] suggested dividing the amplitudes of microseisms observed simultaneously a t various stations by the average for the respective station and to conclude from the region with the highest resulting ratio where high ocean waves existed and where a storm was approaching. Later Gilmore [72] used a similar method to investigate hurricanes in the Caribbean. Westerhausen [86] suggested locating storm centers by use of amplitude ratios of two horizontal components of microseisms together with the observed periods as a function of the amplitudes; he found that both are affected by the distance of the source and that their combination has different characteristics depending on the location of the source. As soon as tripartite stations became available, they were used in attempts to find the direction toward hurricanes from several stations and aided in the location of hurricanes (e.g. [20, 21, 78, 138-1421). The use of microseisms in meteorology was discussed repeatedly during the symposium on microseisms [72]. However, for the detection and investigation of hurricanes the use of airplanes, flying into critical areas, and of radar is now increasingly preferred, The use of microseisms in locating nontropical disturbances approaching a continent may possibly be of advantage in the rare instances when neither meteorological data nor information of heights of ocean waves near the coast are available [78, 142al.

8. MICROSEISMS WITH PERIODS OF TEN SECONDS TO SEVERAL MINUTES 8.1. Microseisms from Surf

Oliver and Ewing [143] have found that microseisms with periods from 11 to 18 sec recorded a t Palisades and at Bermuda are associated with water waves striking the coast near the station. The ground particle motion in all these cases was almost entirely in the horizontal plane. 8.2. Microseisms Connected with Local

Wind

While there is little information about the period of the “tremors” which were recorded by the early long-period instruments and frequently were correlated with strong winds, Milne [2] mentions specificially waves with great amplitudes if the pendulum had a period of about 15 sec. He considered the possibility that they were caused by air currents within

MICROSEISMS

77

the instrument case. Hecker [9] included in his classification microseisms with periods of about 30 sec and believed that they are produced by friction of wind a t the earth’s surface. Zoeppritz [95] observed “disturbances by wind” iu Goettingen with periods of about 15 sec. Galitzin [144] found that microseisms with a period of about 30 sec were recorded with smaller amplitudes by instruments operating in rarified air than by those without protecting cover, so that some direct effect of the wind on the instruments is indicated. Whipple [145] concluded that a t Kew shielding of the instruments did not remove the microseisms (Fig. 9), which had appreciably different amplitudes on the three components, and that strong winds against the walls of the observatory rock the whole building. Similarly, Schunemann [146] found that in Hamburg the amplitudes of these microseisms depended on the direction of the wind relative to the observatory. Addition of wings to the building and removal of trees near it resulted in changes of the microseisms. Tams [147] concluded that in Hamburg the correlation between the local wind velocity and the amplitudes of these microseisms is very high. Landsberg [148] found that such microseisms were recorded at the Taunus Observatory when a window or door was opened slightly. Similar microseisms have been reported at Manila [45] when strong northeast winds accompanied the outbreak of the “Siberian High.” Bernard 11491 concluded that in France microseisms with periods of about 30 sec sometimes originate from strong winds and are propagated through the ground, but that, in addition, convection currents in the recording room and icing of the seismographs may result in similar motion of the instruments. Causes for the 30-sec microseisms were also discussed at the symposium in 1951 (see [ll], pp. 16-18). Ewing and Press [150] could remove long-period “microseisms” from the records at Palisades by constructing a properly compensated seismograph. Similarly, Benioff [151] found that buoyancy variations of the atmosphere played a major part in such “microseisms” recorded at Pasadena. A revision of the instrument reduced these motions to a satisfactory low level; apparently they had been produced by flexure of the base plate. Thus, a t least many L‘microseisms”of the types discussed in this section do not correspond to ground motion.

4

ONE MINUTE

% FIG.9. “Microseisms” with periods of about 36 min, recorded at Kew by NS component of Galitzin seismograph; after Whipple 11451.

78

B. GUTENBERG

8.3. Microseisms with Periods of Several Minutes; Pulsations

The “pulsations” described in the early research by Rebeur-Paschwitz [l], Milne [2], and others (see [5], p. 193) were microseisms with periods of several minutes. They were observed mainly during the winter months. Some of them are probably of the same type as the irregular microseisms with periods of 1 to 3 min recorded by a long-period pendulum in Goettingen (Fig. 10) [36] during the winter months (October to April) from 1907 to 1910. The maxima, average about 100 M during January, occurred early in the morning, the minima of roughly half the maxima during the early afternoon. The relationship between the local temperature t and

_QI

+ONE

MINUTE

FIG.10. Microseisms during frost in Goettingen, January 14, 1908, recorded by NS component of mechanical recording pendulum with free period of 50 sec; from [%I.

the NS component of the microseisms was as follows (n observations) :

t 10 to 6 5 to 1 0 to -4 NS 13 39 66 243 240 n 111

-5 to -9 132 30

=

<-9 178 21

number of

“C p

Gutenberg [36] concluded that freezing of the ground within some distance from Goettingen was probably the cause of these microseisms. Similar microseisms have been observed by Gherzi [152] in Zi-Ka-Wei. Other reported instances [148, 1531 of microseisms with periods between 36 and 1 min may partly belong to the type discussed in Section 8.2; few instruments record relatively small waves with periods of a few minutes. Coulomb [154] has pointed to the possibility that the frost microseisms, like those discussed in Section 8.2, are caused by cold air currents a t the instrument. Microseisms with periods of to 2 min recorded a t Cartuja, Sp:Lin have been attributed by Due Rojo to changes in temperature in the ground. 9. THEORY 9.1, Introduction

9.1.1. The Problems. Most natural microseisms occur when energy of atmospheric disturbances is converted into energy of ocean waves which

MICROSEISMS

79

then transmit a portion of this energy either to the ocean bottom or to a coast. Then elastic waves travel through the earth’s crust and are recorded a t observatories. In some minor types, atmospheric waves may cause directly vibrations of the ground, or the wind may shake buildings, trees, etc., and produce irregular microseisms. The theory involved in the first-mentioned types of microseisms includes problems of transmission of wind energy to waves (compare [24, 122]), of wave energy to the ocean bottom or to the coast, and problems of wave propagation in the earth’s crust. Energy is generated in the atmosphere, while that traveling in the crust is observed. Some of the pertinent theories are outlined in the following sections. 9.1.2. Types of Elastic Waves. Two types of waves travel through the interior of an elastic solid: longitudinal waves, velocity Vp , and transverse waves, velocity V , . Apparently they do not play a role in microseisms. I n addition, there are several types of surface waves, which are conditioned by a discontinuity, especially the boundary between rock or water and air. In a homogeneous half-space their amplitude decreases exponentially from the surface downward, depending on the ratio of the depth below the surface to the wavelength. In pure surface shear waves of t>hefirst mode the particles move in a plane parallel to the discontinuity (in microseisms horizontally) and perpendicular to the direction of propagation. I n a homogeneous body their velocity equals V , . Pure Rayleigh waves have a velocity of about 0.9 V , , depending on Poisson’s ratio v. The ground particles move retrograde in ellipses. Their short axis is in the direction of propagation (horizontal) and their long axis, perpendicular to the discontinuity (vertical), is almost 1.5 the short (horizontal) axis; this ratio again depends on v. If the velocity in the medium changes with the distance from the discontinuity (depth), both types of surface waves show dispersion and the group velocity usually differs from the phase velocity. Moreover, waves with vibrations corresponding to different modes have different properties. Only few results are known about waves of second and higher modes (Section 9.6.4). I n a low-velocity layer (a “channel” containing a surface of minimum velocity), longitudinal or transverse body channel waves may be produced which travel repeatedly up and down inside this layer, either with or without reflections on one or both sides of the channel. If the channel is near the surface, surface channel waves may result. Again, theoretical results for the various types of these “Lg”-waves exist only on simple assumptions [155]. In the regular microseisms with periods between 4 and 10 sec, Rayleigh waves prevail (Section 7.9), but surface shear waves are also observed. The relatively small attenuation which is frequently less than that observed

80

B. GUTENBERG

in earthquake surface waves and the barrier effects are similar to those observed in Lg-surface waves in earthquakes [156, 1571 and suggest [133, 158-1601 that the regular microseisms are a motion of the earth’s surface produced by waves similar to Lg. Channel-surface waves may play a role (see Section 9.5). 9.2. Trunmissimz of Energy from the Surjuce to the Bottom of the Ocean I n a general treatment of the problem, how energy is transmitted from the atmosphere to the solid crust of the earth, the whole system atmosphere-water-crust is considered as one unit. In special investigations each of the three media is treated separately or combined with one of the others. The energy propagation downward through the ocean was discusted by Banerji [I611 with laboratory experiments. Whipple and Lee I1621 a i d others have shown that, contrasting with Banerji’s findings, the firstorder terms for pressure changes in gravity waves in the ocean decrease too rapidly with depth to produce observable microseisms a t the ocean bottom. Scholte [I631 suggested that sufficient energy to produce microseisms travels in elastic waves through the water to the bottom. He concluded that under reasonable assumptions the ratio of the elastic to the gravity wave amplitudes near the surface of the ocean is of the order of 1 :104, that these elastic waves travel to the ocean bottom without much loss of energy, and that the resulting motion of the ocean bottom is large enough to account for microseisms. However, Longuet-Higgins (see [ 1641, p. 2) has expressed doubt about this conclusion since ocean waves are not generated by oscillating pressure as assumed by Scholte but more probably by a systematic difference of pressure between the front and rear slopes of the crests of a wave train. In 1941, Bernard (see [64], p. 47) concluded from observations that interference of ocean wave trains is required for efficient generation of microseisms by ocean waves. In 1944, Miche [123] found theoretically that in stationary ocean waves there are pressure variations of twice the wave frequency which are not attenuated to zero with depth. Longuet-Higgins and Ursell [165] developed this theory, and Longuet-Higgins [164] showed that “variations in the mean pressure over a wide area arise as a result of the interference of groups of waves of the same wave length, but not’ necessarily of equal amplitude, traveling in opposite directions.” Ohservations which include interference of wave trains from two different storm centers as well as between waves coming directly from a storm and waves reflected a t a coast have been reported in support of the theories by Miche and Longuet-Higgins (see Section 7.12.2). The problem will be discussed again in Section 9.6.1.

MICROSEISMS

81

9.3. Waves in the System Ocean-Earth’s Crust The generation and propagation of elastic waves in a two-layered medium consisting of the ocean and the earth’s crust was studied by Press and Ewing [166]. They found that on the assumption of an homogeneous ocean overlying an infinitely thick homogeneous solid the curves giving the wave- and group-velocity as function of the ocean depth in units of the wavelength consist of two branches. Jeffreys [167] has pointed out that waves with periods near a minimum or maximum group velocity carry especially large fractions of the energy. Press and Ewing concluded t#hat on their assumptions the first branch of their dispersion curves for group velocity has one minimum and the second a minimum and a maximum. On reasonable assumptions, the periods corresponding to these extreme velocities are in the range observed in microseisms. In further research [168] Press and Ewing considered the effect of a mud layer in the ocean, of “organ pipe” modes of vibrations of the water layer and of resonant coupling between two media. Some of these questions have been discussed further by Haskell (see [72], pp. 111-112). Moreover, the second mode of vibrations may be involved. Extensive theoretical background for various problems arising in these investigations is contained in the important book on elastic waves in layered media by Ewing et al. [169]. Following the theory of Press and Ewing, Scholte [170] has discussed elastic waves in a two-layered medium consisting of the ocean and the crust, while Coulomb [154] has investigated the problem of maxima and minima of the group velocity and found on changed assumptions an additional minimum. Actually, periods of microseisms seem to show preponderance of certain values [97]. 9.4. Waves in the System Air-Earth’s Crust

Theoretical problems related to the generation of microseisms by impulses and oscillations in the atmosphere have been discussed by Sezawn and Kanai [171, 1721. Haskell [173] has pointed out that air-coupled surFace waves may play a role in microseisms with periods of 0.1 to 0.5 sec. The two-layer problem air-crust has been discussed extensively by Ewing et al. [169]. 9.ii. The Energy Problem

Any useful theory of microseisms must give the correct order of magnitude for their amplitudes. Gutenberg [69] has calculated that 10-3 of the energy of ocean waves driven during a storm against a coast has to be transferred to the coast in order to furnish the energy observed in microseisms. B%th [61] has concluded that the ratio of microseismic to sea wave energy is of the order of Byerly [174] found about 10-7.

82

B . GUTENBERG

Energy calculations by Scholte [170] have been mentioned in Section 9.2. Conditions under which standing ocean waves may be the sources of microseisms large enough to be recorded have been discussed by LonguetHiggins [175]. There are various complications. As mentioned in Section 9.3, waves with periods near a maximum or minimum of the group velocity accumulate energy during their propagation, so that amplitudes of waves with corresponding periods should be noticeably larger than those with other periods. The first mode (see Section 9.3) or the second mode may be involved. Other complications have been mentioned by Ewing ct al. (see [l69], pp. 142 ff.). Furthermore, the source for the energy is not a point. Effects of a line source have been discussed by B%th[115]. The relatively small absorption of microseisms traveling across continents indicates additional complications. The decrease of wave amplitudes by absorption is usually expressed by a factor e-kA where k is the absorption coefficient and A the distance in km. In the usual surface waves of earthquakes [176] approximately Ic = ?r/150 T V . For waves having a velocity V = 354 km/sec and periods of T = 5 sec, this would give an absorption coefficient Ic of over 0.001 per km, or from absorption alone a decrease of the amplitudes to e+ or to less than 0.01, at a distance of 5000 km. Consequently, microseisms originating in Scandinavia could not be observed in Central Asia nor microseisms which have traveled across North America. Microseisms share this relatively small absorption with the Lg-waves in earthquakes. It does not seem likely that the relatively large amplitudes over the fairly wide range of periods (about 4 to 9 sec in microseisms and even wider in Lg) are solely a consequence of the minimum group velocity. This relatively small attenuation is one of the reasons, for which microseisms and Lg-waves have been considered to be types of channel waves. The microseisms then would result from channel surface waves. Where the channel changes only little, the waves would travel with little loss of energy, but the channel waves may undergo appreciable transformation into other wave forms where the channel undergoes changes, as under geologically disturbed mountain areas or in zones between continents and oceans, where additional complications must be expected (see [169], p. 187). 9.6. The Period Problem

The periods of microseisms are considered by some to originate at the source, by others, along the path of the waves, and they may be affected by the structure near the recording station. All these factors may be involved, probably differently for different types of microseisms and depending on their period.

MICROSEISMS

83

9.6.1. Hypotheses Assuming That the Periods are Caused near the Source. Investigations show that the ratio of the periods of ocean waves to those of the corresponding microseisms is frequently close to 2: 1, especially on days with small microseismic amplitudes. However, there is doubt about the prevalence of this ratio during microseismic storms (Section 7.12.2). Moreover, the ratio of ocean wave periods to the periods of microseisms near their source may differ from the ratio based on microseismic periods observed a t a distant station. 9.6.2. Changes in Periods during the Propagation of Microseismic Waves. It has been argued that microseismic waves do not have the complicated structure to be expected if the ground motion near the source differs appreciably between points not far apart. This is to be expected if the motion of the ground is generated by irregular impulses, for example, the surf. However, the regularity of many types of microseisms far from the source could result from greater attenuation of short than of long waves during their propagation. Laboratory experiments, as well as theoretical considerations [176, 1771, favor this possibility for waves traveling in the earth’s crust. The fact that frequently the periods of seismic waves are the longer, the greater the distance over which they have traveled, could be a consequence of actual lengthening of wavelets in addition to greater attenuation of short waves. Practically all attempts to account for attenuation lead to equations which indicate that the wave periods should actually increase with distance (see [Ill, pp. 375-384; [176, 1771). The form of the equations for the increase in period depends on the assumptions [176]. Another source for selection of prevailing periods during the propagation of microseismic waves is the accumulation of energy near extreme values of the group velocity [178] (Section 9.5). Moreover, properties of the channel in which the microseismic waves travel may result in prevalence of certain periods. Finally, free vibrations of layers may play a role in determining prevailing periods. Hardtwig 11791 has tried to show that free vibrations of the “granitic” or “granitic” plus “basaltic” layers of the earth’s crust could explain the observed periods. 9.6.3. Egects of the Geological Structure near the Recording Station on the Periods. Suggested effects of the structure near the station on the recorded periods can be taken as a special case of the effects discussed in the preceding section. In most areas, the structure changes relatively little with distance, so that the observed periods should not differ much. However, for waves with periods of up to a few seconds, local conditions may produce locally different periods of microseisms of a given type. 96.4. Relation between the Mode of the Vibrations and the Periods of the Microseisms. Microseisms with periods of about 2 sec recorded in southern

84

B. GUTENBERG

California (Section 5.2) have properties which can be expected in surface waves of higher modes [43]. In earthquake records, surface waves with periods of less than 10 see, which may involve higher modes, have been observed [180-1821. However, in most of these instances various interpretations as to the type of the vibrations in the observed waves are still possible. If microseisms result from waves of higher modes, the mode in a specific instance is probably affected by the period of the vibrations near the source and by the mechanism of their generation. Periods of maxima and minima of the group velocity play an important role for the amplitudes of vibrations of each mode. Unfortunately, investigations of the modes involved in elastic surface waves in the earth have been started only recently, and no definite answer can be given to the questions, which types of microseisms are a consequence of surface waves of higher modes, or which modes are involved. When the author started his research on microseisms H t y years ago their causes and their theory were a source of extensive controversial discussions. It is still so today. More and more mechanisms have been consideied for the generation of microseismic waves in the earth’s crust, and the theoretical problems of their propagation through the crust have multiplied with the development of the theory of elastic waves in layered media and with the finding of wave types which had not been suspected in the earlier research. LIST OF SYMBOLS

1AB

I

tEC

A trace amplitudes in mm E W east-west component H horizontal component K defined by equation (2.2) N average motion of the sea (Seegang) in Norway NS north-south component P longitudinal waves S transverse waves T period in seconds V velocity of elastic waves in km/sec VP velocity of longitudinal waves in km/sec v s velocity of transverse waves in km/sec z vertical component a ground amplitudes in microns b, c two sides of triangle, formed by tripartite station (Fig. 1) h height of ocean waves in meters k absorption coefficient n number of observations t temperature in “C time differences between arrival of waves at three instruments of I tCA tripartite station

MICROSEISMS

85

v wind velocity in m/sec angle between sides A B arid AC of tripartite station (Fig. 1) 6 azimuth of side AC of tripartite station, measured from direction toward north A distance in km angle between wave front and side AC at a tripartile station (Fig. 1) $ angle between direction from which microseisms arrive and direction toward north (Fig. 1 ) u Poisson’s ratio a

+

REFERENCES 1. von Rebeur-Paschwitz, E. (1894). Horizontalpendel-Beobachtungen auf der Kaiserlichen Universitats-Sternwarte zu Strassburg 1892-1894. Gerl. Beitr. Geophys. 2 , 383. 2. Milne, J. (1908). “Seismology,” 2nd ed., 338 pp. Kegan Paul, Trench, Trubner

and Co., London. 3. Omori, F. (1901). Results of the horizontal pendulum observations of earthquakes, July to December 1899. Publ. Earthquake Invest. Comm. i n Foreign Languages, Tokyo 6 , 1-82. 4. Bertelli, T. (1872). Osservazioni sui piccolo movimenti dei pendoli in relazione ad alcuni fenomeni meteorologiche. Boll. meteorol. osserv. coll. Roma 9, 101. 5. Sieberg, A. (1904). “Handbuch der Erdbebenkunde,” 362 pp. Vieweg, Braunschweig. 6 . Cuenther, S. (1894). Luftdruckschwankungen in ihrem Einflusse auf die festen und flussigen Bestandteile der Erdoberflache. Gerl. Beitr. Geophys. 2 , 109-117. 7 . Macelwane, J. B. (1954). Sketch of the history of microseismology. In “Symposium on Microseisms” (J. T. Wilson and F. Press, eds.), pp. 3-6. National Research Council, Washington, D. C. 8. Gutenberg, B. and Andrews, F. (1952, 1956). “Bibliography of Microseisms,” 2nd ed., two parts. Mimeographed, 134 pp. Seismological Laboratory, California Institute of Technology, Pasadena. 9. Hecker, 0. (1906). Seismometrische Beobachtungen in Potsdam 1905. Veroj’entl. geod. Inst. Potsdam [N.F.] 29. 10. Rapport sur l’unification des symboles A utiliser dans les bulletins des mouvements micros6ismiques. Compt. rend. assoc. sbismol. et phys. intbrieure terre, Brussels, 196.2 10, 91-94. 11. (1951). Semaine d’6tude sur le problbme des micros6isms. Pontif. Acad. Sci., Scripta Varia 12, 402 pp. 12. Benioff, H. (1955). Earthquake seismographs and associated instruments. Advances i n Geophys. 2 , 220-275. See pp. 271-273. 13. Macelwane, J. B. and Sprengnether, W. (1938). A seismograph for microseisms. Trans. A m . Geophys. U n . 19, 128. 14. Sprengnether, W. F. (1946). A description of the instruments used t o record microseisms for the purpose of detecting and tracking hurricanes. Bull. Seismol. SOC.Am. 36, 83-87. 15. Krug, E. D. (1937). Ausbreitung der naturlichen Bodenunruhe nach Aufzeichnungen mit transportablen Horizontalseismographen. 2.Geophys. 13,328-348. 16. Trommsdorff, F. (1939). Untersuchungen uber die naturliche Bodenunruhe mit transportablen Dreikomponentenstationen. 2.Geophys. 16, 304-320. 17. Donn, W. L., Ewing, M., and Press, F. (1954). Performance of resonant seismometers. Lamont Geol. Obs. Tech. Rept. 36, 36 pp.

86

B. GUTENBERG

18. Hecker, 0. (1915). Versuche zur Bestimmung der Fortpflanzungsgeschwindigkeit der Bodenbewegung bei der milrroseismischen Unruhe. Gerl. Beitr. Geophils. 14, Mitt. Zentr. int. seismol. Assoz. 3, 2&33. 19. Shaw, J. J . (1921). Microseisms. 16th Rept. Brit. Assoc. Advance. Sci. Seth. ’4., Seismol. Invest. pp. 10-11. 20. Ramirez, J. E. (1940). An experimental investigation of the nature and origin of microseisms at St. Louis, Missouri. Bull. Seismol. SOC. Am. 30, 35-84, 139-178. 21. Gilmore, M. H. (1946). Microseisms and ocean storms. Bull. Seismol. Soc. A?,,. 36, 90-119. 22. Schuyler, G. L. (1955), Computations of the directions of microseisms :It tripartite stations. Bull. Seismol. SOC.Am. 46, 285-288. 23. Benioff, H., Gutenberg, B., and Richter, C. F. (1951). Progress Report, Scismological Laboratory, California Institute of Technology, 1950. Trans. Am. Geophys. Un. 33, 75c752. 24. Strobach, K. (1957). Stereoskopische Vektorregistrierung. Z. Geophys. 23, 306-315. 25. Roll, H. U. (1957). Oberflachen-Wellen des Meeres. I n “Handbuch der l’hysikEncyclopedia of Physics” (S. Flugge, ed.), Vol. 48, pp. 671-733. Springer, Berlin. 26. Bernasconi, C., Bossolasco, M., and Ignazio, D. (1952). Nuovo metodo di registrazione dello stato del mare e primi risultati ottenuti a Genova. Geojs. Pura Appl. 21, 159-168. 27. Wilson, C. D. V. (1953). The origin and nature of microseisms in the frequency range 4 to 100 c/s. Proc. Roy. SOC.A217, 176-188. 28. Carder, D. S., and Gilmore, M. H. (1945). Ground vibrations. Bull. Seismol. SOC.Am. 36, 13-26. 29. Mintrop, L. (1911). “Ueber die Ausbreitung der von Massendrucken einer Grossgasmaschine erzeugten Bodenschwingungen,” 32 pp. Dissertation, Goettingen. 30. Wilson, C. D. V. (1953). An analysis of the vibrations emitted by some manmade sources of microseisms. Proc. Roy. Soc. A317, 18&202. 31. Essers, E., and Kappes, T. (1927). Bodenerschutterungen durch Kraftfahrzeuye. Z. Geophys. 3, 49-57. 32. Brand, F. (1925). Ein Beitrag zum Studium der Bodenbewegungen nicht seismischen Ursprungs. Z. Geophys. 1, 348-359. 33. Grunmaeh, L. (1909). Ueber neue Methoden und Apparate zur Messung von Erderschutterungen kleinster Periode. Physik. Z. 10, 853-859. 34. Walsh, D . H. (1955). An observational study of the origin of short-period mirroseisms near St. Louis, Missouri. Trans. Am. Geophys. Un. 36, 679487. 35. Chakrabarty, S. K., and Sarkar, D. (1955). Microseisms associated with Norwesters. Compt. rend. Conf. Intern. Assoc. Seismol. and Phys. Earth’s Interior, Rome, 1964 11, 127. 36. Gutenberg, B. (1912). Die seismische Bodenunruhe. Gerl. Beitr. Geophys. 9, 314-353. 37. Belensiefer, E., Buettner, K., Pfleiderer, H., and Wetzel, W. (1939). Untersuchungen ueber die Bodenunruhe auf Sylt. 2. Geophys. 16, 337-364. 38. Omori, F. (1912). Tremor observations on the Usu-san and the Asama-Yama volcanoes. Compt. rend. 4 Conj. Comm. Perm. Sismol., Manchester, 1911 p. 254. 39. Nisimura, E. (1936). Vibrations of the Aso volcanological laboratory building and its surrounding ground. Mem. Coll. Sci. Univ. Kyoto A N , 191-206.

MICROSEISMS

87

40. Sakuma, S., and Nagata, T. (1957). Physical volcanology. In “Handbuch der Physik” 48, p. 997. Springer, Berlin. 41. Caloi, P., Lo Surdo, A., and Ponte, G. (1948). Agitationi microsismiche originati da attivita vulcanica. Ann. geofis. (Rome) 1, 5-9. 42. Lynch, J. (1956). The Great Lakes, a source of two-second frontal microseisms. l’rav. sci. Intern. Assoc. Seismol. and Phys. Earth’s Interior A N , 177-182. 43. Gutenberg, B. (1956). Untersuchungen zur Bodenunruhe in Sudkalifornien. 2.Geophys. 21, 177-190. 44. Donn, W. L. (1953). A comparison of microseisms and ocean waves recorded in Southern New England. Trans. Am. Geophys. U n . 34, 471-476. 45. Alcaras, A., and Kintanar, R. (1952). Pacific microseisms. Publ. Bur. central sismol. intern. Ser. A, 18, 109-125. 46. Caloi, P. (1950). Due caratteristici tipi di microsismi. Ann. geofis. (Rome) 3, 303-314. 47. Giorgi, M. (1949). Su alcuni aspetti caratteristici dei microsismi a Roma in relazione con fattori meteorologici. Ann. Geojs. (Rome) 2, 24-39. 48. Omori, F. (1909, 1913). Report on the observation of pulsatory oscillations in Japan. Bull. Earthquake Invest. Comm. J a p a n 3, 1-35; 6, 109-147. 49. Kishinouye, F. (1951). On the period and the amplitude of microseismic movement. Bull. Earthquake Res. Inst. Tokyo Univ. 29, 483486. 50. Bernard, P. (1938). L’agitation micros6ismique au Maroc. Ann phys. globe France outre-mer 6, (29) 135-136. 51. Banerji, S. K . (1924, 1925). Microseisms associated with SW monsoon. Nature 114, 576; 116, 866. 52. Mukherji, S . M. (1951). Microseisms and sea waves. Bull. Seismol. SOC.A m . 41, 1-4. 53. Gherzi, E. (1926). Microskisms et defkrlement des vagues sur les cbtes. 2. Geophys. 2, 159. 54. Schneider, R. (1909). Ueber die pulsatorischen Oszillationen des Erdbodens im Winter 1907/08 in Wien. Mitt. Erdbebenkomm. Akad. Wiss. W i e n (N. F.] 36, 48 PP. 55. Compt. rend. assoc. intern. sismol., Zermatt, I909 (1910). (R. de Kovesligethy, ed.) p. 22. Budapest, Hungary. 56. Jensen, H. (1957). On the beat-distribution in group-microseisms. Medd. Geod. Inst. Kobenhavn. 3, 27 pp. 57. Romney, C. F. (1953). Discussion in “Symposium on Microseisms” (J. T. Wilson and F. Press, eds.), (Washington), pp. 66-71. National Research Council, Washington, D. C. 58. Gutenberg, B. (1929). Die seismische Bodenunruhe. In “Handbuch der Geophysik,” Vol. 4, pp. 264-298. Borntraeger, Berlin. 59. Galitein, B. (1919). Microseismic movements. Compt. rend. comm. centr. sism. perm. 7, Book 2, 97-185. 60. Whipple, F. J. W., and Lee, A. W. (1931). Studies in microseisms. Monthly Not. Roy. Ast. SOC.Geophys. Suppl. 2, 363-373. 61. Bllth, M. (1949). An investigation of Uppsala microseisms. Meteorol. Inst. Univ. Medd. 14, 168 pp. 62. Mainka, C. (1913). Ueber die Hiiufigkeit einzelner Mi-U.-perioden. Physik. 2. 14, 1285-1286. 63. Milne, J., and Lee, A. W. (1939). “Earthquakes.” Kegan Paul, Trench, Trubner and Co., London.

88

B. GUTENBERG

64. Bernard, 1’. (1941). Etude sur l’agitation microskismique e t ses variations. Ann. inst. phys. globe, Paris 19, 1-77. 65. MurkoviE, B. (1948). MikroseizmiEki Neniir u Zagrebu (resumb in I h n c h ) . Rad GeoJz Zavoda ZL Zngrebu 11, 1, 87 pp. 66. Mukherjec, S. M. (1948). On microseisms recorded i n India arid Ceylon. ZrLd. Meteorol. Dept. Sci. Notes 10(118), 4149. 67. Gutenberg, B. (1931). Microseisms in North America. Bull. Seismol. SOC.A m . 21, 1-24. 68. Carder, D. (1955). Transmission of microseisms across North America and the western North Atlantic. Trans. A m . Geophys. U n . 36, 83&-854. 69. Gutenberg, B. (1951). Observations and theory of microseisms. In “Compendium of Meteorology” (T. F. Malone, ed.), pp. 1303-1311. American Meteorological Society, Boston, Mass. 70. Munk, W. H. (1947). Increase in the period of waves traveling over large distances. Trans. A m . Geophys. Un. 28, 198-217. 71. Darbyshire, J. (1954). Structure of microseismic waves; estimation of direction of approach by comparison of vertical and horizontal components. Proc. Roy. SOC.M23,96-111. 72. Wilson, J. T., and Press, F., ed. (1953). “Symposium on Microseisms.” National Research Council, Washington, D. C. 73. Kammer, E. W., and Dinger, J. E. (1951). Hurricane swell as generator of microseisms. J. Meteorol. 8 , 347-353. 74. Darbyshire, J. and M. (1957). The refraction of microseisms on approaching the coast of the British Isles. Monthly Not. Roy. Ast. SOC.Geophys. Suppl. 7,301-307. 75. Gutenberg, B. (1957). Effects of ground on earthquake motion. Bull. Seismol. SOC.A m . 47, 221-250. 76. Gutenberg, B. (1947). Microseisms and weather forecasting. J . Meteorol. 4, 21-28. 77. Gilmore, M. H., and Hubert, W. E. (1948). Microseisms and Pacific typhoons. Bull. Seismol. SOC.Am. 38, 195-228. 78. Gutenberg, B . , and Benioff, H. (1956). An investigation of microseisms. Final Rept. Contract AF 19(122)436, 39 pp. California Institute of Technology, Pasadena, Calif. 79. Kishinouye, F., and Shida, I. (1956). Tripartite observation of microseisms a t Sakata. Bull Earthquake Res. Inst. Tokyo Univ. 34, 301-306. 80. Gutenberg, B. (1921). Untersuchungen uber die Bodenunruhe mit Perioden von 4“-10* in Europa. Veroffentl. Seismol. Assoz. (Strassburg), 106 pp. 81. Lee, A. W. (1934). A world-wide survey of microseismic disturbances recorded during January, 1930. Meteorol. O$., London, Geophys. Mem. 62, 1-33. 82. Bonchkovsky, V. F. (1946). Microseismic disturbances and their causes (in Russian, with English abstract). Publ. Inst. Seismol. Moscou 120, 40 pp. 82a. Roth6, J. P. (1952). Etude du mouvement microskismique B Strasbourg. Pontif. Acad. Sci., Scripta Varia 12, 19-61. 83. Blth, M. (1953). Comparison of microseisms in Greenland, Iceland and Scandinavia. Tellus 6 , 109-134. 84. Kammer, E. W., and Dinger, J. E. (1952). Results of field experiments during 1951 on microseisms as related to storms a t sea. N R L Mem. Rept. 3, 10 pp. 85. Murphy, L. (1947). Geological effects on microseisms in the Caribbean. Trans. A m . Geophys. Un. 28, 528-533. 86. Westerhausen, H . (1954). Ueber die Ortung mikroseismischer Unruheherde. Ann. geofis. (Rome) 7 , 71-124.

MICROSEISMS

89

87. Blth, M. (1952). The problem of microseismic barriers with special reference to Scandinavia. Geol. Fbren. i Stockholm Forhandl. 74, 427449. 88. Lacaze, J. R. (1953). Applications g6ologiques d’6tudes sur les micros6ismes h Alger. 19. Internat. Geol. Congress, 1952. Abstract in Geophys. 18, 219. 89. Gutenberg, B. (1953). Microseisms, microbaroms, storms and waves in western North America. Trans. A m . Geophys. U n . 34, 161-173. 90. Gutenberg, B. (1924). “Die seismische Bodenunruhe.” Borntraeger, Berlin. 91. Wadati, K., and Masuda, K. (1935). On pulsatory oscillations of the ground. Geophys. Mag. 9, 299-340. 92. Whipple, F. J. W. (1934). Notes on Mr. A. W. Lee’s investigation “A world-wide survey of the microseismic disturbances, January 1930.” Publ. bur. central. assoc. intern. sismol. Ser. A, 10, 127-135. 93. BPtth, M. (1957). Written communication. 94. Schneider, R. (1927). Untersuchungen uber die seismische Bodenunruhe kureer Periode. 2 . Geophys. 4, 103-109. 95. Zoepprite, K. (1908). Mikroseismische Bewegung; in: Seismische Registrierungen in Goettingen im Jahre 1906. Nach. Ges. Wiss. Goettingen Math-phys. K1. pp. 2-10. 96. Mendel, H. (1930). Die seismische Bodenunruhe in Hamburg und ihr Zusammenhang mit der Brandung. 2 . Geophys. 6 , 3241. 97. Geussenhainer, 0. (1921). Ein Beitrag aum Studium der Bodenunruhe mit Perioden von 4 sec. bis 10 sec. Jahrb. Phil. Fakultlit Gbttingen Part 2, Ausaiige aus Dissertationen, No. 18, 73-80. 98. Meissner, 0. (1931). Ueber die tiigliche und jiihrliche Periode der mikroseismischen Bewegung in Eskdalemuir und Kew. 2 . Geophys. 7 , 193-195. 99. Lacoste, J. (1934). Observations sur le mouvement micros6ismique il Strasbourg. Trav. sci. bur. central. seismol. Intern. A, 10, 45-48. 100. Lacoste, J. (1932). Dix ann6es d’observations sur les mouvements micros6ismiques Strasbourg. Trav. sci. bur. centra. sbismol. intern A7, 16-35. 101. ZAtopek, A. (1956). Sur les micros6ismes de Praha. Trav. sci. bur. central shismol. Intern A19, 183-191. 102. Kishinouye, F. (1940). A supplement to A. W. Lee’s paper ‘‘A world-wide survey of microseismic disturbances recorded during January, 1930.” Bull. Earthquake Res. Inst. Tokyo Univ. 18, 507-513. 103. Blth, M. (1948). Some long period variations of microseismic activity. Geojis. Pura Appl. 12, 6 pp. 104. Malone, T. F., ed. (1951). “Compendium of Meteorology,” pp. 819,840. American Meteorological Society, Boston, Mass. 105. Gherzi, E. (1924). Etude sur les micros6ismes. Notes de Sismol. O h . de Zi-Ka-Wei 5 , 16 PP. 106. Gherzi, E. (1932). Cyclones and microseisms. Gerl. Beitr. Geophys. 36, 20-23. 107. Bradford, D. C. (1935). On a study of microseisms a t Sitka, Alaska. Bull. Seismol. SOC.A m . 26, 323-342. 108. Gilmore, M. H., and Hubert, W. E. (1948). Microseisms and Pacific typhoons. Bull. Seismol. SOC.A m . 38, 195-228. 109. Leet, L. D. (1948). Microseisms in New England. Bull. Seismol. SOC.A m . 38, 173-178. 110. Macelwane, J. B. (1946). Origin of microseisms. Science 104, 300-301. 111. Van Straten, F. W. (1954). Storm and surf microseisms. In “Symposium on Microseisms” (J. T. Wilson and F. Press, eds.), pp. 94-101. National Research Council, Washington, D. C.

90

B. GUTENBERG

112. Dinger, J. E. (1954). Discussion in “Symposium on Microseisms” (J. T. Wilson and F. Press, eds.), p. 87. National Research Council, Washington, D. C. 113. Hardtwig, E. (1951). Untersuchungen iiber Mikroseismik in Deutschland wahrend des zweiten Weltkrieges. Ann. geojis. (Rome) 4, 95-106. 114. BMh, M. (1952). The microseismic time lag problem. Pontif. Acad. Sci., Scripta Varia, 12, 279-289. 115. Bllth, M. (1951). The microseismic importance of cold fronts. Arkiv Geofys. 1, 267-358. 116. Leet, L. D. (1947). Microseisms in New England. Geophysics 12, 639-650. 117. Donn, W. L. (1951). Frontal microseisms generated in the western North Atlantic Ocean. J . Meteorol. 8, 406-415. 118. Donn, W. L. (1957). A case study bearing on the origin and propagation of 2- to 6-second microseisms. Trans. Am. Geophys. Un. 38, 354-359. 119. Deppermann, C. E. (1951). Father Jose AlguB, 8. J., and microseisms. Bull. Seismol. SOC.Am. 41, 301-302. 120. Wiechert, E. (1904). Verhandlungen der zweiten Internationalen Seismologischen Konferenz. Gerl. Beitr. Geophys. Erganzungsband 2, 4143. 121. Banerji, S. K . (1930). Microseisms associated with disturbed weather in the Indian seas. Phil. Trans. Roy. SOC.London A229, 287-328. 122. Munk, W. H. (1951). Ocean waves as a meteorological tool. I n “Compendium of Meteorology” (T. F. Malone, ed.), pp. 1303-1311. American Meteorological Society, Boston, Mass. 123. Miche. M. (1944). Mouvements ondulatoires de la mer en profondeur constante ou dbcroissante. Ann. ponts et chausstes 114, 25-87, 131-164, 270-292, 396-406. 124. Deacon, G. E. R. (1948). The generation of microseisms. ddrd Rept. Brit. Assoc. Advance. Sci., Seismol. Invest. pp. 7-8. 125. Darbyshire, J. (1949). The correlation between microseisms and sea waves. Compt. rend. assoc. intern. sbismol., 9 bis, 19-20. 126. Darbyshire, J. (1950). Identification of microseismic activity with sea waves. Proc. Roy. SOC.A202, 439448. 127. Longuet-Higgins, M. S. (1950). A theory of the origin of microseisms. Phil. Trans. R o y . SOC.London A243, 1-35. 128. Geddes, A. E. M. (1958). A survey of microseisms recorded at Aberdeen in 1955, together with a review of the meteorological conditions under which they may have arisen. Bull. Seismol. SOC.Am. 48, 65-76. 129. Dinger, J. E., and Fisher, G. H. (1955). Microseisms and ocean wave studies 011 Guam. Trans. Am. Geophys. U n . 36, 262-272. 130. Inouye, W., Hirono, T., and Murai, G. (1954). Microseisms and surf. Geophys. Mag. 26, 175-183. 131. Kishinouye, F. (1951). Microseisms and sea waves. Bull. Earthquake Res. Inst. Tokyo Uniu. 29, 577-582. 132. Donn, W. L. (1953). A comparison of microseisms and ocean waves recorded in southern New England. Trans. Am. Geophys. U n . 34, 471476. 133. BIlth, M. (1953). Comparison of microseisms in Greenland, Iceland and Scandinavia. Tellus. 6, 109-134. 134. Linke, F. (1909). Die Brandungsbewegungen des Erdbodens und ein Versuch ihrer Verwendung in der praktischen Meteorologie. Abh. Ges. Wiss. GoLtingen Math. phys. KZ. [N. F.] VII(3), 58 pp. 135. Tams, E. (1953). Einige Korrelationen zwischen seismischer Bodenunruhe in Hamburg und der Brandung in West- und Nord-Europa. 2. Geophys. 9, 23-31.

MICROSEISMS

91

136. Jung, K . (1934). Ueber mikroseismische Bodenunruhe und Brandung. 2 . Geop h p . 10, 325-329. 137. Gutenberg, B. (1924). “Die seismische Bodenunruhe,” p. 56. Borntraeger, Berlin. 138. Gilmore, M. H . (1947). Tracking ocean storms with the seismograph. Bull. A m . Meteorol. SOC.28, 73-85. 139. Gilmore, M. H. (1956). Microseisms used in hurricane forecasting. Trav. sci., Intern. ASSOC. skismol., Al9, 203-215. 140. Jones, 0. A. (1950). The early detection of hurricanes. Univ. Queensland Papers, Dept. Geol. [N. S.]41. 141. Macelwane, J. B. (1951). Practical application of microseisms to forecasting. In “Compendium of Meteorology” (T. F. Malone, ed.), pp. 1312-1315, American Meteorological Society, Boston, Mass. 142. Deacon, G. E. R. (1954). Sea waves and microseisms. Brit. Assoc. Advance. o f S c i . 11,356-357. 142a. Iyer, H. M. (1958). A study on the direction of arrival of microseisms a t Kew Observatory. Geophys. Jour. Ast. SOC.1, 3243. 143. Oliver, J . , and Ewing, M. (1956). Ultra-long-period microseisms. Earthquake Notes 27, 21. 144. Galitzin, B. (1910). Sur les mouvements microseismiques. Comptes rend. assoc. Intern. sismol., Zermatt, 1909 p. 68. 145. Whipple, F. J. W. (1928). The action of wind on seismographs. 2. Geophys. 4, 417419. 146. Schunemann, H. (1931). Die seismische Bodenunruhe zweiter Art in Hamburg und ihre Ursache. 2 . Geophys. 8 , 216226. 147. Tams, E. (1935). Seismische Bodenunruhe in Hamburg und ortlicher Sturm. 2. Geophys. 11, 9-15. 148. Landsberg, H . (1933). Beitrag zum Thema: Seismische Bodenunruhe, 2 . Geophys. 9, 15&161. 149. Bernard, P. (1947). Sur la cause des “micros6ismes” A grande p6riode. Ann. gbophys. 3, 96-100. 150. Ewing, M., and Press, R. (1953). Further study of atmospheric pressure fluctuations recorded on seismographs. Trans. A m . Geophys. U n . 34, 95-100. 151. Benioff, H., Gutenberg, B., Press, F., and Richter, C. F. Progress Report, Seismological Laboratory, California Institute of Technology, 1956. Trans. Am. Geophys. Un. 38, 248-254. 152. Gherzi, E. (1929). Etude sur les micros6ismes causes par le froid. Notes de sbismol. Obs. de Zi-Ka-Wei 10, 1-7. 153. Gutenberg, B. (1928). Bodenunruhe durch Brandung und durch Frost. 2 . Geophys. 4, 246-250. 154. Coulomb, J. (1956). L’agitation microskismique. “Handbuch der PhysikEncyclopedia of Physics” (S. Flugge, ed.), Vol. 47, p. 141. Springer, Berlin. 155. Jeffreys, H. (1957). Elastic waves in a continuously stratified medium. Monthly Not. Roy. Ast. SOC.Geophys. Suppl. 7 , 332-337. 156. Bbth, M. (1954). The elastic waves Lg and Rg along Euroasiatie paths. Arkiv Geophys. 2, 295-342. 157. Oliver, J., Ewing, M., and Press, F. (1955). Crustal structure of the Arctic regions from the Lg phase. Bull. Geol. SOC.Am. 66, 1063-1074. 158. Ewing, M., and Press, F. (1952). Propagation of elastic waves in the ocean with reference t o microseisms. Pontif. Acad. Scrzpta Varia 12, 121-127.

92

B . GUTENBERG

159. Gutenberg, B. (1955). Channel waves in the earth’s crust. Geophysics 20,283-294. 160. Bllth, M. (1956). Some consequences of the existence of low-velocity layers. Ann. geojis. (Rome) 9, 411-450. 161. Banerji, S. K . (1935). Theory of microseisms. Proc. Znd. Acad. Sci. 1(10),727-753. 162. Whipple, F. J. W., and Lee, A. W. (1935). Notes on the theory of microseisms. Monthly Not. Roy. Ast. Soc. Geophys. Suppl. 3, 287-297. 163. Scholte, J. G. (1943). Over het verband tussen eeegolven en microseismen. Ned. Acad. Wetensch. Afd. Natuurk. @(lo), 669-683. 164. Longuet-Higgins, M. S. (1950). A theory of the origin of microseisms. Phil. Trans. Roy. Soc. London A243, 1-35. 165. Longuet-Higgins, M. S., and Ursell, F. (1948). Sea waves and microseisms. Nature 162, 700. 166. Press, F., and Ewing, M. (1948). A theory of microseisms with geologic nssurnptions. Trans. Am. Geophys. U n . 29, 163-174. 167. Jeffreys, H. (1925). On the surface waves of earthquakes. Monthly Not. ROM. Ast. SOC.Geophys. Suppl. 1, 286292. 168. Press, F., and Ewing, M. (1953). The ocean as an acoustic system. In “Syniposium on Microseisms” (J. T. Wilson and F. Press, eds.), pp. 109-113. National Research Council, Washington, D. C. 169. Ewing, W. M., Jardetzky, W. S., and Press, F. (1957). “Elastic Waves in Layered Media.” McGraw-Hill, New York. 170. Scholte, J. G. (1954). On theories of the origin of microseisms. In “Symposium on Microseisms” (J. T. Wilson and F. Press, eds.), pp. 114-123. Nationd Research Council, Washington, D. C. 171. Seeawa, K., and Kanai, K. (1935). The nature of microseisms of local type. Bull. Earthquake Res. Inst. Tokyo Univ. 13, 729-738. 172. Sezawa, K., and Kanai, K. (1939). Microseisms caused by transmission of ntmospheric disturbances. Bull. Earthquake Res. Inst. Tokyo Univ. 17, 19&207, 548-558. 173. Haskell, N. A. (1951). A note on air-coupled surface waves. Bull. Seismol. SOC. A m . 41, 295-300. 174. Byerly, P. (1942). Microseisms at Berkeley and surf on near-by coasts. Bull. Seismol. Soc. A m . 32, 277-282. 175. Longuet-Higgins, M. S. (1954). Can sea waves cause microseisms? In “Symposium on Microseisms” (J. T. Wilson and F. Press, eds.), pp. 74-86. National Research Council, Washington, D. C. 176. Gutenberg, B. (1958). Attenuation of seismic waves in the earth’s mantle. Bull. Seismol. SOC.A m . 48, 269-282. 177. Fortsch, 0. (1956). Die Ursachen der Absorption elastischer Wellen. Ann. geojis. (Rome) 9, 469-524. 178. Mitra, M. (1957). Rayleigh waves in a multi-layered medium with applications t o microseisms. Monthly Not. Roy. Ast. SOC.Geophys. Suppl. 7. 324-331. 179. Hardtwig, E. (1957). Ueber die Entstehung der Mikroseismik. Z . Geophys. 23, 83-112. 180. Oliver, J., and Ewing, M. (1957). Higher modes of continental Rayleigh w a v ~ s . Bull. Seismol. SOC.A m . 47, 187-203. 181. Oliver, J., and Ewing, M. (1957). The second mode of continental Love wives. Earthquake Notes 28, 15. 182. Oliver, J., and Ewing, M. (1958). Normal modes of continental surface waves. Bull. Seismol. SOC.Am. 48, 33-49.

THE SIZE AND SHAPE OF THE EARTH R. A. Hirvonen Institute of Technology, Helsinki, Finland, and The Ohio State University, Columbus, Ohio

Page 1. Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

93 2. Triangulation. . . . . . . . . 2.1 The Lay-Out.. . . . . 2.2 Base-Line Measurement . . . . . . . . . 94 2.3 Measurement of An 2.4 The Computation of Triangulations 97 2.5 The Intercontinental Connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 3. Astronomical Coordinates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Spirit Leveling., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Normal Gravity.. ............................................ 105 5.2 Iletermination of t . . . . . . . . . . 111 5.3 New Ideas.. . 112 List, of Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . .

1. INTRODUCTION

The determination of the size and the shape of the earth is the task assigned to the science called-incorrectly-geodesy, “division of the land.” The proper name should, of course, be geometry. The scientific achievements of geodesy have usually been employed for the pursuit of practical needs such as mapping and charting, navigation, engineering, or prospecting for oil and ore. The increased practical demands und the rapid technical progress have, during the last decades, led to a great number of important advances in the construction of geodetic instruments and in the methods of observation. Without going into technical details, these advances will be mentioned here only in cases where they contribute to scientific geodesy. The principal geodetic observations, on which the most precise knowledge of the form of the earth is based, are according to [l]: (1) triangulation, (2) astronomical fixations, (3) spirit leveling, and (4) gravity measurements. 93

94

R. A. HIRVONEN

In the following sections we shall discuss each of these groups separately in order to give a general review of the problems concerned and especially of the precision obtainable with the modern instruments and methods. The rapid accumulation of the observational material has caused a reformation in the theoretical treatment of the combination of these four groups. The development of the new theory is not yet quite finished. However, the outlines of this new general theory of geodesy will form the final section of this review. 2 . THIANGULATION 2.1. The Lay-Out

The mapping of each country is based on triangulations. The triangulation bench marks are situated on hills which are mutually visible in such a way that the triangles form a network, a chain, or a chain-network covering the whole country. The requirements of precision being balanced against those of economy, the average lengths of the sides in a triangulation of the first order are made about 30 km long. In exceptional cases, when very high mountains are available triangulation sides of several hundred kilometers could be used, but because of the weather conditions and other practical difficulties longer sides than 60 km are usually avoided. The lines connecting Spain and Africa are about 200 km. Wider oceans than that between these territories cannot be bridged by ordinary triangulations. The networks of different continents have, so far, remained isolated from each other. The observational work in a triangulation consists of two operations: measurement of base lines, and measurement of angles. Geometrically, one base line in each network is enough. The sides of all triangles can then be trigonometrically computed by the aid of the angles. The rapid accumulation of the errors of the measured angles compels, however, the insertion of new base lines with spacing of 10 to 20 triangles (200 to 400 km). The error in azimuth must be controlled by astronomical observat’1011s. As explained in Section 3, these observations are carried out in order to determine the deflections of the vertical. But if both the azimuth and the longitude have been observed astronomically a t the same station, the deflection of the vertical can be eliminated and the azimuth obtained can be used for the corroboration of the triangulation. Stations a t which azimuth is controlled in this way are known as Laplace stations. The general trend in most, countries has been to establish one Lnplare station for cnch base line. 2.2. Base-Line Measurement

The base lines are usually short, 3 to 6 km, and they must be connected to the nearest side of the main triangulation by a specially observed exten-

SIZE AND SHAPE O F THE EARTH

95

sion net. The precision obtainable in the base-line measurement is about 1 mm/km. Several inoar wires, usually 24 meters long, are used, and these must be checked against changes a t standard base lines before and after each measurement. The standard base lines of neighboring countries are compared with each other. It is of minor importance to have an absolute comparison with the international metric system. Certainly this can be done and has been done with the precision of one in ten millions, using the interference apparatus of Y. Vaisala [2, 31. 2.3. Measurement of Angles

The angles are measured with theodoliles that usually must be mounted on observation towers. Wooden towers are most common; sometimes concrete buildings have been used as well. Nowadays the movable steel towers have often been found to be the most rapid and economical contrivance, in cases where the first-order triangulation does not have to be immediately followed by the second-order triangulations for the mapping purposes. The surrounding towers are furnished with electrical lamps or heliotropes reflecting sunlight toward the observer. The theodolites are in principle similar to those that have been used through the centuries. Of course, many technical improvements have been made, up until the last years, with the result that the modern theodolites are smaller, and more rapid to use, but hardly more precise than the old models. The atmospheric disturbances seem to set a limit of precision that cannot be transgressed just by using better instruments. This precision is about 0.3” or, in radians, 1.5 to a million, thus slightly inferior to that of the base lines. The sides of the triangles being about 30 km can be computed from the base lines and from the angles with an accuracy of about 6 cm, but the accuracy decreases rather rapidly with the distance from the base lines. 2.4. The Computation of Triangulations The purpose of triangulations is to give coordinates of the bench marks in some suitable three-dimensional system. The first step is to compute the differences of coordinates for neighboring stations. The length and the azimuth of each side correspond to these differences in two dimensions. The third dimension-corresponding to the differences in the altitudecould be computed from the vertical angles of the side. The atmospheric disturbances, however, make the observation of the vertical angles ineff ective. Certainly the vertical angles can be used for mapping purposes, but usually they have no scientific value. The uncertainty of the trigonometric altitudes in a first-order triangulation can be several meters. Therefore the triangulation can give precise coordinates only in a two-

9G

R . A. HIRVONEN

TABLE I. Some important ellipsoids Name

Date

a

1lP

Bessel Clarke Clarke Hayford International Krassowski Jeff reys

1841 1866 1880 1909 1924 1940 1948

6377 397 6378 206 6378 249 6378 388 6378 388 6378 245 6378 099

2'39.15 294.98 293.47 297.002 297. 298.3 297.10

dimensional system as along a surface. Which particular surface should be selected is more or less a matter of convention. Of course, it should correspond to the actual surface of the earth-or rather to that of the oceans-as closely as possible, but it must be a regular mathematical surface with simple formulas for the computation. Furthermore, all countries should use the same reference surface. For this purpose the international ellipsoid was agreed upon by the International Union of Geodesy and Geophysics in 1924. However, several countries prefer to continue their triangulations in the old systems or to introduce new systems that are believed to be more correct than the international ellipsoid. Table I shows the most important ellipsoids now in use. I n addition to the selection of the reference surface its orientation must also be established a t the initial point of the triangulation. The coordinates of the first point as well as the azimuth of the first side must be approximately known, because the triangulation gives only the differences of coordinates. We shall return to this question in Section 3. The actual measurements that have been made a t the physical surface of the earth (i.e., in some altitude above the ellipsoid) must be corrected to be valid a t the latter surface. Often these corrections can be neglected, and in fact they cannot be definitely secured before the observations have been dealt with over large areas around the entire network of triangulation. We shall return to this question in the last section. The observed bearing from a point A to point B is supposed to be a normal section, i.e., a curve pertaining to the plane that is perpendicular to the ellipsoid a t A. The normal section from B to A does not coincide with that from A to B. The procedure accepted for the practical computations is to reduce both normal sections to the geodetic line, which is the shortest curve from A to B along the surface. Using the geometry of the geodetic lines at the ellipsoid first, the length of each triangle side is computed and then the ellipsoidal coordinates. It is important to note that these coordinates, the geodetic latitude and longitude, mean the direction of the normal of the reference ellipsoid. Latitude

SIZE AND SHAPE OF THE EARTH

97

is the angle between the normal and the equatorial plane, longitude is the angle between the meridian plane of the station and a fixed meridian plane; e.g., that of Greenwich. As a third quantity, the back-azimuth of each side is obtained. The measured angles give then the azimuths toward the next stations. These routine computations, which offer many interesting but merely technical problems to the geodesists, can be disregarded here. There are, however, a couple of principal questions that require some attention. The observed quantities contain, as mentioned before, usually six reliable digits, but it is advisable to carry out the computations to eight significant digits. The number of observed quantities is always made essentially greater than the number of coordinates to be computed. This causes the appearance of discrepancies within the extra decimal places. Therefore an adjustment is necessary in order to make the observed quantities compatible up to the last decimal place used in the computations. It is believed that the observational errors compensate each other in some degree in the adjustment and that this compensation is more probable if large networks are adjusted in one block. An adjustment by the method of least squares requires the solution of a system of linear equations, the number of which increases with the network. Earlier it had been practically impossible to solve great systems but the modern high-speed electronic computers can deal with thousands of equations. In this way the triangulations of several adjacent countries have, in recent times, been adjusted in one block [4,51. The measured quantities to be adjusted are the angles, the base lines, and the astronomical azimuths at the Laplace stations (or, strictly speaking, differences of azimuths between two Laplace stations). The methods used and the precision obtained in different countries and in different times have been very unequal, and especially the base lines and Laplace stations have caused much confusion in the recent international adjustments. It seems that this particular and essential part in the determination of the earth’s size and shape is still waiting for a satisfactory solution. But even when all connected triangulations of each continent have properly been adjusted the coordinate systems of different continents are still separated. The coordinates within one system have only a relative character: a t the initial point of the network the coordinates are more or less arbitrary. 2.6. The Intercontinental Connections

The connection of the present triangulations to one solid system over all continents will perhaps be realized some time in the distant future. The obstacles retarding it, e.g., the continuation of the European triangulations through Siberia and Alaska are economical and political rather than

98

It. A. HIRVONEN

geographical. But even then a direct connection over the Atlantic Ocean is very desirable. There are some modern modifications of triangulations which could aid in bridging the oceans in the near future. They can be enumerated as (1) flare triangulation, (2) electronic trilateration, and (3) lunar methods. The observation of artificial satellites is a combination of all three of these ideas. The $are triangulation still uses optical observations with theodolites. Instead of steady lamps located on high mountains, flares are dropped from airplanes or shot to very high altitudes. Hence, they are visible to distances of several hundreds of kilometers. The observations must be made simultaneously a t all observing stations; e.g., two on one, two on the other coast of the sea which can thus be about a thousand kilometers broad. Using the existing islands of the northern Atlantic, a chain of triangles can be observed between Great Britain and Greenland [6]. So far only isolated islands have been connected with the continents by the aid of this method. Another modification of this method is to photograph the flares against the background of stars. No experiments have hitherto been made [7]. The electronic trilateration has already been used for the network covering all of Canada [S]. Instead of optical theodolites, electronic range finders like shoran have been used. Using airplanes a t high altitudes as targets the distances to be measured can be the same as in the flare triangulation. Trilateration is the accepted name for triangulation when no angles but all sides of the triangles are measured. The electronic range finders are less accurate than the optical theodolites. In recent years optical range finders-e.g., geodimeter and tellurometer-have been constructed. This might very soon render the optical trilateration competitive with the standard triangulation of first order [9, lo]. The lunar methods use the moon as a triangulation point a t high altitudes. The distance of the moon being about 380,000 km, the standard accuracy of one to a million could give the coordinates with a precision of 380 meters. The direction toward the moon can, however, be observed with a better precision in special circumstances like solar eclipses and star occultations. In the beginning and a t the end of a total or annular eclipse of the sun, photographs can be taken with movie cameras, and the relative position of the moon with respect to the position of the sun can then be measured from the films obtained. Of course, the time of each exposure must be recorded too by the aid of radio time signals. The motions of the moon and of the sun must be. specially computed for this purpose, because the positions given in the astronomical ephemerides do not furnish the required precision. The main source of uncertainty of the results seems to be in the topography of the moon, which is not yet known with sufficient accuracy. Reference [Ill tells about the new mapping of the profile zone of the moon.

SIZE AND SHAPE O F THE EARTH

99

Another modification of the eclipse method is to record the flash spectrum caused by the sickle of the sun during a few seconds before and after the totality. A third modification is to record only the intensity of the sun’s light. It was hoped that an accuracy of about 30 meters could be attained with all these methods. Unfortunately, all attempts to use this method during the eclipses of 1945, 1947, and 1954 that were visible on both sides of the Atlantic failed because of the weather conditions and instrumental difficulties. References [12-141 contain the results obtained. No definite reports on the attempts in the Pacific area have so far been published. An occultation of a star by the moon-at least the disappearance behind the dark limb of the moon-can be registered with a very high precision by the aid of a photocell. The higher precision seems to compensate the drawback that only one observation is obtained, while the eclipse of the sun offers a great number of observations a t different points of the moon’s limb. The effect of the topography of the moon can be diminished by selecting the stations so that the same smooth spot of the moon’s limb passes over the star. The U. S. Army Map Service tested this method in 19491950 for distances from 400 to 1000 km [15]. The comparison with the geodetic distances indicated that the precision of the occultation method is about 11 meters. This remarkable precision can hardly be attained for the transoceanic distances. The establishment of a network between two continents requires many suitable occultations with paths that cross each other at some islands. The laborious preparations can be frustrated by the slightest misfortune. In. 1956 the occultation method was used in order to put the island Palau correctly “on the map” [16]. The most promising variation of the lunar methods is the simple photographing of the moon and background stars with the aid of the Markowitx camera especially designed for the elimination of the relative motion and the excessive brightness of the moon [17]. During the year 1958, twenty stations around the earth photographed the moon whenever it was visible. From the collected material geodesists expected to obtain, in addition to some facts concerning the motion and the shape of the moon itself and the irregularity of the rotation of the earth, the coordinates of the stations in an intercontinental system. Experimental photographs indicate that a precision of about 40 meters is not beyond reach.

3. ASTRONOMICAL COORDINATES When the vertical axis of the theodolite or the horizontal axis of a transit instrument is controlled by the level, the direction of the actual gravity in relation t o the star-universe can be determined by the astronomical fixes. At the same time, the direction of the rotational axis of the earth can be determined from the daily rotation of the sky. In this way the

100

R. A . HIRVONEN

astronomical coordinates of the observation point can be obtained, latitude referring to the angle between the true vertical and the equatorial plane, and longitude referring to the angle between the actual meridian and n fixed meridian; e.g., that of Greenwich. As a third quantity the azimuth or the angle between the normal section to some adjacent terrestrial point and the true meridian is obtained. All these quantities are absolute or independent of any prescribed reference system. The geodetic coordinates that indicate the direction of the normal of the reference ellipsoid do not necessarily coincide with the astronomical coordinates indicating the direction of the true vertical. The difference, called the deflection of the vertical, is defined by two components: the north-south component is denoted by f and the east-west component is denoted by 7. Denoting latitude by cp, longitude by A, and azimuth by a, astronomical coordinates by primes, and geodetic coordinates without the prime, we have

a ! ‘ - a! = 7 tan p (3.3) The elimination of 7 from the last two equations gives the equation of Laplace

(3.4)

a‘

-a

=

(A’ - A)sin Q

that can be used for the controlling of the azimuths in the triangulation. The deflection of the vertical is caused by (1) the mass irregularities of the earth’s crust (e.g., by the mountains near the station), and (2) the dimensions and the orientation of the reference ellipsoid. Usually the orientation and sometimes the dimensions are varied to make the square sum of f and q at all astronomical stations as small as possible. Nevertheless, these so-called astro-geodetic deflections of the vertical of two separated triangulations cannot be compared without the connections described in the previous section. The precision obtainable by the astronomical fixes is of the order one in a million-as in the triangulation. However, as the radius of the earth is about 6400 km, the precision of the astronomical coordinates is about 6 meters, thus essentially lower than the precision of the differences of the geodetic coordinates between adjacent points. Nevertheless, when the geodetic network covers distances that are many hundreds of kilometers long, the astronomical fixes could serve as valuable “super-control-points,” if the deflection of the vertical could be eliminated [HI. The deflection of the vertical is usually only a few seconds of arc-

SIZE AND SHAPE OF THE EARTH

101

corresponding along the earth’s surface to distances that are of the order of 100 meters. The largest values observed near great mountains amount to one minute or a couple of kilometers. These differences are easily recognizable in the maps of the border regions between two countries. No consistent maps can be based on an astronomical network only, but a triangulation is inevitable. If a dense and extensive network of astro-geodetic deflections of the vertical is available, a surface can be constructed so that it is everywhere at right angles to the actual plumb line. Such a surface is called level surface, nowadays also geop. The level surface that coincides with the mean level of the oceans is called geoid (see Fig. 1). The geoid comes rather close to the reference ellipsoid, the difference being probably always less than a hundred meters [19]. The base lines that have been measured along the physical surface of the earth must always be corrected so as to represent an arc along the reference ellipsoid. Because the altitude of the base line above the sea level is known, it is easy to perform this reduction provisionally, but the reduction of the base line from the geoid to the ellipsoid can be completed only when the undulations of the former have been determined. In the same way, the theodolite bearings that have been aimed a t the hilltops must be reduced in order to correspond to the points of the reference ellipsoid. This cannot be done before the deflection of the vertical has been determined a t each station. For this purpose the entire triangulation should already be provisionally but carefully computed. A second computation of the triangulation can begin after the definitive reduction of the base lines and theodolite bearings. At this time international geodesy has progressed so far that this second computation has become an actuality. At the same time the adjustment will be revised and expanded in continental proportions. As stated above, the principal cause of the deflection of the vertical is the attraction of the mass irregularities of the earth’s crust. A great number of attempts have been made to compute the attraction of great mountains on the basis of their topography and assumed densities. Usually the results computed are much larger than the deflections observed, as if there were “caves inside the mountains.” Nowadays, as all geophysicists know, the suggestion of these caves has been replaced by various forms of the isostatic compensation. 4. SPIRITLEVELING

As previously mentioned, the atmospheric disturbances prevent optical methods over long distances (just as the measurement of vertical angles) from giving the precise altitudes of terrain stations. The triangulated

102

R. A. HIRVONEN

heights certainly have a great practical value for mapping, but for scientific purposes only the results of spirit leveling can be used. I n this method the sightings are strictly horizontal and short, preferably not longer than 50 meters. It is essential that the sightings backward and forward be equally long. Then the curvature of the earth and most instrumental errors will be eliminated. The effect of the refraction that still remains above a sloping terrain can be allowed for by reading the temperature at different altitudes [20]. The quantity which is usually wanted in the height measurements is the orthometric height or the metric height above the sea level (geoid) (see Fig. 2 ) . The difference of the two readings of the rod is certainly the orthometric height difference of the two sightings, but the sighting lines themselves, although horizontal, are not parallel with the sea level. Hence the orthometric height of the sighting line a t the next rod forward is not necessarily the same as a t the rod behind. It is impossible to compute the precise correction of the sighting line, because the direction of the geoid depends on the local densities within the earth’s crust and these densities remain always unknown [21]. The only exact result that can be obtained from the spirit leveling is the difference of geopotentials. Geopotential is the mechanical work required for carrying the unit mass from the sea level to the elevated point. This quantity is independent of the path used for the spirit leveling, and it can be computed only if the gravity has been measured along the path too. Denoting geopotential by W , gravity by 9, and the orthometric height by h, we have dW = -g d h ; this expression must be integrated along the path: h

(4.1)

AJV

=

gdh

The geopotentials were introduced into scientific geodesy in 1954 by the general assembly of the International Association of Geodesy. Earlier the dgnamic heights were used for the same purpose. Dynamic heights indicate the altitudes of level surfaces at a fixed central point. The networks of spirit leveling that have been finished in neighboring countries are usually connected, and an international adjustment of the combined nets has been initiated in Europe. In this way, the normal zero points or the mean sea levels of different countries can be brought to the same system. The tides and the secular or seasonal variations of the sea level are recorded by automatic mareographs that must be connected to the bench marks of leveling. The exact rate of the land rise, which is conspicuous in the regions of post-glacial rises, can be determined inland only from repeated levelings. The first results of this method have been obtained in Finland a few years ago [22].

SIZE AND SHAPE OF THE EARTH

103

Spirit leveling is defined as a leveling of high precision, if the random error does not exceed 1 mm/km and the systematic error 0.2 mm/km. While crossing broad rivers and narrow sea belts it is difficult to attain this precision; several hundreds of readings over Bundefjord which is 5 km broad gave an accuracy of 5.7 mm [23]. Therefore the hydrostatic leveling is a necessary completion of the spirit leveling. A tube must be filled with water and sunk to the bottom so that both ends stand vertically up for the observation of the free surfaces of the water. The first experiments in Denmark gave an astonishing accuracy of 0.09 mm per 22 km [24]; the unknown systematic errors, however, might be considerable. 5. GRAVITYMEASUREMENTS

Referring to Woollard’s article, “The Earth’s Gravitational Field and Its Exploitation” in Volume 1 of Advances in Geophysics, I shall here only summarize the mathematical treatment of gravity observations and its applications to the determination of the earth’s shape. 5.1. Normal Gravity

For the gravimetric determination of the geoid a reference surface is also necessary. It is advisable to use the same surface as for the astrogeodetic determination; e.g., the international ellipsoid. In this case, however, we have to imagine that the ellipsoid is filled with matter which is so distributed that the combined potential of the attraction of the matter aiid of the centrifugal force caused by the rotation of the body is constant along the surface. A first question arises: Is such a distribution physically possible and in accordance with the observed facts? The attraction of a homogeneous flattened ellipsoid has been one of the favorite problems of mathematicians, and the solution has, a t last, been put in a closed formula by several authors; see, for example, reference [25]:

+

3.fM ( 6 cot6(l - 6 cot 6 ) f sin”[S - 3 cot 6(1 - 6 cot S)]) 4c where f = the constant of attraction = 6.67 X lo-* gm-’ cm3 sec-2, M = the mass of the ellipsoid (for the earth 5.977 X loz7gm), c = the distance of the focal points of meridian ellipses from the center (for the international ellipsoid c = 52,297,608.714 em). The potential is valid outside the ellipsoid-and also a t the surface itself-for any point with the coordinates (5.1)

V

=

c

_

x

=

c cosec 6 cos p cos X

y = c cosec 6 cos

(5.2)

p sin X

I

[ x = c cot 6 sin

0

Here X is the customary longitude, p is the so-called reduced latitude, aiid

104

R. A. HIRVONEN

sin 6 means the eccentricity of the ellipsoid that goes through x, y, z and which has the same value of c as the attracting ellipsoid. As the expression of V does not depend on the dimensions of the attracting ellipsoid save the factors M and c, the same potential will be obtained for all “confocal” ellipsoids with the same values of M and c, without regard to the actual size and shape of the ellipsoid. This offers a possibility for replacing the homogeneous ellipsoid by any ellipsoid with a variable density, if only the surfaces with a constant density are confocal ellipsoids. Certainly the eccentricity of these confocal ellipsoids increases toward the central disk which is bordered by the focal points, while in the real earth the eccentricity must decrease toward the central point. Wolf has shown that it is possible, a t least approximately, to construct the ideal earth using the densities that have been derived from seismological ohservations and letting the eccentricity decrease downwards [26]. For the time being, we can also accept the international ellipsoid as mathematical fiction without any real physical significance. The mathematical treatment shows that the total potential of the attraction of the ellipsoid, without regard to the mass distribution, and of the centrifugal force can be expressed in a closed formula [27]: (5.3)

fM6 w2c2 [ u=--.+cos c 2 sin2 6

2

p

61 + (sin p - -31) qoqsin2 sin2 60 2

~

where w = 0.72921 15147 X lo-* sec-’ is the angular velocity of the earth and q is an auxiliary function that is defined by 1 2

g = - [6

(5.4)

- 3 cot 6(1

- 6 cot a)]

but in practice it is better to compute it by the series: (5.5)

1 2

-q =

1 tan3 6 3.a

; 7

2 tans 6 + -tan 3 - 5.7 7.9

7

6-

...

At the surface of the international ellipsoid, we have cos 60 = 1 - p

=

296/297

60 = 0.082084 03642

tan 6o =

e =

0.082268 88961

q o = 0.0000 73813 03293

From the formula (5.3), the value of the normal gravitg y at any point outside the ellipsoid can be derived. The following closed formula gives

105

SIZE AND SHAPE OF THE .EARTH

the gravity at the surface of the ellipsoid : (5.6)

yo =

a2wo

(3 sin2 p

wo

€4; - 1) - cos2p)

6qo

1

where

- sin2 60 cos2p

(5.7)

w,”= I

(5.8)

qo’ = 3(1

(5.9)

401 = 6

- 60 cot 6,) 60

cosec260 - 1

1 tan460 + 1 -tan6 6n - . .

5.7

7.9

.)

For the international ellipsoid, we obtain I

(5.10) At the equator /3

“O

- 1.002895 675

3qo=

0 the normal gravity yo becomes

(5.11)

As numerical value of this constant, the International Association of Geodesy accepted in 1930 (5.12)

ya =

978.049 cm sec-2 (the unit is also called “gal”)

Nowadays this value, according to the latest determinations of the absolute gravity, is believed to be 10 to 15 mgal too great. The normal gravity a t any latitude of the international ellipsoid or above it can be computed either by the aid of the closed formula or, preferably, by the series : y = 978.049(1

+ 0.0052 88376 sin’q - 0.0000 05885 sin2 29

(5.13)

+ O.OOO0 00008 sin23q) -0.30855 h -0.00022 h cos 2q

+0.000072 h2 where h stands for the altitude in kilometers. The three first terms with only seven decimal places for the coefficients have been accepted as the

106

R. A. HIRVONEN

official International Gravity Formula. The terms with h signify the normal free-air reduction. 6.6. Determination of the Geoid

The differences Ag = g - y of the observed gravity and the normal gravity are called gravity anomalies or, to be exact, free-air anomalies, if the above formula for the normal gravity has been used. There has, however, been some ambiguity as to the application of the free-air reduction. Usually the reduction is added to the observed gravity, instead of subtracting it from y o . The anomaly becomes certainly the same, but it has been assumed that it is valid a t the “sea level.” In order to reduce the observed gravity down to the geoid the radius of curvature of the geoid should be known, and not only that of the geoid itself but of each equipotential surface between the geoid and the observed station. Because the densities of the earth’s crust below the station are essentially unknown, the reduction down to the geoid cannot be precisely performed. On the other hand, the altitude h in the free-air reduction of the normal gravity has two defects. First, as mentioned in Section 4, the orthometric altitudes cannot be observed or computed with full precision, but the original results of the precision leveling should be expressed as geopotentials. Second, the orthometric altitudes have been reckoned from the “sea level” or from the geoid, but the reduction of the normal gravity implies altitudes from the ellipsoid. The elevation of the geoid above the ellipsoid, denoted by N in the following discussion, is exactly the quantity to be determined in this problem and it is not available a t this stage of the procedure. Therefore, either the geopotentials AW or the potential altitudes h’ should be used :

hI

(5.14)

= -AW -_ 746

where the constant 7 4 5 = 980.629394 represents the normal gravity a t the latitude 45” of the international ellipsoid. Using the potential altitude the normal gravity can be accurately reduced by the formula y = 70 -

(5.15)

0.308555 h’

- 0.001041 h’ cos 2p

+ 0.000004 h’ + 0.000024 h”

COS‘

2p

This value, however, is not valid at the observed station but at an equipo-

107

SIZE AND SHAPE O F THE EARTH

I c

Ellipsoid

FIG.1. The level surfaces of the earth. At the actual level surfaces (geoid arid geops GI, Gz)the observed geopotential W has the same constant value as the potential U of the ellipsoid at the refererice surfaces (ellipsoid and spherops SI , Sz respectively).

tential surface called spherop. At the spherop the potential U of the normal gravity has the same constant value as the potential W of the actual gravity a t the observed station (see Fig. 1). The equipotential surface of the actual gravity is called geop. The altitude of the geop above the spherop will be denoted by N' (see Fig. 2). The gravimetric determination of the shape of the earth is based on a formula which was published by Stokes in the year 1849 [as].I n this formula

I R

(5.16)

S(*) stands for Stokes' function S(*)

(5.17)

=

*+

cosec 2

*

1 - 5 cos 9 - 6 sin 2

- 3 cos

* ( ;+ In sin -

.">

sin 2

The integration must be carried out over a sphere u with radius 1 on which the values of N and the anomalies Ag are situated according to the geographic coordinates of the points where N will be computed or where Ag is observed; J. stands for the arc between the points of N and Ag. In other words, in order to compute N a t one point, the anomalies must be known everywhere over the earth's surface. The function S(+), however,

108

It. A . HIRVONEN

Y,

90

FIG.2. The relations between the level surfaces. The classical theory renders N = elevation of geoid G above ellipsoid E , and the deflection of the vertical a t G . The new theory renders N' = the elevation of geop L above spherop S and the deflection of the vertical a t the physical surface P . The orthometric height h is the elevation of P above G along the curved vertical line.

has such a form that the anomalies of the remote regions have a minor and smoothly undulating effect on N . The practical application of Stokes' formula was, of course, not possible before there was a passable number of observations around the earth, especially on the seas. The first attempts were initiated by Heiskanen and performed by Hirvonen in 1934 [29] and Tanni in 1948 [30]. At the present time the immense number of the gravimetric observations necessitates continuous work for the compilation and for the processing of the material'. The first results were published by Heiskanen in 1957 [31]. These are s h o w in Figs. 3 and 4. According to them, the geoid rises above the internatioi1:il ellipsoid 40 meters in Spain, 15 meters in the East Indian Archipelago, 20 meters in Wyoming, and sinks below it 40 meters around Puerto Itico, 25 meters in India, and 5 meters in the Eastern Pacific. The line of zero elevation runs from Newfoundland to Southern Africa, with a turn over the Caspian Sea to the north-east. Another zero line encircles the western states of the United States following the Pacific coastline, turning from Mexico to Lake Superior and returning to Vancouver. The third zero line crosses Sumatra toward the north-east, encircles Japan, and turning south-east 1 This work is going on in the Mapping and Charting Research Laboratory of The Ohio State University.

SIZE AND SHAPE OF THE EARTH

FIG.3. The Columbus Geoid-after Trans. of the Am. Geophys. Union.

109

Heiskanen. Courtesy W. A. Heiskanen and

diverges to form a smooth normal region all over the Pacific Ocean. These general features can be locally disturbed by irregularities that at some mountain point can reach 20 meters. The precision of the N values obtained depends on the density of the observed stations. In the most favorable areas, as in Europe and in North America, the uncertainty caused by the gaps of the gravity net, can be estimated to be about 10 meters [32]. In the southern hemisphere the observations are still too scarce to justify any attempts to determine the undulations of the geoid. The undulations of the geoid on the northern hemisphere are of a regional character and do not reveal any distinct systematical features that could be explained by a spheroidicaldeviation from the ellipsoid or by a triaxiality of the latter. The anomalies compiled so far do not give any reliable cor-

110

R . A. HIRVONEN

FIG. 4. The Columbus Geoid-after Trans. of the Am. Geophys. Union.

Heiskanen. Courtesy W. A. Heiskanen and

rection to the flattening of the international ellipsoid. Heiskanen obtained 297.4; the uncertainty of this value seems to be about f0.5. In the year 1928, Vening Meinesz published a derivation of Stokes' formula which gives the deflection of the vertical from the gravity anomalies [33]. This formula:

depends even less on the anomalies of the remote regions of the earth than the formula of N , and the results are much more of a local character. Several applications have been made in different countries, (see, for example, [34]). The precision obtainable, is f 0 . 8 5 seconds of arc in Europe and North America [32]. Usually the results are compared with the astro-geodetic

SIZE AND SHAPE O F THE EARTH

111

deflections of the vertical in order to determine the systematic part of the latter. I n this way any isolated triangulation can be computed in an absolute world system of coordinates [18]. The direct transoceanic connection of the triangulations of different continents, however, cannot be made superfluous by the gravimetric determination of the geoid, for the latter gives only the shape and not the size of the earth. The distance of two continents remains always on a hypothetical basis without direct measurements. 5.3. New Ideas

As the application of Stokes’ formula during the last decades has become an actuality, there has been an increased interest in its theoretical revision and improvement. Several authors have derived the formula again using different methods, especially avoiding the spherical harmonics, and they have succeeded in showing that the formula is more accurate than Stokes himself stated. Using Lame’s functions, Zagrebin even corrected Stokes’ formula to an accuracy that exceeds the present practical demands [35]. But one drawback remained. Stokes’ formula presumes that the observed anomalies are valid a t the geoid and that there are no attracting masses outside the geoid. This presumption is, of course, not met with in practice, as the continents protrude above the sea level and the observations are made a t the physical surface of the earth. Therefore, a very complicated reduction of the observed values of the gravity is necessary. First, the protruding masses must-in imagination-be removed or transferred below the sea level, and then the observed station must be transferred into the geoid. The first procedure changes the potential, and the original constant value of the potential is now to be found a t another surface of equilibrium, called cogeoid. The indirect effect of this displacement of the geoid to the gravity must be allowed for. The second procedure is identical with the free-air reduction of the observed gravity and, as we have seen above, it cannot be carried out with full precision. The reduction of the observations has one more practical purpose. After thc reduction, the anomalies must run as smoothly as possible in order to make an interpolation between the observed stations reliable. If the protruding masses are transferred just below the sea level, the anomalies show a positive correlation with the heights of the stations. If the masses are transferred very deep or entirely removed, as in the Bouguer reduction, the correlation becomes negative. The isostatic reductions have the advnntage that the correlation vanishes aiid the anomalies run smoothly. But these reductions are very laborious aiid cause great indirect effects. Jeffreys pointed out that these serious drawbacks of Stokes’ theory could be avoided, if the integral formula were derived not for the geoid or for

112

R. A . HIRVONEN

the cogeoid but for the physical surface of the earth [36]. The idea was fully developed by Molodenskij, and the new method has since been esclusively used by the Russian geodesists [37]. In the last years different applications of the idea have been suggested by de Graaff-Hunter [l], Arnold [38], and Ledersteger [39]. According to Levallois [40],the formula of Stokes which gives N in an explicit form must, in this new theory, be replaced by an integral equation (5.19)

where D is the distance from the point of N’ to the point of Ag. In order to compute N’ in one point we should know N‘ everywhere on the earth. Nevertheless, using Stokes’ formula (5.16) as the first approximation of N’, the equation (5.19) can be solved numerically. The advantages of the new method are obvious. There are no reductions with orthometric altitudes, with uncertain radii of curvature, and with indirect effects. Only the normal gravity must be reduced upwards which can be done with mathematical precision by the aid of the geopotentials. The terrain correction of the old method which implies subterraneaii densities can be replaced by a purely geometrical slope correction. The results, distances N‘ between the real physical surface of the earth and the mathematical surface spherop, define the earth’s shape better than the undulations of the hypothetical geoid. The gravimetric deflections of the vertical will be obtained for the physical surface of the earth where they can be immediately compared with the astro-geodetic ones, without any reduction from the geoid (see Fig. 2). LIST OF SYMBOLS a, b C

d

f 9 h , h‘

P Pt

P’

2, Y, W

D G M N N‘ R

equatorial and polar radius of ellipsoid distance of focal points from center infinitesimal part or differential gravitational constant observed gravity orthometric and potential height, flattening of ellipsoid auxiliary functions rectangular coordinates auxiliary function metrical distance mean gravity mass of earth elevation of geoid above ellipyoid elevation of geop above spherop mean radius of earth

SIZE AND SHAPE OF THE EARTH

s 17

V W a,a’

A, X’ PI rp’

B Y

6 0

5, 9 U

$ W

A

113

Stokes’ function total potential of ellipsoid attraction potential total potential of earth geodetic and astronomical azimuth geodetic and astronomical longitude geodetic and astronomical latitude reduced latitude normal gravity parameter of confocal ellipsoids second eccentricity of international ellipsoid components of deflection of vertical spherical surface with radius 1 angular distance angular velocity small but not infinitesimal difference REFERENCES

1. de Graaff-Hunter, J. (1957). Legitimate deductions from geodetic observations. “Festschrift C.F. Baeschlin,” pp. 57-63. Orell Fussli Verlag, Zurich. 2. Honkasalo, T. (1950). Measuring of the 864 m-long Nummela standard base line with the Vaisala light interference comparator and some investigations into invar wires. Veroffentl. Finn. Geod. Znst. 37, 88 pp. 3. Kukkamaki, T. J., and Honkasalo, T. (1955). Base de comparaci6n Buenos Aires internacional standard para medidas lineales. Inst. geograf. militar. Buenos Aires Publ. tecn. 26, 65 pp. Reprinted (1957) : Die Eichstrecke in Buenos Aires. Deut. Geod. Forsch. Inst. tfbersetz. 16. 4. Tardi, P., Lambert, W. D., Hough, F. W., Whitten, C. A., Bonsdorff, I., Heiskanen, W., and Rune, G . A. (1947). Compte rendu des S6ances. Rbunion de la commission d’6tude de la compensation du r6seau g6od6sique europ6en. Bull. gBod. 7 , 91 PP. 5. Olander, V. R., (1949). Adjustment of the Baltic Ring. Baltic Geod. Comm. Spec, Pubt. 10, 68 pp. 6. Berroth, A. (1950). Die Bedeutung der geodatischen Astronomie fur die Uberbruckung der Ozeane. GeoJis. pura e appl. 18, 10 pp. 7. Vaisala, Y. (1946). An astronomical method of triangulation. Silt ber. Finn. Akad. Wiss. pp. 99-107 8 . Department of Mines and Technical Survey (1950). Geodetic applications of Shoran. Survey of Canada 78, 189 pp. 9. Bergstrand, E. (1950). Determination of the velocityof light. Arkiv Fysik 2, 119. 10. Wadley, T, L. (1957). The tellurometer system of distance measurement. Em.pire Survey Rev. XIV, 100-111, 146-160, 227-230. 11. Watts, C. B. A. (1950). Sky and Telescope IX, 6. 12. Kalaja, P., Pesonen, U., Hirvonen, R. A., and Kukkamaki, T. J. (1955). The solar eclipse in 1945. Veroffentl. Finn. Geod. Inst. 46, 175-214. 13. Kukkamaki, T. J., and Hirvonen, R. A. (1954). The Finnish solar eclipse expeditions to the Gold Coast and Brazil 1947. Veroffentl. Finn. Geod. Inst. 44, 71 pp. 14. Brein, R., Jelstrup, H. S., Nottarp, K., Sandig, H.-U., and Sigl, R. (1957). Beobachtung zur Sonnenfinsternis 1954 in Sudnorwegen. Mitt. Znst. angew. Geod. 16, 52 PP-

114

R. A. HIRVONEN

15. O’Keefe, John A., and J. Pamelia Anderson (1952). The Earth’s equatorial radius and the distance to the Moon. Astr. J . 67, 219-247. Reprinted (1953): Bull. gbod. 29. 16. Henriksen, S. W., Genatt, S. H., Marchant, M. Q., and Batchelor, C. I). (1057). Surveying by occultations. Trans. Am. Geophys. U n . 38, 651-656. 17. Markowitz, W. (1954). Photographic determination of the moon’s position, and applications to the measure of time, rotation of the earth, and geodesy. Astr. J . 69, 69-73. 18. Heiskanen, W. A. (1951). On the world geodetic system. VerojJentl. Finn. Geod. Inst. 39, 25 pp. Reprinted (1951) : Publ. Inst. Geod. Photogram. Cartogr., Ohio State Univ. 1. 19. Bornford, G. (1954). Liste des stations de deviation de la verticale rattachkes au rkseau europ6en. Presented at the 10th General Assembly, Assoc. Geod. IUGG, Rome. Unpublished. (see also Wolf, H . (1957). Astronomisch-geodatische Lotabweichungen im mittleren Europa. Deut. Geod. Komm. B39, 15 pp. 20. Kukkamliki, T. J. (1939). Formeln und Tabellen zur Berechnung der nivellitischen Refraktion. Veroffmtl. Finn. Geod. Inst. 27, 18 pp. 21. Jung, F. Rudolf (1957). Potentialdifferenzen und orthometrische Hohen. “Festschrift C.F. Baeschlin,” pp. 105-126. Orell Fussli Verlag, Zurich. 22. Kiiiiriainen, Erkki (1953). On the recent uplift of the earth’s crust in Finland. Veroffentl. Finn. Geod. Inst. 42, 106 pp. 23. Jelstrup, G. (1955). Crossing of fjords with precise levelling. Bull. gbod. 38, 55-63. 24. Norlund, N. E. (1945). Hydrostatisk Nivellement over Store Baelt. M b m Inst. Giod. Danemark VI (3), 122 pp. 25. Eichhorn, H. (1957). Bemerkung zum Potential eines homogenen Rotationsellipsoids. Ast. Nachr. 283, 249-250. 26. Wolf, H. (1953). Zur Bestimmung von Abplattungswerten im Erdinnern aus einer vorgegebenen Dichte-Verteilung. 2. Vermess. 78, 386-389. 27. Pizzetti, Paolo (1894). Sulla espressione della gravita alla superficie del geoide, supposto ellissoidico. Atti accad. naz. Lincei 3, 166. 28. Stokes, George G. (1849). On the variation of gravity a t the surface of the earth. Trans. Cambridge Phil. SOC.Math. Phys. 2, 131-171. 29. Hirvonen, R. A. (1934). The continental undulations of the geoid. Veroffentl. Finn. Geod. Inst. 19, 89 pp. 30. Tanni, L. (1948). On the continental undulations of the geoid as determined from the present gravity material. Ann. Acad. Sci. Fennicae AIII, N o 16. Reprinted (1948): Publ. Isostatic Intern. Assoc. geod., 18, 78 pp. 31. Heiskanen, W. A. (1957). The Columbus Geoid. Trans. Am. Geophys. TJn. 38, 841-848. 32. Hirvonen, R . A. (1956). On the precision of the gravimetric determination of the geoid. Trans. Am. Geophys. U n . 37, 1-8. 33. Vening Meinesz, F. A. (1928). A formula expressing the deflection of the p l u m b line in the gravity anomalies and some formulae for the gravity field and the gravi t y potential outside the geoid. Verhandl. koninkl. Nederl. Akad. Wetenschap 31, 315-331. 34. Rice, Donald A. (1952). Deflections of the vertical from gravity anomalies. Bull. giod. 26, 285-312. 35. Sagrebin, D. W. (1952). Die Theorie des regularisierten Geoids (in Russian). Inst. Theor. Astr. USSR. German translation (1956): Geod. Inst. Potsdam 9, 129 pp. 36. Jeffreys, H. (1931). An application of the free-air reduction of gravity. Gerlands Beitr. Geophys. 31, 378-386.

SIZE AND SHAPE O F THE EARTH

115

37. Molodenskij, M. S. (1945). Basic problems of geodetic gravimetry (in Itussian). Central. Res. Inst. Geod. Air Survey and Carlogr. 43, 107 pp. 38. Arnold, K. (1957). Die Co-Geoide der Freiluftreduktion. Gerlands Beitr. Geophys. 66, 181-198. 39. Ledersteger, K. (1957). Eine Modifikation der Freiluftreduktion. “Festschrift C.F. Baeschlin,” pp. 155-164. Orell Fussli Verlag, Zurich. 40. Levallois, J. J. (1957). Sur la formule de Stokes et celles qui en d6rivent. “Festschrift C.F. Baeschlin,” pp. 165-183. Orell Fussli Verlag, Zurich.

This Page Intentionally Left Blank

OCEANIC TIDES

. .

A T Doodson Liverpool Observatory and Tidal Institute. Liverpool. England

Page 1. Introduction and General Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 1.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 1.2. General Deductions, True and False . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 1.3. Effects of Natural or Free Periods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 1.4. Forces Due t o the Earth’s Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 1.5. Amphidromic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 2 . Tidal Charts Obtained by Empirical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 2.1. Charts by Whewell and Airy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 2.2. Charts by Harris . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 2.3. Charts by Prufer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 2.4. Charts by Sterneck . . . . . . . . . . . . ................................... 125 2.5. Charts by Dietrich and Villain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 2.6. Illustrations for the Pacific Ocean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 2.7. Discussion of charts by Dietrich and Villain for the Pacific Ocean. . . . . 127 2.8. Comments on Illustrations of General Principles ....................... 128 3 . Mathematical Investigations for Oceans Encircling the Earth . . . . . . . . . . . . . . 129 129 3.1. The Equations of Motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Laplace’s Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 3.3. Hough’s Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..................... 132 3.4. Goldsbrough’s Solutions for Polar and Zonal Oceans . . . . . . . . . . . . . . . . . . . 133 4 . Mathematical Investigations for Oceans Bounded by Meridians . . . . . . . . . . . . . 133 133 4.1. Oceans on a Nonrotating Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 4.2. Goldsbrough’s Solutions for a Narrow Ocean ........................... 4.3. Ocean Bounded by a Complete Meridian: Proudman and Doodson . 4.4. Doodson’s Solutions for Narrow Oceans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 140 5 . Mathematical Methods Applied t o Actual Oceans . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Limitations of Methods . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . 148 6.4. 6.5. 6.6. 6.7.

The Effect.s of Internal Tides . . ............ The Effects of Mutual Gravitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Effects of Earth Tides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . General Considerations ................................................ 117

148 148 148 149

118

A. T. DOODSON

List of Symbols.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iteferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

149

150

1, INTRODUCTION AND GENERALREMA~ZXS 1 .l. Introduction

I n many geophysical subjects difficulty arises in formulating theories which will explain, even qualitatively, the observations of natural phenomena, and the theories may require much justification before they can be accepted. This was the state of the subject of oceanic tides prior to the time of Newton, but since his work on the tides it can be said that, there has been no mystery concerning the forces which give rise to tidal motion. Since the forces a t any point of the earth’s surface must be represented vectorialIy it is simpler to give the data for the forces in expansions of the potential from which the components of force in specified directions may be obtained by differentiation. Such expansions have been given, in incrensing detail and accuracy, by Kelvin, Ferrel, Darwin, and Doodson [1].The last one has been adopted internationally. It is an expansion which expresses the potential in far greater detail than is usually required, but it exists to cover any unusual phenomena of tidal origin that may be found from observation, for periodic oscillations in oceans are known to exist from causes other than tidal forces, and it is desirable that the possibilities of tides as causes should be investigated. Normally, the subject of oceanic tides is not dependent upon minor variations in the forces, for the difficulties of investigation of the most important harmonic terms are so great that for the main oceans it can be truly said that we have no exact knowledge of the distribution of the tides over any one of them. It is impracticable to consider the tide as a whole in an ocean, for the harmonic terms vary in amplitude ratios with the corresponding terms of the forces, and so also the phases are not simply related with the phases of the terms in the forces. It is usual to consider some of the principal harmonic terms defined as follows: M z the principal lunar semidiurnal constituent with angles increasing by 28.9841 degrees per mean solar hour Sz the principal solar semidiurnal constituent, with angles increasing by 30.0000 degrees per mean solar hour Kz the luni-solar declinational semidiurnal constituent, with angles increasing by 30.0821 degrees per mean solar hour K 1 the luni-solar declinational diurnal constituent, with angles increasing by 15.0411 degrees per mean solar hour O1 the lunar declinational diurnal constituent, with angles increasing by 13.9431 degrees per mean solar hour.

OCEANIC TIDES

119

The constituent denoted by Mz is usually the most important, and tidal charts purporting to represent the tides of a sea or ocean, as deduced from ohscrvations, relate to this constituent :LS a rule, but theoretical investigtttioris find it more vonvenient to work with the constituent Kz as its rate of increase of angle is simply related to the rate of rotation of the earth, and this permits a simplification of the equations of motion. For similar reasons the diurnal tides under theoretical investigation will be most easily represented by the constituent K1. The results of investigations for these constituents give general guidance for the other constituents. These remarks relate principally to the oceans rather than to small seas for it is possible to find charts of tides in small seas which are given for all principal tidal constituents. 1.2. General Deductions, True and False

The subject of the tides has suffered from general or empirical reasoning, partly because of the difficulties of determining observations in mid-ocean and partly because the treatment of the subject by mathematical or other scientific methods is very difficult. Inasmuch as some erroneous theories are frequently quoted even to this day it is well to comment upon them in an essay such as this. It was a direct consequence of Newton’s investigation of the tide-generating forces to consider the tides as responding instantaneously to the forces, or, more accurately, the forces were considered on water at rest on an earth wholly covered with water so that the shape of the fluid surface was that of an ellipsoid with its longer axis pointing to the moon, if the sun were ignored, or as a combination of two such ellipsoids if both lunar and solar forces were considered. The elevation of the surface over the undisturbed or mean surface under these conditions is proportional to the potential, and the variations of the surface under the changing positions of sun and moon were immediately deducible from the potential. Such a tidal system was called the equilibrium tide. It could be considered also to be the tide on a rotating earth wholly covered with water of great depth if the water retained its gravitational properties but had no inertia. Such a tide would appear to traverse the earth from east to west as a progressive wave, and it would have all the characteristics of tides as shown in springs and neaps, diurnal tides, and long period tides. Newton had no illusions as to the effects of land barriers and he rightly recognized that the potential was of value principally in showing the general characteristics of the tides and something too of the laws of variations. One deduction that was made by later theorists was that where the tide existed in a belt of water encircling the globe as is the case in the Antarctic regions, then it would be well able to maintain itself fully without losses due to land barriers, and even to feed the Atlantic and other oceans run-

120

A. T. DOODSON

ning northward from this belt. Even the great Sir George Darwin, as h t e as 1911, regarded such a theory as valid. It is a very remarkable fact that early observations :dong the west cwahts of Africa arid Europe showed a northerly progression of the tide in :ipp:ircwt accordance with the wave theory. Another fact which seems to support the theory is that the constituent Sz has a phase relationship with M Swhich increases by one degree per mean solar hour and consequently if the two constituents had the same phase in the Southern Ocean then they would conspire a t increasingly later times after the times of new and full moon as the waves traveled northward. In Europe this time interval is about 36 hr and it was thought that an explanation of the so-called age of the tide was thus furnished by this theory. The equilibrium and wave theories still feature largely among the nonscientific attempts to explain the tides even though the validity of these theories has been disproved for a century or more. As this essay has in view a number of scientific workers who may not have made a study of the tides i t is considered to be necessary to give them warning as to the inexactness of these theories and to consider some of the reasons for this. 1.3. E$ects of Natural or Free Periods

I n the year 1738, Bernoulli formulated an equation which is extensively used in hydraulics. It is strictly applicable to steady motions and it is essentially an extension of the law of conservation of kinetic and potential energy. A tidal wave is so long as compared with the depth of the water that the velocity of the water may be taken as the same from top to bottom apart from the effects of friction which may be ignored in deep water. If g is the gravitational constant and { the elevation of the wave above the mean surface, then g t is the potential due to gravity. We may write P for the potential of the external tide-producing force, and u for the mean velocity in a vertical line, taken as relative to the earth. Then, if we tr:ivel with the wave a t a velocity c, the apparent velocity of the water is (16 - c). Bernoulli’s equation then gives for the surface (1.1)

+ + P = constant

>S(u - c ) ~ g t

From the rate of transport of volume across a section of a canal of unit width, we obtain (1.2)

(u - c) (h

+ t)

=

constant = ch

where h is the depth, and the average value of the product on the left is ch. If we take t / h to be small, we obtain (1.3)

(u

- c)

= -c(l

- {/h)

OCEANIC TIDES

121

and the substitution of this in (1.1) gives, with c constant (1.4)

Inasmuch as f and P are harmonic terms then the constant is zero. Also, as the equilibrium tide occurs when c is very small, the final value is (1.5)

elevation r = the equilibrium (1 - c2/gh)

This formula is frequently encountered in tidal investigations, and can be obtained by more general methods. The reason for dealing with it here is to introduce the concept of the part played by the depth. For a very deep channel the equilibrium tide is a close approximation to the actual tide but if the depth is critical, so that gh = c2

(1.6)

there is resonance with the free waves. In actuality, infinite values of elevation would not be obtained because frictional and other forces would come into play, but relatively large values of tidal elevations would occur. One result of the above simple investigation, therefore, is that even general considerations of tidal motion must have regard to the depth of water, and another result is that for a canal on or near the equator the value of c2is greater than the value of gh for depths smaller than about 13 mi, since c is the rate of travel, at the equator, of about 25,000 miles per day. This shows that the tide in a re-entrant canal would be “inverted”; that is, low water, instead of high water as is assumed by exponents of the equilibrium theory, would occur “under the moon.” Thus, in waters of less depth than the critical value indicated, the equilibrium theory, on this account alone, is misleading. It is instructive to return to equation (1.4) which can be written as (1.7)

g{

- cf/h

=

-P

The first term is the variable part of the potential energy due to gravity, while the variable part of the kinetic energy is given by the second term. For the critical case there is a balance between the two variable parts so that no external energy is needed to keep the wave in being; that is, we then have a free or natural wave. When we consider a system of such canals parallel to the equator there would be one such canal a t a certain latitude which might give resonance, so that direct tides would appear in an adjacent canal and inverted tides in a canal on the opposite side of the one in which resonance occurs. If the walls of the canals were destroyed then there would be a rush of water

122

A. T. DOODSON

from one to the other of the canals and resonance would not actually occur because it depends upon a fine balance between the potential and kinetic energies, and this balance would be destroyed when motion took place across the broken-down walls. The transition would then occur a t zero range, corresponding to a nodal line. Such a set of canals, though it might be considered as giving an approach to tidal conditions for an ocean covering the whole earth could not give trustworthy results, not only because of the reasons given above but also because the tide-generating forces have components north and south as well as east and west, so that the currents are not always parallel with the equator. Also, such a theory takes no account of forces due to the earth’s rotation.

i.4. Forces Due to the Earth’s Rotation The theory of tides in an open sheet of water is seriously complicated by the earth’s rotation. It has been known from the year 1735 that a particle having a motion in latitude tends to keep its angular momentum around the earth’s axis unchanged, so it alters its motion in longitude. This was recognized by Hadley in his theory of the trade winds. Similarly, if a particle is moving eastward along a parallel of latitude its real angular velocity around the axis of the earth is greater than that of a fixed particle upon the surface, and therefore the centrifugal force upon it is greater than that due to its position, so that it will move outward and the constraint of gravity will force it along the earth’s surface southward until it reaches equilibrium. This geostrophic force in each case, exercised upon a particle of unit mass with a velocity V a t a point in latitude X on an earth with angular velocity Q is expressed for each component of velocity, and therefore in general, by (1.8)

2VQ sin X

The force operates to the right of the particle’s path in the northern heniisphere, and to the left in the southern hemisphere. The importance attached to this geostrophic force cannot be overemphasized. It applies to all fluid motions on the earth, and the effect of the force was clearly apprehended by Laplace when he formulated the equations of fluid motion for a fluid upon the earth’s surface about the year 1776, and it was included by Airy in his investigations, using the Laplacian equations, in the year 1842, so that the geostrophic forces have been included as a matter of course in the tidal equations since the end of the eighteenth century. In recent years, meteorologists and others unfamiliar with this history have become aware of the importance of the forces but have attributed their introduction into scientific usage to Coriolis about

OCEANIC TIDES

123

the year 1835 and his name is now used by many writers to express the force. Kelvin, Ferrel, Darwin, Lamb, Harris, and other noted exponents of tidal theory do not mention the name, a fact which needs to be borne in mind when considering the literature lest it be thought that these so-called Coriolis forces are a modern discovery. We shall follow tidal usage. The effects are most apparent in the large oceans, where the gradients set up by the geostrophic forces may be fully developed. The currents involved in setting up these gradients are themselves affected by the geostrophic force so that there is a reaction upon the primary currents due to the tidal forces and hence this complex interaction is a deterrent to exact reasoning except by mathematical methods, although attempts to draw charts from observations on the coasts may be guided empirically to some extent by the knowledge of these forces. 1.6. Amphidromic Systems

The general character of tidal motions is profoundly affected by land boundaries. The simplest effects are seen in the reflection of progressive waves so that the resulting oscillation loses its character of apparent progression and becomes a standing oscillation, as in a narrow sea where the water oscillates about a nodal line, so that high water occurs simultaneously on one side of the nodal line, and on the other side low water occurs simultaneously. In a basin which is not very narrow we may have another effect. Firstly, the tide-generating forces can set up independent oscillations, if we ignore the rotational effects, and each of these may have a nodal line. Where the two lines cross is a point of no tidal range. Inasmuch as the components of the tidal forces are in phases 90" apart it follows that the vertical oscillations will appear to circulate around the point of no vertical motion. Consequently, the system is called an amphidromic system and its center is called an amphidromic point. The system described above is derived from two simple standing oscillations without taking into account the rotational forces, but it is evident that each standing oscillation will be affected by the geostrophic forces so that the two oscillations are not independent. For the oceans these are the main causes of amphidromic systems. The apparent rotation of the amphidromic system may be either clockwise or counterclockwise and, in an ocean where many such systems abound, both kinds of systems may, and indeed must, exist even in the same hemisphere north or south of the equator, though generally for the semidiurnal tide the clockwise rotations are more common than the counterclockwise rotations in the northern hemisphere, with the reverse in the southern hemisphere.

124

A. T. DOODSON

2. TIDAL CHARTSOBTAINEDBY EMPIRICAL METHODS

In this Section the term ‘‘empirical” is used to describe charts which nrp based upon observations of tides on the coasts and on islands. It is not intended to imply that no theory is used by the authors in compiling their charts, although in the case of some charts the theories may be rejected. The later charts may use qualitatively the results of modern mathematical methods, but they are empirical in the sense that they are not obtained hy rigid or precise mathematical methods. 2.1. Charts by Whewell and

Airy

The first attempts to describe charts in oceans were entirely based upon coastal observations with some few observations from islands. Such charts were given in 1833 by Whewell [a] and were confined to the resultant semidiurnal tides on the days of new moon and full moon. Amphidromic systems were not visualized, although in the year 1836 Whewell gave one such system for the lower part of the North Sea, but it was repudiated by Airy [3]. The wave theory was dominant to such an extent that in a channel the front of the wave was highly convex in the middle of the channel; that is, it was supposed that the wave traveled faster in the middle than a t the sides. Also, it was supposed that local deptths had direct influence upoii the shapes of the cotidal lines. 2.2. Charts by Harris

The wave theory implied that in a wide gulf the wave must either circulate within the gulf or else have all its energy dissipated at the closed end of the gulf. It is to Harris’ [4] lasting credit that he utilized the concept of standing oscillations about nodal lines, and the influence of depth upon the tidal oscillations. I n a closed region, free oscillations occur with periods which depend upon the depth and, if the region is very long there may be several oscillations occurring together, each having a nodal line; in a crude way we can visualize the simple oscillations set up as dishes set up end to end in a chain, and then for the barriers to be removed when the oscilIntions have been synchronized. If these oscillations could be maintained by forces of the right period then resonance could occur. Considering thus n continuous series of basins whose natural periods correspond with thr period of the tidal constituent, we have the main basis of Harris’ theory. I n applying it, he considered basins of unusual shapes and also combinations of basins a t right angles, but the rotation of the earth was not taken into account, or a t least it was not adequately considered. His theory, as a theory, was not very well received, but the charts he produced, as far as the oceans are concerned, were much in advance of those previously exist-

OCEANIC TIDES

125

ing, and his work was a great stimulus to others. For the first time the predominance of amphidromic systems in the oceans was made evident. 2.3. Charts by Prufer

In order of principles rather than of date we may mention that similar principles seem to have been applied by Prufer [5], in 1939, for the Indian Ocean, in that he considered simple basins as representing parts of that ocean and then he used the results as a guide in drawing charts, which he produced for seven principal tidal constituents. He was the first to give co-range lines as well as cotidal lines for an ocean. 2.4. Charts by Sterneck

Charts for all oceans were given in 1921-1922 by Sterneck [Ci, 71 for the tidal constituent Mz and for a composite diurnal tide. For many regions his charts corresponded broadly with those given by Harris. Thus, for the North Atlantic Ocean, both authors agreed that the semidiurnal tides could be represented by an amphidromic system in the main part of the basin, with smaller systems on the outskirts, but there were differences in the assignment of the amphidromic points and in the distribution of the cotidal lines. For the Pacific Ocean there were considerable differences between the two charts. The basis of Sterneck’s work is not very clear but it is judged that he considered sets of nodal lines; in each of two sets the lines were nearly parallel and they crossed the lines of the other set as nearly orthogonally as possible. The intersections of the lines gave the positions of the amphidromic points, but the distributions of the cotidal lines around the points were then conjectural. 9.5. Charts by Dietrich and Villain

Cotidal charts for the oceans have been produced by Dietrich [8] for two semidiurnal constituents, M 2and SZ,and also two diurnal constituents, K 1 and 01.Charts have also been given by Villain [9] for the constituent M I . Both these authors had far more material than was available to earlier authors. As the charts were published in 1944 and 1951 respectively, they are the most modern charts for all the oceans, and these efforts involved very great attention to details and the consideration of previous charts. Both authors give cotidal lines but not co-range lines. 2.6. Illustrations for the Paci$c Ocean

I t is unnecessary to give examples of all the charts referred to above or to give charts for all the oceans. A choice has been made for the Pacific Ocean, and Figs. 1 and 2 give charts by Dietrich and Villain. Both have been redrawn and relettered, and much detail has been omitted for channels

126

A . T. DOODSON

FIG.1. Cotidal lines for the Pacific Ocean, after Dietrich. Times in mean lunar hours after the equilibrium tide M I on the Greenwich meridian. near the coasts of Australia and Asia, as the objects of the illustrations w e as follows: (a) to show something of the complex distribution of the tides in a large ocean; (b) to show the degree of difference between charts produced by highly competent authors; and (c) to demonstrate some of the principles which have been expounded in this article. For details which have been omitted, reference should be made to the original publications. It is desirable to emphasize that these charts have been produced by what we call empirical methods and thus there is no conspicuous attempt on t,hc part of the authors to force conclusions to agree with any special theory.

OCEANIC TIDES

127

FIG.2. Cotidal lines for the Pacific Ocean, after Villain. Times in mean lunar hours after the equilibrium tide M 2 on the Greenwich meridian.

2.7. Discussion of Charts by Dietrich and Villain for the Pacific Ocean

For the purpose of discussion letters A to J have been placed adjacent to systems given by Dietrich. The numbers on the lines in each chart represent the times of high water in mean lunar hours after the transit of the mean moon over the meridian of Greenwich, this being the time of high water of the equilibrium constituent Mz on that meridian. It will first of all be noted that Villain had before him all the data used by Dietrich, but he has not felt justified in extending his results in a large region west of the southern part of South America, so that system I is not given by him. While no doubt is explicitly castJupon Dietrich’s chart for

128

A . T. DOODSON

that region, we thus discern some measure of uncertainty which exists in drawing charts such as these. Apart, however, from that, there is (%lose agreement in the distribution of the main amphidromic systcnis. Thus thc systems A , B, D, h’,G , H , and J , apart from variations iii the cotidtll lines, are suggested also by Villain. System E, however, is affected in that lines numbered 12 and 1 go to system C, and lines 10 and 11 go to system H . Other lines also are affected, such as the line for hour 7 from E to H ; for this hour Villain prefers a line from E to D and from H to G. The question may be asked as to why such a difference may occur. It should be remembered that a t amphidromic points there is zero range of tide and if two amphidromic systems occur then the range of tide between them may also be small, so that the determination of the time of high water may be a little uncertain. Also, large areas may have almost simultaneous tides as may be seen between E, B, and F where the high waters over a very large area evidently occur about the hour 2. Very little change in the times could determine an apparently large change in a cotidal system, hence these differences may often have little practical importance. The systems F and C are merged by Villain but are not represented by a single true amphidromic system. He draws lines which, if produced, meet on the land. Doodson has given the name “degenerate amphidromic system” for such a case as this. It is not uncommon; thus system G is of this class at the western end. Doubts exist as to whether system A is real or degenerate, but very accurate observations would be needed to settle the matter, as the ranges of tide in the vicinity are very small. Again, therefore, what may appear to be a fundamental difference has little practical significance. Evidently a system of co-range lines would reveal reasons for discrepancies, for in the ultimate recourse of computation a phase angle depends upon the tangent yielded by the ratio of two small quantities when the range is small. 9.8. Comments on Illustrations of General Principles

We may conclude this discussion of empirical charts by some comments on the general principles referred to in the first Section. In system J the cotidal lines are bunched together, and in effect there is a broadened-out nodal line separating two cotidal areas. Because the range of tide must be small the apparent time shifts of four hours in a short distance are of little practical significance, but the system does demonstrate the importance of standing oscillations. We may even say that the systems A , B , F and/or the systems D, E , F and/or the systems B, E , H may have originated from nodal lines so that these lines of zero range cross one another a t amphidromic points, and the effects of the rotation of the earth, and of other causes, are seen in the fanning out of the cotidal lines.

OCEANIC TIDES

129

It is interesting to note that systems A , D, F , H , and I have clockwise rotations, while systems B, E , and G have counterclockwise rotations. There is a principle that when two lines are common to two systems there must be rotation in opposite directions, but where there are no lines in common or only one, both systems rotate in the same direction; thus B, which is joined to A and F , differs from these in rotation, while E and B with no lines in common have the same direction of rotation. 3. MATHEMATICAL INVESTIGATIONS FOR OCEANSENCIRCLING THE EARTH

Two objects are before the geophysicist, the first being the production of charts for actual oceans, using observations, and the second being the quantitative explanation and description of the tides without recourse to observations. The latter ideal is very far from being attained, though steady progress has been made toward it during the present century. It is only by the use of mathematical methods that the physical processes may be understood. There are two kinds of mathematical methods, one of which may be fairly described as classical in that it uses well-known mathematical functions, and the other is mainly an adaptation of finite differences expressive of the differential equations. Little use has been made of the latter methods except for work based upon coastal observations. The mathematical methods will be dealt with in three parts; firstly for oceans which encircle the earth, secondly for oceans bounded by meridians, and thirdly for the composite methods which use both mathematical methods and observations. 3.1. The Equations of Motion

The general equations for fluid motion in shallow water can be simplified for deep oceans by the neglect of terms of the second order of small quantities. Even with a deep ocean it is still possible to consider a tidal wave as a long one in the sense that the depth of the ocean is very small compared with the length of the wave, and it is therefore possible to take the velocity of the water as the same from top to bottom, but with actual oceans it may be necessary to consider the effects of internal tides to which reference will be made in the last Section. The symbols used for the equations for tides on a rotating earth are as follows: a the mean radius of the earth h the depth of the ocean, not necessarily constant the angular speed of rotation of the earth e the colatitude, zero at the North Pole x the longitude east of a standard meridian

130

A. T. DOODSON

u,v the components of current-velocity in the directions of increasing 0, x respectively, taken as means in a vertical line the elevation of tide above the mean surface f the equilibrium elevation

c

{’

=s-f

the time the speed of the tidal motion, = 2?r/period The equilibrium tide is used to represent the tidal forces. Then the equations to be solved are: t

u

au _ at -

(3.1)

av + ( at

(3.2)

-

( 2 cos ~ e)v

- 9--at’ a

ae

2 5 2 ~ 0 =~ ~ --.--)g ~ 1 a sine

- (hu sin 0)

(3.3)

=

+ax a (hv)} =

at ax ac -at

The factor (20 cos 0) corresponds to the geostrophic factor given in (1.8). The solution in general for all contours is highly complex and even for simplified basins it is difficult. Only simple basins will here be considered, such having coasts which can be represented by simple mathematical functions. The depth is usually taken as constant or as a function of latitude. 3.2. Laplace’s Solution

Laplace [lo] solved the equations for an ocean covering the whole carth. Since a semidiurnal constituent of the equilibrium tide has a phase (at 2 ~ ) it follows that in the absence of boundaries all variables are harmonic expressions in (at ax).Following the usual artifice we remove the common factor

+

+

(3.4)

el(rrt+zX)

,

> = -1

and use the same symbols for the variables. The solutions obtained by Laplace were for

(3.5)

u =

20,

h

=

constant

and for values of the parameter

(3.6)

@ =

4Qza2/gh= 10, 20, 40

It is desirable to explain the reason for taking

u =

2Q. If we perform the

131

OCEANIC TIDES

differentiations in (3.1) and (3.2), we obtain

(3.7)

and from these we may eliminate v, say, to obtain

and the choice of u gives

(-ae + 2 cot

u sin2 e = 2 a t

(3.8)

ua

e.j-1)

A similar formula is obtained for v, and the advantage of this choice of u is that j-’ is then readily expanded as a power series in sin28.The choice is equivalent to the choice of Kz rather than Mz or Sz: as a representative of the semidiurnal tide, and it is a choice which has been made by several workers. The determination of the coefficients in the series for Laplace’s solution, writing x = sin%, (3.9)

j-’ = Ao

+ A ~ +x A& +

m

.

0

+ A,xn +

is made by the use of a recurrence equation, after substitution in the fundamental equations, which is given by (3.10)

n(n

+ 6)An+z - n(n + 3)An+1 + @An= 0

Clearly, when n is very large there are two solutions to this equation

’ converThe solution (a) is not convergent forx = 1, but that given by (b) is gent. Laplace obtained his solution for (3.9) by starting with (b) for a large value of n and then working backwards. His solution was doubted by Airy, but was vindicated by Thomson [ll]. It is desirable to mention these facts for they are of importance in the development of the subject. The solution was examined and amplified by Doodson [12] who showed that the recurrence equation could be used twice in a forward fashion, with and without the inclusion of f, so obtaining a particular integral and a complementary function, and that the coefficients An in the two series rapidly approached

132

A . T. DOODSON

a limiting ratio. When the two solutions were combined so that the complementary function, multiplied by the limiting value of the ratio, W:~S subtracted from the particular integral, then a convergent series was 01)tained and this was in agreement with the one obtained by Laplace. An independent method, using numerical integration, also gave two nonconvergent solutions which could be combined so as to give u = 0 on the equator, the same ratio being obtained. Laplace’s solution gave results corresponding to a progressive wave round the earth, owing to the absence of all boundaries, but the great importance of depth was manifested, as may be seen from the following equation, which gives the ratio of elevation a t the equator to that of the elevntion of the corresponding equilibrium constituent. (3.12)

r

p

=

h

=

20 10 29,040 14,520 {/f = 11.259 -1.821

40 7260 f t -7.434

It is evident that resonance occurs somewhere between p = 10 and p = 20, and that resonance for a smaller value of p may be expected. Laplace also considered the diurnal tide K 1 and obtained the remarkable result that for constant depth there are, for this case of an ocean covering the whole earth, no vertical oscillations for this constituent but there are tidal currents. 3.3. Hough’s Solution

Though Laplace is hardly considered as one of the modern writers whose work is before us in this article, yet the fact is that he had no successor until the year 1897, when Hough [13] developed the solution for the same problem of an ocean covering the whole earth. The special character of his method was the use of solutions in series of Legendre functions so as to allow for the effect of the mutual gravitation of the water; a matter which will be referred to in the last section. Hough’s use of tesseral harmonics enabled him to obtain more general results than those of Laplace and he also obtained solutions for the lunar tide Mz . He showed that when the period of a forced oscillation differs from that of one of the free oscilla t’ions by as little as one minute, the forced tide may be nearly 250 times as great as the equilibrium tide, but if the difference in period is five minutes the amplifying factor is only 10. Hough’s work is of great importance as there is no more recent work than his in some of the problems studied by him, but it is limited to the highly idealistic type of ocean covering the whole earth.

OCEANIC TIDES

133

3.4. Goldsbrough's Solutions for Polar and Zonal Oceans

The next development consisted of a restriction to a polar ocean and to an ocean bounded by two parallels of latitude, solutions for which were given in the years 1913 and 1914 by Goldsbrough [14, 151 for a uniform depth. Since there are no boundaries hindering east and west motion in a polar ocean the problem is very similar to that of Laplace and similar methods were used. Goldsbrough's results are very thorough for the three species of tides. For the zonal oceans it was necessary to consider solutions of the equations which are not finite a t the poles in order to satisfy the two boundary conditions. The results again show how boundary conditions and depths profoundly affect the character and magnitude of the motion. Doodson [12] applied the method of complementary function and particular integral to the polar ocean in the same manner as was done for Laplaee's ocean. In the year 1927, Goldsbrough [16] also investigated the tides in a polar ocean for a depth comparable with that of the Atlantic Ocean and he obtained quite small tides; he also showed that large oscillations could only occur with very small depths so that he disproved that the Southern Ocean could provide large tides to be the source of large tidal oscillations in adjacent oceans. 4. MATHEMATICAL INVESTIGATIONS FOR OCEANS BOUNDED BY MERIDIANS

4.i. Oceans on a Nonrotating Earth The methods and results described in the previous Section are very instructive; but, apart from the possible application to the Southern Ocean, the basins are not comparable with those which actually occur on the earth. It was obvious that the next approach to reality should be by the consideration of oceans bounded by meridians and a start was made in 1927 by Proudman and Doodson [17] for the tides on a nonrotating earth. Even in the simple case of constant depth the solution in general had to be expressed in infinite series of tesseral harmonics, but for certain cases these series were finite and they were computed and illustrated for /3 = 20; the solutions are given for semidiurnal and diurnal tides for oceans bounded by meridians 60", 90"' 120", and 180" apart. The results showed in all cases amphidromic systems at each pole, two on each central meridian, and four On the bounding meridians, all symmetrically disposed with regard to the equator. The chart,s obtained are very like those which were later obtained for rotating oceans, as illustrated in Figs. 3 tjo 8, and they were indicative of what might be expected for those oceans.

134

A. T. DOODSON

4.2. Goldsbrough's Solutions for a Narrow Ocean

A number of theoretical solutions for oceans bounded by meridians on a rotating earth has been investigated by Goldsbrough. I n 1937, he published his first paper of the series [16] which dealt with an ocean bounded by two meridians 60" apart, and extending from pole to pole. A depth varying as the square of the cosine of the latitude was taken, and the semidiurnal tide was represented by K2 , in order to simplify the equations. The method was applied in the later paper by Goldsbrough and Colboriie [lS] to an ocean bounded by meridians 60" apart and having a uniform depth of 12,700 ft. Such an ocean would fairly represent the Atlantic Ocean if it were completely land-locked instead of being open to the Southern Ocean a t one end and partially open a t the other end. The tidal constituent chosen was M S , and the results are illustrated in Fig. 3. In this work the elevations were defined by

1 p

(4.1) FIG.3. Cotidal and corange lines for an ocean bounded by meridians 60" apart, after Goldsbrough and Colborne. Constant depth 12,700 ft. Times in full lines after the equilibrium tide M S on the central meridian. Semi-range lines, broken, require factor 37.6 H .

m

=

f =

C C,~P,~(COS n=s

6)er(uL+sx)

m

C y;p;(COYe ) e l ( u t + * x ) n=8

where Plls (cos 6) is an associated Legendre function, Cp, and yn8 are constants to be determined, n and s are integers because the range of x is a fraction of T,and the values of s are multiples of 3. Inasmuch as f, by definition for a semidiurnal tide, involves s = 2, a special development of f had to be made, and this was done by a "null function" which is everywhere zero and which was added to the proper expression for f. This null function involved the expansion of sines in series of cosines, and the expansion of cosines in series of sines, both series being infinite, and one of them nonconvergent a t the boundaries. A great deal of mathematical development follows by transference of the associated Legendre functions into infinite series of ordinary Legendre functions. The solution was deveIoped for a few terms only and the boundary conditions were not perfectly satisfied. The velocity transverse to the boundary contained a group

OCEANIC TIDES

135

of positive terms and a group of negative terms, and the inequality between these groups was less than 4 % of either, which was considered to be a good enough approximation. One conclusion from the investigation is that there is a very large amplification of tide due to partial resonance, the ratio between the largest amplitude of tide in this ocean and the corresponding largest amplitude in the equilibrium tide being over 37. Colborne [19] applied Goldsbrough's methods to give the diurnal tide K 1 for the same ocean as in the previous paper. The amplitudes are small and the changes in phase are small, in partial agreement with Laplace's result of zero diurnal amplitude for an ocean of constant depth. The solution is therefore not illustrated here. I n a much later paper, in 1949, Goldsbrough [20] describes a method which uses a double series of orthogonal functions, each of which satisfies the boundary conditions, and he found it possible to write explicitly in algebraic form the terms of the double series expressing the tidal height. The method is elaborated for Kz with a special law of depth varying as the square of the cosine of the latitude. As in all his work, the value of the parameter Po for the depth is not assigned in the theoretical formula, and so he found it possible to obtain an equation for P O , the critical values of which determine the depths a t which resonance takes place. For an ocean 60" wide the mean depth for resonance is given as 16,600 f t; in his 1928 paper i t was given as 15,500 f t for a similar ocean. 4.3. Ocean Bounded by a Complete Meridian: Proudman and Doodson

I n 1917, Proudman [21] published an account of a general method of treating the dynamical equations of the tides in which the ordinary differential equations were transformed into an infinite sequence of algebraic equations. One of the chief features of the treatment is that an attempt is made to deal rigorously with questions of convergence. It was pointed out a t that time that the method could easily be applied to basins of simple shapes, but it was a disadvantage that the algebraic equations are naturally arranged in a double sequence and not a single one. Consequently, Proudman considered the case of a flat semicircular sea and that of a hemispherical ocean bounded by a complete meridian, both basins having uniform depth. For the former case the method was completely successful, but for the latter case i t was first necessary to evaluate a number of functions analogous to Bessel's functions. The investigation was taken over by Doodson [22]; he tabulated the functions and then proceeded to use them. The unknowns of the algebraic equations had to be determined by the vanishing along the equator of a function of longitude represented by a trigonometrical series and he soon found that this was impracticable.

136

A. T. DOODSON

Proudman [23] then showed that for the nonrotating ocean the corresponding series do not converge, but the difficulty would not arise for an ocean bounded by the meridian and a parallel of latitude other than the equator. Doodson then developed new methods (see later) and Goldsbrough’s work was published, in which he uses a singly infinite sequence of algebraic equations, while in 1931 and 1933, for small seas, Goldsbrough uses doubly infinite sequences of equations to discuss free tidal oscillations; those equations are practically the same as those resulting from Proudman’s method. Proudman [24] applied the general ideas of his 1917 paper, but introduced two important changes by using coordinates 0 and x referred to a pole on the equator and by the use of subsidiary functions of zero dimensions. Questions of expansion and convergence remained the same as in the previous paper. The equations were particularized for an ocean bounded by a complete meridian and with infinite depth, and 716 coefficients involved in the equations were tabulated. The computations for these numerical coefficients required the evaluation of the following integrals (4.2)

/4r

’ sin20

sin eP,”P,” do,

0

Pm-1

P:ae, tr

sin 0 cos BP,”P,” dB for n, s up to 12. The subsidiary functions cp, $ are such that da -=o,

(4.3)

an

lj=o

at the coastline, a/an denoting differentiation along a normal to the coastline. The functions are defined in relation to components of velocity by

at 1 a21j ----ae at sin e ax at

(4.4)

a The solution is expressed in terms of an infinite number of values of r in cp, , ljr which are terms in series for p, $. The application of the method was made by Doodson [25] for a diurnal tide K 1 and the solution involved simultaneous equations, the variables in which were limited in number to 63, related by six sets of equations. These were considered to be sufficient to give an adequate accuracy. The solutioii of so many equations, without the use of high-speed machines, was a formidable task, but the solution was sought in the most general way possible,

OCEANIC TIDES

137

so that each minor variable was expressed in terms of the principal ones, and these again in terms of the two most important variables. At each stage the coefficients were series in powers of the parameter p. By these means a resonance-equation was derived, from which the critical depths for resonance could be readily evaluated. The solution was adequately illustrated for four depths, two of them being critical, and these suffice to illustrate the change in tide with depth, from an infinite depth to a depth smaller than the mean depth of the oceans occurring in nature. Actually, there is little of outstanding interest in the charts until /3 = 20, when two amphidromic systems have developed upon the central meridian, and again for p = 40, when four amphidromic systems exist, one in each quadrant. The movements of the amphidromic points as p increases are investigated and described. As was Goldsbrough's experience, difficulty was found in the computations owing to the lack a t that time of tables for the associated Legendre functions, and a further difficulty occurred owing to the need for integra-

FIU.4. Cotidal and co-range lines for an ocean bounded by meridians 180" apart, by Doodson. Constant depth 16,130 ft. Phase lags relative t o equilibrium tide K 2 on the central meridian given by full lines. Co-ranges given by broken lines. Divide phase lags by 30" t o give hours. Factor for semi-ranges 6.03 H.

138

A. T. DOODSON

tions of these functions over a range of 0 less than T. This difficulty was overcome by computing the functions as Fourier series in 0, and the vn1unl)le tables obtained are given in the paper cited. Much more varied and interesting results followed from a further application of the theory to the tidal constituent Kz . The equations for the 63 variables were solved by Doodson [26]for 20 values of the parameter p, from 1 to 20. Though a part of the work was common to all these cases, in effect 20 sets of 63 equations were solved. The results are fully illustrated in the original paper, and the genesis and development of the amphidromic systems, as the depth changes, can be fully traced. The possible systems :ire of great variety, and the type of system changes so rapidly with depth that a single illustration is of doubtful value for comparison with an actual ocean. Figures 4 and 5, for /? = 18 and 19, show this change. The numbers on the cotidal lines should be divided by 30 to give hours of high water :LS in previous diagrams. The critical depths for which resonance occurs are

FIG.5. Cotidal and co-range lines for an ocean bounded by meridians 180" apart, by Doodson. Constant depth 15,280 ft. Phase lags relative t o equilibrium tide K? on the central meridian given by full lines. Co-ranges given by broken lines. Divide phase lags by 30" t o give hours. Factor for semi-ranges 5.57 H .

139

OCEANIC TIDES

as follows:

(4.5)

(p

=

\h

=

1.896 5.87 9.41 12.69 15.62 153,200 49,500 30,900 22,880 18,590 ft

This is the most elaborate investigation yet made concerning the variation of tide with depth; the paper gives tables of components of tidal currents, components of elevation, and amplitudes and phase-lags.

4.4. Doodson's Solutions for Narrow Oceans I n the early part of the previous sub-section, reference was made to methods developed by Doodson [27] while the above investigations were being made. These methods are very unlike those described above, in that no developments in infinite series of tesseral functions were used. The first of these methods was concerned with very narrow oceans with a view to tracing the effects as the ocean was widened, and solutions were given in terms of the semiwidth of the ocean, a! in longitude, as a parameter, so that by taking (4.6)

{ =

cos at

+ fZ sin at

with corresponding expressions for other variables, we write =

(4.7) \{2'

=

zo+ a!2zz+ a 4 ~ 4+ . . . + a3Z3 + ...

where 2, is a function of 0, the colatitude, and $ = x/a!. The equilibrium expressions are readily expanded, giving, with well-known values for C, ,

s,

f1

=

H sin2 0 cos 2x

=

1

H sin2e T

(4.8)

fz

C, - $'a!' r!

( r even) 1

= -Hsin2Bsin2x = - H s i n 2 e z S , - $ ' a '

r!

( r odd)

Putting the above expressions into equations (3.1), (3.2), and (3.3), we may write

( r even) (4.9)

( r odd)

It is then easily found that recurrence expressions for the functions 2, , ZTf2, exist, from which it was a simple matter to obtain the functions in finite series of harmonic terms. The solution was illustrated for p = 10,

140

A . T. DOODSON

but the method could not be used for oceans wider than 30" in longitude. It gave results for an investigation of nonresonant but free oscillations for p = 20. The second method, developed in the same paper [27], investigated the application of finite differences. In a sense it was experimental and was intended as preliminary to applications of finite differences to tides in actual oceans. The great difficulties inherent in the use of the common mathematical functions, due to slow convergence of series, appear to set a limit to their use, even for simple basins, whereas methods of finite differences can find application, and should be usable, for any type of variation of ocean depths and contours, so that these methods have undoubtedly a large field of use. In the paper the methods were successfully applied to the evaluation of the semidiurnal tide Kz in oceans of constant depth, up to 90" wide, for /3 = 10 and /3 = 20. Though the solutions may not be of very great accuracy, yet the variations in character as the width of the ocean increases are instructive. This method supplements the foregoing method. An important point was in the choice of x = sin20 as the variable representing the latitude; the differences with regard to x are much simpler than those with regard to 8 or sin 8, for the equations involve only third powers of x in place of sixth powers of 0 or sin 8. Another important point is that inasmuch as finite differences are given in terms of elevations at specified points, then contributions from one of these variables can be expressed separately from the rest. The chosen variables were taken on the meridians f x = 5" and the intervals used were 62 = 0.1,

ax

=

10"

Special consideration was needed at the equator as x does not exceed unity. The boundary conditions were readily found by the use of meridians 5" on either side of the chosen bounding meridian, and full use was made of the conditions of symmetry and asymmetry. The result was to obtain 20 simultaneous equations for each of five cases and the solutions of these sets of equations gave results on f x = 5", 15", 25". . .for all values of L. The solutions for three cases where the widths of the oceans are 50", 70", and 90" in longitude are given in Figs. 6 , 7, and 8. These are for p = 20 and there is a measure of agreement with Fig. 3, but the amphidromic systems are fewer in number. 5. MATHEMATICAL METHODS APPLIEDTO ACTUALOCE.4NS 5.1. Limitations of Methods

It is evident that the methods of deduction from limited observations such as were considered in Section 3, even though guided by general

141

OCEANIC T I D E S

principles, are not by themselves wholly satisfactory because different persons may and do get diffcreiit results, but they have an advantage over the purely mathematical methods of Section 4 in that they are able to take account of all the vagaries of the coasts and the depth. The purely mathematical methods have hitherto been applied only to simple basins, and it is evident that very great labor has been expended upon them. Methods applicable to more complex basins will be referred to in Section 6. Many of the same problems had to be considered for seas such as the North Sea, and ultimately the solution was found by Proudman and Doodson [28] to be the application of the differential equations, of which equations (3.1), (3.2), and (3.3) are simplified versions which omit terms necessary for shallow seas, and to use these from coast to coast utilizing coastal data and currents. Special importance is attached to the tidal currents for they imply a gradient of the surface and this can be computed by means of the equations; then the gradients can be integrated from coast to coast, or fitted to the coastal observations. For the oceans, however, the tidal currents for all depths below the surface cannot easily be obtained, largely because of the difficulties in maintaining position a t one point while taking observations in very deep water. Other methods have to be pursued, in which mathematical methods may be applied to actual observations, and these methods may be termed “conjoint methods.”

1.0

FIG.6. Cotidal and co-range lines for an ocean bounded by meridians 50” apart, by Doodson. Constant depth 14,520 f t , K z , factor for semi-range 7.9 H .

5.9. Application of “Narrow Sea” Methods Defant [29, 301 developed methods of calculation which he had exploited for narrow seas and channels, and he applied them to the Atlantic Ocean which has an elongated basin. The ocean was divided into narrow canals transverse to the central line of the ocean, and step-by-step integration was applied along this line to give mean values of elevations over the sections. The integrations were commenced by assuming the tides at the junction with the Antarctic Ocean and allowance was made for the openings in the north, this also depending upon observation. Then geostrophic gradients were introduced, and several other adjustments were made. His final chart showed an amphidromic system in the North Atlantic, similar to, but differently placed

142

A. T. DOODSON

\

FIG.7 FIG.8 FIG.7. Cotidal and co-range lines for an ocean bounded by meridians 70" apart, by Doodson. Constant depth 14,520 ft, K 2 , factor for semi-range 4.9 H . FIG.8. Cotidal and co-range lines for an ocean bounded by meridians 90" apart, by Doodson. Constant depth 14,520ft, Kz , factor for semi-range 6.3 H .

from, those given by Harris and Sterneck. This effort was certainly :L step in the right direction, even though the methods are somewhat imperfect. 5.3. Application of Kelvin and Poincarb Waves

A different approach to the subject was adopted by Proudman [31] who made use of certain special types of waves which had been studied by Kelvin and PoincarB. These waves satisfy the equations of motion and, broadly speaking, they may be considered as complementary functions to be added to the particular integral so as to satisfy the elevations on the coasts. Proudman applied the method to the South Atlantic Ocean but he found that it was not capable of refinement to allow for all the details of the tmsiii

143

OCEANIC TIDES

beyond a first approximation; the method served a useful purpose in assessing the possibilities of applying mathematical methods for the open ocean. 5.4. Proudman's Tidal Theorem

As far back as 1925, Proudman [32] gave a theorem in tidal dynamics which seemed to have great possibilities if the difficulties concerning extensive calculations could be overcome. It depends upon the fact that two of the tidal equations involve - f) while the third one involves [. Use is made of Green's theorem and of certain auxiliary functions U , V , 2, (analogous to u, v , p ) which satisfy equations related to the tidal equations by changing the sign of the rotation of the earth. The theorem is expressed in its simplest form for a land-locked ocean as follows:

(r

(5.1)

1

hp'N d s -

/ hZv

ds =

LU//

fZ dS -

9

I/ ( F U + GV) dS

where N is the component of the vector ( U , V ) along the outward drawn normal to and a t the coast, and v is a similar component of velocity. The single integrals with respect to s are taken around the coast or other boundary and the double integrals with respect to S are taken over the surface of the enclosed ocean. The theorem provides for frictional forces, F , G for use in shallow water. The symbol I arises from the use of a time factor err'. The functions U , V , Z are independent of the boundaries and they can be calculated by standard methods, such as those used by Doodson [12], who calculated a number of the functions for the use of this method. It will be observed that as Z and 3' are known over the ocean then the double integral of their product can be calculated; the single integrals also involve quantities which are known over the boundaries. If, however, we consider a small island of negligible area around any point & of the ocean then the first integral, taken around an enclosing small circle with center &, can yield a limiting quantity (5.2)

a1yr:I&

where 1 denotes a convenient length introduced to maintain correct dimensions and serving as an arbitrary constant multiplier for the functions. Thus in its simplest form the theorem gives the value of p' and therefore p on any chosen point, if the elevation and current normal to the boundaries are known and if frictional forces may be neglected in the ocean. The theorem thus involves integrations for constant or variable depth and for observed quantities so that it is not liable to the errors which affect methods depending upon differences of depths or of observed tides. It could be applied to a number of selected points in an ocean bounded by

144

A. T. DOODSON

land. It should be noted, however, that the point Q is a source and that the function N is infinite there, if (5.2) is valid. A much more useful adaptation of the method was provided by Proudman for an area bounded by a coast and a line across the open sea, resulting in Fourier series representing the elevation along that line and the current normal to it, these series being derived from the auxiliary functions, which are finite. This form of the theorem is therefore of very great importance, for the open entrances to oceans cause very great difficulties in deducing tides ill oceans from coastal observations only. The theorem has been applied, after many years, to the northern part of the Indian Ocean by Fairbairn [33] for one line only, and he has used the results for this line so as to assist the drawing of cotidal and co-range lines within the area enclosed by the coasts and the line. This, of course, still leaves a personal element in the drawing of the charts, and, strictly speaking, the computations should be made for other lines across the area. Secondary processes have been applied in relation to the offshoots from the ocean, such as exist for the Red Sea and the Persian Gulf. Despite the residual element the chart may be considered to have a much more trustworthy basis than was previously possible. 6.6. Finite Diference Methods

I n recent years much progress has been made in solving complex differential equations by means of finite differences, and the methods have becii applied to tidal problems. In a sense, Defant’s method used finite differences in one direction only, but modern methods are applied to a network covering small elements of the ocean surface, either in the form of small triangles or approximate rectangles. At each point of the network the differential equations may be replaced by finite-difference equations, and so at each point there exist two linear equations for two phases of the tide, and these connect elevations a t the point with elevations a t neighboring points of the network, and so ultimately many equations are obtained. At the boundaries there are special equations to satisfy the necessity for zero currents across the boundaries where these are coasts. These equations caii be solved by normal methods suited in these days to high-speed machines or they can be solved by iteration. As was only wise, such methods have been first applied to small seas for which the tides are well known, and Hansen [34, 351 applied them to the North Sea with considerable success, although it was necessary to use or assume data of tidal currents for the open boundaries of his chosen wen. An iteration method was applied to the Irish Sea for similar purposes by Doodson et al. [3G], using only coastal observations. The agreement with more exact methods, using data for the open sea as explairied in Sec-

OCEANIC TIDES

145

tion 5.1, was quite good except near the open entrance to the ocean. It was concluded that unless very accurate observations of coastal data near the entrance were available no great accuracy could be expected for charts in the vicinity: Hansen afterwards applied his method to the North Atlantic Ocean [37] and his results are shown in Fig. 9, in which certain details are omitted, and the original paper should be consulted for these. The network of points is shown by small black circles for which elevations were not known, and by small open circles at points near the coasts, but sufficiently far from the coasts to fit in with the network and to be considered reasonably free from shallow-water effects. Some deductions must have been made for elevations a t these points. The results shown are for the two elevations for phases 90" apart, and T2 for M z , and the lines give values of equal elevations for one or the other of the two phases. Where the two lines of zero elevation cross

FIG.9. Tidal chart for the North Atlantic Ocean, after Hansen. Constituent M2 relative to equilibrium Mz On the meridian of Greenwich. Full lines give {I , broken lines give TZ . Heights in cm.

146

A. T. DOODSON

there must be an amphidromic point. Where the lines for the two phases run closely parallel with one another the cotidal lines must be bunched together, so that we have, in effect, an expanded nodal line. In many ways charts for {I and c 2 give more information than the usual charis, and from computation the latter charts must be derived from the former. The results are only in fair agreement for islands, where more detailed methods are necessary, but the method has been shown to be feasible and (‘conjoint methods” using coastal data with the equations of finite differences are likely to be much exploited in the future. Hansen [38] developed his method for the whole of thc Atlantic Ocean and his results are shown in Fig. 10. The cotidal lines for the constituent Mz are given in solar hours, hence the original chart gives lines for times

FIG.10. Tidal chart for the Atlantic Ocean, after Hansen. Constituent M Z. Full lines give times of high water in mean solar hours after the high water of the equilibrium tide M z on the Greenwich meridian. Broken lines give ranges in meters.

OCEANIC TIDES

147

1200 and 1225, but for the sake of clarity the last line has been omitted in the illustration here given. The co-range lines are given a t intervals of 0.2 meter in range. (The co-range lines in Figs. 3 to 8 give co-range lines in terms of the amplitude or half-range.) This is probably the most exact chart yet produced for a large ocean. One matter of great importance in these methods is that of the scale of the network. Unless there is a suitable relation between the lengths of the bounding sides of the elements of the network there are risks of uncontrollable oscillations being set up in the calculations. The free periods in the ocean can be a source of difficulty.

6. FUTURERESEARCH This Section is not intended to offer any plans for future researches, but it will make comments on certain difficulties which will have to be overcome in the exploitation of methods and it refers to several matters which have been noted in the article. 6.1. Imperfect Coastal Data

All methods which use tidal observations are liable to give imperfect results for, although there are copious data, some are poor and are generally given in a form which is not that which is most useful. When data are given in nonharmonic constants it is not easy to derive the data required for any harmonic constituent. The effects will be most apparent where differences of the data are used, and this specially applies to the methods of finite differences. If such differences are used uncritically in equations which are submitted to what might be called “brute-force methods” of an impersonal nature, such as might be used with high-speed machines, then curious results might be obtained. Methods of iteration permit and even encourage a critical modification of data when the need is revealed. It is always wise to submit the coastal data to careful smoothing, but excess zeal in smoothing is to be avoided. The integrations used in Proudman’s method are not as susceptible as other methods are to the effects of localized errors in data. 6.2. The Effects of Capes, Bays, and Islands

The smoothing of coastal data should not ignore the effects of bays and capes. It was shown by Proudman [39] that cotidal lines tend to converge from a bay and to diverge from a cape, and that islands can distort the cotidal lines. Where it is necessary to commence with stations in deep water then the tides there have to be deduced from coastal observations, and this procedure requires considerable knowledge and skill, and on some of these difficulties reference might be made to the paper [28].

148

A. T. UOODSON

6.3. The Use of Tidal Currents

The paper last quoted may also be referred to for the use of coastal currents in deducing the direction of cotidal lines and co-range lines away from the coast. Apart from this, it is not generally possible to make use of' tidal currents in the deeper water of the oceans. Reference has previously been made to the difficulties of observation in deep water, and it may also be noted that as the currents in deep water are small then the need for accurate observation is greater than for shallow water. The deduction of harmonic constants from observations limited to one or two days is not a simple matter, but if the currents can be obtained then their value is enhanced by the methods of integration with which they are used.

6.4. The Efects of Internal Tides Within the scope of this article it has not been found possible to consider what are known as internal tides which occur in stratified water, and the distribution of these tides over an ocean may not be susceptible to illustration by charts of permanent value. Within the layers large vertical oscillations may occur, much larger than those perceived at the surface, and they are accompanied by large currents. It is therefore necessary to take t~ number of observations from top to bottom in order to get a mean value which will represent the astronomical tide. 6.5. The Efects of Mutual Gravitation

Reference was made in Section 3.4 to the effects of mutual gravitation of the water affected by the tides. This is not easily included even in mathematical methods except for very simple basins. Hough has been the only one to give much attention to this problem, but for the tides in more restricted basins than the one considered by him the effects would be much more complex. It is hardly necessary to consider it when conjoint methods used for the special mathematical methods are simply elaborate methods of interpolation between coastal data which will already show the effects of mutual gravitation. 6.6. The Efects of Earth Tides

Though mathematical calculations €or water tides are usually based upon the supposition of a rigid earth, yet in actuality the earth itself yields under the tidal forces and the effects upon the apparent height of tide are not negligible. No mathematical method has yet applied the theory to the charts of cotidal lines, and the conjoint methods hardly need to do so; for, as was just remarked, they are essentially interpolation methods applied to coastal data, which already include the effects of the earth tides.

OCEANIC T I D E S

149

6.7. General Considerations

It is very doubtful whethcr much can now be gained by the devcloprncnt of purely mathematical methods for simple basins. The methods dcscrihcd

in this article have served their purpose in demonstrating the types of motion which may occur, but this remark would become inappropriate if an entirely new method were evolved. It is proper a t this point to remark that Poincar6 [40] and Proudman [41] have given mathematical developments which theoretically apply to any systems of oceans, but the numerical work involved has been admittedly prohibitive of their application. Whether the use of high-speed machines will permit the exploitation of these methods is a matter for the remote future, but a glance a t the charts for the Pacific Ocean will show that mathematical methods must have their limitations. The original charts by Dietrich and Villain show that between Tasmania and Australia, for instance, the cotidal lines are closely bunched together. Finite-difference methods on the scale suitable to an ocean would need very great refinement to cope with the complexities introduced by islands and channels. Thus it appears once again that a great part of the problem is to get coastal data used so as to step out into the main ocean, where the data deduced may be put into the oceanic system of equations. For the oceans in general the methods of finite differences or that which uses Proudman's theorem are likely to be most useful. LIST OF SYMBOLS the mean radius of the earth coefficients in a series in Section 3.2 the rate of progression of a wave = (gh)*'a a numerical constant used in Section 4.4 a numerical constant used in Section 4.2 frictional terms used in Section 5.4 the acceleration due to gravity the depth, not necessarily constant where h = h, sinV the maximum amplitude of a tidal constituent in f a unit of length, used in Section 5.4 one of a series of numbers length along a normal t o a boundary, used in Section 4.3 component of vector ( U , V ) ,used in Section 5.4 the potential of a tidal force, used in Section 1.3 a Legendre Function Associated Legendre functions one of a series of numbers a numerical constant, used in Section 4.4 a length and an area, used in Section 5.4 the time components of velocity along increasing 8, X , respectively

150

u,v,z

V X

Zr 01

P Po

Y2

s” .t 51

9

?l

,f 2

L

0

x V

9

X X

**

Q

special functions, used in Section 5.4 a velocity, used in Section 1.4 sin%, used in Section 4.4 a function in a series, used in Section 4.4 the angular semiwidth of an ocean, used in Section 4.4 the value of 4Q2a2/gh the value of 4Qxa*/gh, a numerical constant, used in Section 4.2 the elevation of tide above the mean surface the elevation of the equilibrium tide above the mean surface the value of p functions of position used with 5 = r1 cos at r2 sin at functions of position used with = cos at j l ~sin at the colatitude = 35 k an angular coordinate used with a special pole in Section 4.3 the value of d ( - l ) the speed of a tidal constituent whose period is 2z/u the latitude component of velocity normal to a boundary, used in Section 5.4 a special function of position, used in Section 4.3 the longitude east of a chosen meridian an angular coordinate used with a special pole in Section 4.3 a special function of position, used in Section 4.3 the ratio x/a, used in Section 4.4 the angular speed of the earth’s rotation

r

I‘ e e

A. T. DOODSON

I2

+ +

REFERENCES 1. Doodson, A. T. (1921). The harmonic development of the tide-generating potential. Proc. Roy. Sac. (London) A100, 305-329. 2. Whewell, W. (1833). Essay towards a first approximation to a map of cotidal lines. Trans. Roy. Sac. (London) pp. 147-236. 3. Airy, G. B. (1842). Tides and Waves. Zn “Encyclopzdia Metropolitana,” Vol. 5, pp. 241-396. Griffin, London. 4. Harris, R. A. (1904). Manual of tides, IV B. U. S. Coast Geod. Survey Rept., App. 5, pp. 313-400. 5. Prufer, G. (1939). Die Gezeiten des Indischen Ozeans. Ver6ffentl. Znst. Mcercsk. Univ. Berlin [N.F.] A37, 1-56. 6. Sterneck, R. (1920). Die Gezeiten der Ozeane. Sitzber. Akad. Wiss. W i e n , A b f . , ZIa 129, 131-150. 7. Sterneck, R. (1921). Die Gezeiten der Ozeane. Sitzber. Akad. Wiss.Wien, Abt. IZa 130, 363-371. 8. Dietrich, G. (1944). Die Schwingungssysteme der halb- und eintagigen Tiden in den Ozeanen. Veroffentl. Znst. Meeresk. Univ. Berlin A41, 1-68. 9. Villain, C. (1951). Cartes des lignes cotidales dans les oceans. Ann. hydrog. (Paris) [4] 3, 269-388. 10. Laplace, P. S. (1775). Recherches sup plusieurs points du systkme du monde. Mbm. Acad. Roy. Sci. 88, 75-182; (1776). Ibid. 89, 177-267. 11. Thornson, W. (1875). On an alleged error in Laplace’s theory of the tides. Phil. Mag. [4] 60, 227-242.

OCEANIC TIDES

151

12. Doodson, A. T. (1927). Application of numerical methods of integration t o tidal dynamics. Monthly Not. Roy. A s t . SOC.Geophys. Suppl. 1, 541-557. 13. Hough, S. S. (1897). On the application of harmonic analysis to the dynamicitl theory of the tides. Trans. Roy. Sac. (London) A189, 201-257; (1899). Ibid. A191, 139-185. 14. Goldsbrough, G. R. (1913). The dynamical theory of the tides in a polar basin. Proc. London Math. Soc. 14, 31-66. 15. Goldsbrough, G. R. (1914). The dynamical theory of the tides in a zonal ocean. Proc. London Math. SOC.14, 207-229. 16. Goldsbrough, G. R. (1927). The tides in oceans on a rotating globe, I. Proc. Roy. SOC.(London) A117, 692-718. 17. Proudman, J., and Doodson, A. T. (1927). On the tides in an ocean bounded by two meridians on a non-rotating earth. Monthly Not. Roy. A s t . SOC.Geophys. Suppl. 1 , 468483. 18. Goldsbrough, G. R., and Colborne, D. C. (1929). The tides in oceans on a rotating globe, 111.Proc. Roy. SOC.(London) Al26, 1-15. 19. Colborne, D. C. (1931). The diurnal tide in an ocean bounded by two meridians. Proc. R o y . SOC.(London) A131, 38-52. 20. Goldsbrough, G. R. (1949). The tides in oceans on a rotating globe, V. Proc. Roy. SOC.(London) A2OO. 191-200. 21. Proudman, J. (1917). On the dynamical equations of the tides, I, 11, 111. Proc. London Math. SOC.18, 1-68. 22. Doodson, A. T. (1927). Application of numerical methods of integration to tidal dynamics. Monthly Not. Roy. Ast. SOC.Geophys, Suppl. 1 , 541-557. 23. Proudman, J. (1929). The forced tides in an ocean bounded by a complete meridian on a non-rotating earth. Monthly Not. Roy. Ast. SOC.Geophys. Suppl. 2, 209-213. 24. Proudman, J. (1935). Tides in oceans bounded by meridians, I, Ocean bounded by complete meridian: General equations. Trans. Roy. SOC.(London) A236,273-289. 25. Doodson, A. T. (1935). Tides in oceans bounded by meridians, 11,Ocean bounded by complete meridian. Diurnal tides. Trans. Roy. SOC.(London) A236, 290-333. 26. Doodson, A. T. (1937).Tides in oceans bounded bymeridians, 111,Ocean bounded by complete meridian: semidiurnal tides. Trans. Roy. Sac. (London) A237, 311373. 27. I)oodson, A. T. (1940) Tides in oceans bounded by meridians, IV, Series solutions in terms of angular widt.h of ocean: semidiurnal tides in narrow oceans. V, Solutions by use of finite differences: semidiurnal tides. Trans. Roy. SOC.(London) A238, 477-512. 28. Proudman, J., and Doodson, A. T. (1924). The principal constituent of the tides of the North Sea. Trans. Roy. SOC.(London) A224, 185-219. 29. Defant, A. (1924). Die Gezeiten des Atlantischen Ozeans und des Arktischen Meeres. Ann. Hydrog. (Berlin) 62, 153-166, 177-184. 30. Defant, A. (1932). Die Gezeiten und inneren Gezeitenwellen des Atlantischen Ozeans. Wiss. Erg. Deut. Atlant. Exped. “Meteor” 1965-1987, 7(1), 1-318. 31. Proudman, J. (1944). The tides of the Atlantic Ocean. Monthly Not. Roy. A s t . SOC. 104. 244-256. 32. Proudman, J. (1925). A theorem in tidal dynamics. P h i l . Mag. [S] 49, 570-579. 33. Pairbairn, L. A. (1954). The semidiurnal tides along the equator in the Indian Ocean. Trans. Roy. SOC.(London) 247, 191-212. 34. Hansen, W. (1948). Die Ermittlung der Gezeiten beliebig gestalteter Meeresgebiete mit Hilfe des Randwertverfahrens. Deut. Hydrog. 2. 1 , 157-163.

152

A. T. DOODSON

35. Hansen, W. (1952). Gezeiten und Gezeitenstrome der halbtiigigen Hauptmondtide M2in der Nordsee. Deut. Hydrog. Z . Erganzungsh. 1, 1-46. 36. Doodson, A. T., Rossiter, J. It., and Corkan, R. H. (1954). Tidal charts based on coastal data: Irish Sea. Proc. Roy. SOC.Edinburgh A 64(1), 90-101. 37. Hansen, W. (1949). Die halbtagigen Gezeiten im Nordatlantischen Ozean. Dcul. Hydrog. Z . 2, 44-51. 38. Hansen, W. (1952). Gezeiten des Meeres. I n Landolt-Bornstein, “Zahlenwerte und Funktionen,” Vol. 3, p. 516. Springer, Berlin. 39. Proudman, J. (1925). On tidal features of local coastal origin and on sea-seiches. Monthly Not. Roy. Aat. SOC.Geophgs. Suppl. 1, 247-270. 40. Poincar6, H. (1910). Th6orie des marbes,” Vol. I11 of “Leqons de M6canique c6leste,” pp. 13-19. Gauthier-Villars, Paris. 41. Proudman, J. (1917). On the dynamical equations of the tides, I, 11, 111. Proc. London Math. SOC.18, 1-68.

ULTRAVIOLET ABSORPTION PROCESSES IN THE UPPER ATMOSPHERE K. Watanabe Hawaii Institute of Geophysics and Department of Physics, University of Hawaii, Honolulu, Hawaii

Page

1. Introduction

...........................

.............................

160

3. Solar Ultraviolet and X-Rays.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Extension of Solar Spectrum 3.2. Intensity Measurements.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.3. Nz . . . . . . . . . . . . . . . . . . 4.3.1. Spectral Regio

............................ .............................

.............................

4.7. NzO.. . . . . . . . . . . . . . . .

.................................................... 4.9. 0 and N Atoms.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

153

189

200 200 201

154

K . WATANABE

4.10. Other Minor Constituents. . . . . . . . . . . ........................... ........................... 4.10.1. C H I . . . . . . . . . . . . . . . . . . . . . . . . . ........................... 4.10.2. NH3.. . . . . . . . . . . . . . . . . . . . . . . . ........................... 4.10.3. GO.. . . . . . . . . . . . . . . . . . . . . . . . . 4.10.4. Hzand Rare Gases.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.10.5. Na . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Some Atmospheric Absorption Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................................ 5.1. General.. . . . . . . . . . . . . . . . . . . . . . 5.2. Penetration of Solar Ultraviole tion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Ozone Layer., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. O2Dissociation Layer.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5. D Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6. E Layer, . . . . . . . . . . . . . ............................................ 5.7. F Layers.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of Symbols.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

201 202 202 202 202 202 202 202 203 206 207 209 209 210 210

1. INTRODUCTION 1.1. General

The upper atmosphere appears to be extremely complex. For example, its composition is apparently altered significantly or even radically by a number of processes such as photochemical reactions, recombination processes, mixing, diffusion, and intrusions of extraterrestrial particles, However, since about 1946, when rockets became available for fundamental studies, research efforts have increased; consequently, considerable advances have been made in our knowledge of the various properties of the upper atmosphere. The present review is confined to only a part of upper atmosphere research, namely, the study of absorptjon processes due to solar radiation of wavelengths from about 2900 A to 1 A. F r o q laboratory studies, it is known that photons in the extreme ultraviolet region of the spectrum are required to dissociate or ionize molecules such as 0 2 , Nz , and NO. Therefore, it is usually assumed that nearly all primary photochemical processes in our atmosphere are produced by solar ultraviolet radiation, including soft x-rays ; these processes have been studied by theoretical calculations, laboratory experiments, and rocket measurements. Direct determination of atmospheric composition a t high altitudes, particularly above 100 km, is difficult because atmospheric densities are low, duration of a rocket flight is short, and rockets are costly. However, it is possible to obtain quantitative information on atmospheric composition by theoretical calculations, provided sufficient data are available. In 1930, Chapman pioneered this type of calculation when he deduced the distribution of atmospheric ozone 111 and the transition layer for the dissociation

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

155

of molecular oxygen [2]. Since then, many theories on photochemical processes in the atmosphere have been published. Although often these theoretical studies have led to different conclusions, they have been valuable guides to experiments, and in many cases they provide the only information. This review attempts to cover the decade from 1946 to the beginning of the International Geophysical Year, or from the first rocket experiments to the first artificial satellite. Detailed discussions of earlier studies on upper atmosphere are found in a treatise by Mitra [3] and a monograph by Nicolet [4]. Early rocket experiments are described in a book by Newel1 [5]. 1 .Z. Absorption Processes

This subsection briefly summarizes several types of absorption processes important in the study of the upper atmosphere. For more complete treatments on atomic and molecular absorption processes, one may refer to treatises such as the one by Herzberg [6, 71. Included below, however, are some new results not found in these standard references. The minimum energy required to remove an electron from an atom or molecule is usually called the ionization potential. In the case of a molecule, this energy refers to the 0-0 transition involving the molecular and ionic ground states. The ionization potentials of some known and possible at-

Molecule

IP 15.426 15.580 12.08 9.25 14.013 11.1 13.18 15.13 13.79 12.59 10.47 10.15 12.3 12.90 12.80 12.34 12.99 10.51 13.91 13. S?

Reference

Atom

IP

H He Li Be B C N 0 F Ne Na

13.595 24.580 5.390 9.320 8.296 11.264 14.54 13.614 17.42 21.559 5.138 7.644 5.984 8.149 11.o 10.357 13.01 15.755 13.996 12.127

Mg

A1 Si P S c1 A Kr Xe

Reference

1%

K. WATANABE

mospheric constituents are shown in Table I. Spectroscopic and photoionization values are usually more accurate than electron impact, values, since the latter often do not correspond to “adiabatic” or 0-0 transitions. Values for OH, CN, HCN, and O3 are based on recent electron impact measurements. Results for 0 2 , NO, NH3, NzO, S02, CH4, and H2O are from photoionization measurements, while the others are spectroscopic values. Early spectroscopic values were revised by using recent physical constants [24]. The dissociation energies of some diatomic molecules which exist or possibly exist in the atmosphere are shown in Table 11. Many values are not definitely established. For example, the dissociation energies of Nz , NO, and CO have been in controversy for many years, but recent studies tend to support the so-called “higher values” which are listed in Table 11. The dissociation energies of NO+, CO+, and OH+ were obtained by cycles as follows

(14

+ I ( 0 ) - I(N0) D(CO+) = D(C0) + I(C) - I(C0)

(1.3)

D(OH+) = D(0H) 4- I(H) - I(0H)

(1.1)

D(NO+) = D(N0)

In order to study absorption processes, it is necessary or at least desirable to use tables or diagrams of energy levels. For example, Fig. 1 shows potential curves of known electronic states of the 0 2 molecule and 0 2 + ion. This diagram may be used to illustrate several types of absorption processes. In Fig. 1, transition (i)32: +-- ”2, and transition (ii)3ZL +-- ” Z , exemplify excitation processes which are represented by the reaction

(1.4)

0 2

+ hv

+ 02*

where 02*is the excited molecule. Processes of this type correspond to TABLE11. Dissociation energy D of some diatomic molecules in electron volts. Molecule

D

N2 N2+

9.76 8.73 5.115 6.65 6.49 10.85 11.11 8.36

0 2 02+

NO NO+

co co+

Reference

Molecule

D

Reference

Hz H2+ OH OH+ NH CN CH CH+

4.476 2.648 4.35 4.76 3.8 7.6 3.47 3.6

[281 1281 1281 1271 I28 I

“31 1281 P81

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

o+(~s) +o

157

18.73ev

208ev

'\

(v)

I 708ew

511ev

INTERNUCLEAR DISTANCE

(A)

FIG.1. Potential curves of the observed states of 02 and OZ+.

transitions to discrete excited electronic states, and in absorption spectra such transitions appear as discrete bands. Transition (i) gives the Herzberg hands, and transition (ii) gives the Schumann-Runge bands. Molecules in excited states usually return by emission of light to the ground state in a very short time of the order of 10-8 sec, but some excited states have much longer life. In any case, photochemical reaction may result if a molecule in an excited state collides with another molecule.

158

K. WATANABE

Dissociation of 02 as represented by the reaction (1.5)

0 2

+ hv

---f

0

+0

results in the case of transition (iii) 311u+ ” Z , in Fig. 1, since the upper level of the transition lies on the continuous portion of the 311ustate (above 5.115 ev). Both 0 atoms are in the atomic ground state 3P.Transition (iu) also leads to dissociation, but one of the 0 atoms is left in the ID metastable state. I n absorption spectra these transitions appear as dissociation continua. Transition (iii) gives the Schumann-Runge continuum, but a continuum for (iv) has not yet been definitely established. According to Wilkinson and Mulliken [29], the 31’Iustate “crosses” the state 32; at the point indicated by X , since the transition to this point, which corresponds to the 12-0 member of the Schumann-Runge bands, showed perturbation in the form of rotational line broadening. Predissociation as represented by the two-step reaction (1.6)

0 2

+ hv -+

02*

+0

+0

is expected to occur in such absorption processes involving perturbation between a discrete and a continuous state. Predissociation occurs frequently in molecular absorption, and in pronounced cases the absorption spectrum shows very diffuse bands with no detectable fine structure. Serious errors may result if such absorption processes are neglected in photochemical studies. I n Fig. 1, transition (v) illustrates a photoionization process

(1.7)

0 2

+ hv

+02+

+e

Molecular absorption of this type gives a continuous spectrum with a rather abrupt break a t the wavelength corresponding to the ioniz.‘1t‘1011 potential of the molecule [lo]. In photographic absorption spectra, such a continuum is nearly always masked by overlapping dissociation contiimi and absorption bands; however, recent studies by the photoionization method have yielded quantitative data and even revealed vibrational fine structures as exemplified by nitric oxide [lo]. Ionization may be produced by a preionixation process such as

+

+

O2 hv -+0 2 * + Ozf e (1.8) in a manner analogous to predissociation. For example, the Hopfield bands . have been shown to be definitely preionized of O2in the region 900 to 1000 & [9]. There is also the possibility of ionization by formation of an ion pair according to the reaction (1.9)

0 2

+ hv -+ o++ 0-

ULTRAVIOLET ABSORPTION PROCESSES I N UPPER ATMOSPHERE

159

This type of process has not been observed in the case of 0 2 but has been inferred in the case of BrZ and IZ . Collision processes involving both neutral and ionized particles are extremely important in photochemical reactions, since they occur simultaneously with primary absorption processes. However, types of collision processes are so numerous that only specific cases will be dealt with in Section 5. 1.5. Some DeJinitions and Equations

For the equation of state of gases in the upper atmosphere, one usually assumes the perfect gas equation

P

(1.10)

=

nKT

where P is the pressure in dynes em+, n is the number of particles K is Boltzmann’s constant, and T is the ambient air temperature in degrees Kelvin. One also assumes, as a first approximation, the hydrostatic equation (1.11)

dP

=

-pgdz

where p = nm = ambient density in gm m = average molecular mass in gm, g is the gravitational acceleration, and z is the altitude above sea level in cm. Combining equations (1.10) and (1.11), one obtains 1 dP mg -Pdz KT --

(1.12)

The quantity KT/mg is often called the scale height H . Although energy level diagrams, such as Fig. 1, inform us that certain processes are energetically possible, they do not give us the magnitude of the transition probability. Selection rules are very helpful, but they give only qualitative information. Usually the intensity of an absorption process is measured experimentally and expressed in terms of absorption crosssection. For a given wavelength of light, the absorption cross-section u in cm3 is defined for a gas by the equation (1.13)

I

=

I @-unL

where I , is the incident and I the transmitted intensity of monochromatic light in number of photons sec-1, and L is the layer thickness of gas in em. Equation (1.13) may be modified by means of equation (1.10) into the following form (1.14) I = Ioe-unox where no is Loschmidt’s number and x in em (or atmos-cm) is the equiva-

1GO

R. WATANABE

lent optical path (or layer thickness reduced to normal temperature and pressure (NTP)) . The quantity ano is usually called the absorption coegicient k , expressed in cm-l. The differential form of equation (1.13) is

d l = -IundL

(1.15)

The latter is used in calculating the penetration of solar ultraviolet radiation, because n is a function of altitude x and dx = - d L . 1 ./t. Rate of Absorption

According to Einstein’s law of photochemical equilibrium, the number of particles of a given species which absorb light quanta is equal to the number of absorbed photons for a given absorption process involving monochromatic light. Hence, using the units and symbols given in Section 1.3, we have

dni - AL = -IuniAL dt

or (1.16)

where the subscript i indicates a particular constituent. This equation is the basis of theoretical calculation of primary absorption processes. However, the application of equation (1.16) is complicated by the fact that other reactions, such as recombination, occur simultaneously with the absorption process. Theoretical calculation of photochemical reactions generally involves several variables, parameters, and equations. Equation (1.16) shows that the rate of particle transformation can be studied if sufficient data are available for the quantities n, I , and U. As Zni gives the total particle concentration, data on atmospheric densities are important and are discussed in Section 2. The I is discussed in Sect.ion 3 in terms of solar radiation and values of u are reviewed in Section 4. Finally, several theories on atmospheric photochemistry are described i n Sectioii 5. 2. DENSITY,TEMPERATURE, AND COMPOSITION OF THE UPPERATMOSPHERE 2 . i . General Some data on densities and temperatures in the upper atmosphere were available before direct measurements became possible. In 1947, the National Advisory Committee for Aeronautics (NACA) [30] presented a tentative model of the atmosphere up to 120 km. In the light of present knowl-

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

161

edge, this model gave a fairly accurate picture. It is remarkable that the NACA curve for temperature distribution showed a maximum at about 50 km and a minimum a t 80 km; however, the temperature in the region above about 40 km appears to be consistently higher than recent values. Detailed descriptions of some indirect methods used in the study of atmospheric structure have been described by Mitra [3]; and in particular, meteoritic techniques have been reviewed by Whipple [31]. In addition, Elterman [32] has reported the results of a search-light technique. The first successful direct measurement of pressure in the upper atmosphere was reported in 1946 by Best et al. [33] for the altitude region from 50 to 90 km attained by a V-2 rocket. Since then, these investigators and their co-workers [34-361 have reported a number of experiments carried out with rocket-borne vacuum gauges, such as a Pirani gauge and a Philips coldcathode ionization gauge. In most cases, pressure was the primary quantity measured as a function of altitude, and in some cases, density; whereas temperature was derived from equations (1 .lo) and (1.11) by assuming values of average molecular mass. Other direct methods have been used successfully to measure pressure, density, and temperature in the upper atmosphere. Ference, Stroud, Walsh, Weisner, and co-workers [37-391 have studied sound from exploding rocket grenades to determine temperatures and winds a t altitudes of 30 to 80 km. Byram et al. [40] have determined atmospheric densities in the region from 95 to 130 km by measuring absorption of solar x-rays with photon counters. Sicinski et al. [41], Bartman [42], and Liu [43] a t the University of Michigan have used several different techniques to obtain upper air pressure, density, and temperature. On the whole the results are consistent, and present knowledge of atmospheric structure appears quite reliable for altitudes up to about 70 km, somewhat uncertain in the region from about 70 to 150 km, and more or less speculative for higher altitudes. Data obtained up to 1952 have been and a comprecompiled, averaged, and reported by the Rocket Panel [44], hensive review of the work done during the period 1946-1954 has been reported by Newel1 [45]. It is desirable that the reader refer to these reports, because the present review will give emphasis to very recent papers. In the following sections, some of the data on atmospheric structure are tabulated and discussed in order to set up an atmospheric model which will be used for subsequent discussions of atmospheric absorption and photochemistry. 2.2. Data on Density and Temperature

In Table III are shown some of the published values of atmospheric densities for various altitudes in the region from 50 to 220 km. Values in

162

K. WATANABE

TABLE 111. Atmospheric densities

p

in gm ~ r n -at~ various altitudes.

Altitude in km Havens et al. [34] Rocket Panel

I441

50

60 70 80

90 100 110 120 130 140 150 160 170 180 190 200 210 220

1.3 X 3 . 8 x 10-7 1.2 x 10-7 2.5X 4.0 x 10-9 8 X 1Wlo 2.0 x 10-10 5.0 X lo-" 2.0x 10-11 7 x 10-12 3.0X 1.5X

1.16 X 3.49 x 9.73 x 2.11 x 4.08 x 8.61 X 2.07 X 5.64 X 1.91x 7.60X 3.40 X 1.66 X 8.67 X 4.81x 2.81 X 1.71 x 1.08X 7.02 X

Byram et al.

[401

Horowita and LaGow [36]

10-6

10-7 10-8 10-8 10-9 10-lO 10-11

10-11 10-l2 10-12

10-l2 10-13 10-13

10-13 10-13

3.0X 10-lo 6.2 X 10-11 1.6 X 10-"

2.5X 5.0 X 1.2x 3.3 x 1.2x 6.6X 4.3 x 3.0x 2.3x 1.8x 1.4x 1.1 x 9.0 x

lo-" 10-11 10-12 10-12

10-13

10-13 10-13 10-13 10-13 10-13 10-14

the second column were reported in 1952 by Havens et al. [34], who based their results on data obtained with several rocket flights. Values in the third column are averaged results which the Rocket Panel [44] recommended as a "representation of the combined results to January 1952" rather than as a standard atmosphere. Recent solar x-ray measurements seem to show that densities recommended by the Rocket Panel for the region about 100 km are too high by a factor of two to ten; values shown in the fourth column were obtained by Byram et al. [40] with an x-ray photon counter having a Mylar window. In the last column are shown the results reported recently by Horowitz and LaGow [36]. Their values are as much as a factor of six lower than those by the Rocket Panel. I n order to decide between the high and low values in Table I11 for altitudes in the region from 100 to 150 km, it is necessary to consider possible observational errors as well as the possibility of real variations in the atmospheric densities. One of the main sources of error in measurements using pressure gauges is the effect of outgassing from the rocket and the gauge assembly, particularly for pressures below mm Hg [45]. In this connection, the work of Byram et al. [40] is significant, since outgassing has only a negligible effect in their type of measurements. Although Horowite and LaGow [36] used a Philips gauge, they made corrections for residual

ULTRAVIOLET ABSORPTION

PROCESSES IN UPPER ATMOSPHERE

ALTITUDE

163

(KY)

FIG.2. Atmospheric densities in the altitude region from 60 t o 150 km.

gas and velocity ram. Thus the lower values for the region near 100 km in Table I11 apparently are more reliable. If we accept the lower value a t 100 km, it is necessary to extrapolate the data down to lower altitudes. A fairly smooth extrapolation can be made down to about 65 km where the curve can be made to agree with the Rocket Panel curve, because a t this altitude the latter curve is considered reliable. Such an extrapolation is shown in Fig. 2 by the broken curve. Further discussion of this curve is given in Section 2.4. Most of the data on atmospheric temperatures for altitudes in the region from 50 to 220 km, as shown in Table IV, are derived from the data in Table I11 or similar pressure data. Values in the second column are by the Rocket Panel [MI, the third column by Johnson [46], and the fourth column by Horowitz and LaGow [36]. Values in the last column are by Stroud et al. [39] who used the method of rocket grenade. 2.3. Composition

The study of atmospheric composition and the study of absorption of solar radiation are directly related and usually proceed together. For example, it is possible to determine atmospheric composition from absorption measurements, and reciprocally, data on composition assist calculations of absorption processes.

164

K. WATANABE

TABLEIV. Atmospheric temperatures T in degrees Kelvin a t various altitudes. Altitude in km 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220

T Rocket Panel [44]

Johnson [46]

270.8 252.8 218.0 205.0 217.0 240.0 270.0 330.0 390.0 447.0 503.0 560.0 618.7 676.9 734.9 792.5 849.8 906.6

27 1 245 210 185 183 200 230 280 423 594 737 846 925 979 1043 1061 1074 1082

~-

Horowitz and LaGow [36]

Stroud et al. [39] 268 250 224 200

210 228 270 428 700 880 980 1050 1070 1070 1070 1070 1070

TABLE.^. Abundance of atmospheric constituents in atmos-cm. Constituent

Nz 0 2

A

coz Ne HI3

CHI

Abundance 624,600 167,600 7440 320 14.6 4.2 1.2

I

Reference Constituent [471 [471 [471 1521 [481 ~481 [521

Kr NzO Hz 0 3

Xe

co

Hz0

Abundance 0.8 0.4 0.4 0.2-0.3 0.06 0.06-0.15 10~104

Reference [471 ~521 [471 [521 [47I 1521 [521

Table V shows the abundance of some known atmospheric constituents expressed in atmos-cm. Values by Paneth [47] and Gluckauf [48] for rare gases and rather “permanent” gases are based on analyses of samples obtained near the earth’s surface and on the assumption of complete mixing. This assumption is an acceptable one, as (1) 99.9 % of the total atmosphere lies below about 50 km, (2) diffusive separation apparently is negligible below about 70 km [49-511, and (3) even gases like 0 2 are dissociated only to a very small degree below 100 km. The abundance of gases such as COz , NzO, and CHI was reported by Goldberg [52] and is based on infra-red ab-

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

165

sorption measurements. It appears that N20 and CH4 gases are uniformly mixed in the lower atmosphere [52]. On the other hand, ozone is produced photochemically in the atmosphere, and its maximum concentration occurs in the altitude range from about 20 to 30 km. Although water vapor is known to be confined below about 20 km, traces of it may escape through the temperature minimum at this altitude. Among the minor constituents of importance to the upper atmosphere, ozone has been studied most thoroughly, and publications on this subject have been reviewed very thoroughly [3, 53-56]. Recently, measurements have been extended to altitudes of up to 70 km by Johnson et al. [57, 581. According to them, ozone appears to be in photochemical equilibrium a t altitudes from about 40 km up to a t least 70 km. The results show that ozone concentration a t 40 km is about 4.6 x loll molecules per cm3 or about one part per 2 X lo6, and a t 70 km, 6 X lo8 molecules per cm3 or about one part per 3 X lo6.Thus, the ozone concentration decreases with altitude faster than the total concentration, but may still be significant at altitudes near 100 km. Minor constituents of such small concentrations may play important roles in the atmosphere. For example, the D layer (60 to 90 km)is generally ascribed to the ionization of nitric oxide; however, this gas has not been definitely observed in the D layer. The region of 0 2 dissociation predicted by Chapman [2] was investigated recently by Byram et al. [59]. They measured the solar radiation in the spectral region from 1425 to 1500 by means of photon counters a t altitudes between 110 and 130 km. To obtain 0 2 concentration, they used a mean absorption coefficient of 386 cm-l for 0 2 in the above spectral region. Their results show that O2 appears to be about 45% dissociated a t 110 km, but that the transition layer for 0 2 dissociation is much broader than those predicted by theories assuming photochemical equilibrium. Evidently there is considerable mixing and diffusion. Further discussion of this topic will be deferred to the section on photochemistry. Townsend, Meadows, Pressly, Johnson, and Heppner [60-641 have studied the composition of neutral and ionic constituents in the upper atmosphere by means of Bennet type, radio-frequency mass spectrometers. They reported observation of 02+,0+, CN+, NO+, Na+, and possibly OH+ in one flight which reached 219 km [61], while in another flight Johnson and Heppner [64]observed negative ions, probably N O z , 02, and 0-, but no positive ions. In spite of several successful flights, their interpretation of data is beset by some difficulties, particularly that arising from probable contamination by gases from the rocket. Numerous investigators have measured the electron densities in the upper atmosphere by the method of radio wave propagation. In his book, Mitra [3] has given a thorough treatment of the theoretical basis of the

166

K . WATANABE

method, the apparatus, and the results of measurements. Here, it should be mentioned that radio propagation methods have been carried out with the aid of rockets by Seddon, Jackson, and co-workers [65-681 and by Lien et al. [69, 701. Furthermore, ion densities have been measured with rocketborne probe instruments by Hok et al. [71].

2.4. Model Atmosphere Our concept of “model atmosphere” has changed considerably during the past decade with the addition of new experimental data. Early rocket experiments [33, 341 showed that atmospheric densities in the region above 70 km is lower than those of the NACA model [30] by a factor of 3 or 4. As described in Section 2.2, Byram et al. [40] and Horowitz and LaGow [36] have found in the region around 100 km still lower densities. Furthermore, different mean molecular weights have been assumed for the region above 100 km by various investigators, and different theories have been employed for the region above 200 km where experimental data are almost entirely lacking. Accordingly, different model atmospheres have been proposed and used from time to time. It is hoped that experiments carried out with rockets and artificial satellites will soon reduce these uncertainties. I n the study of photochemical problems, it is usually convenient to use the total particle concentration rather than the pressure or densities. Furthermore, reliable data on dissociation of the major atmospheric constituents, O2 and Nz , are useful in estimating mean molecular weight. Particle concentrations of O z , 0, Nz, and N have been estimated, for example, by Bates [72], Havens et al. [73], and by Kallmann el al. [74]. However, as yet there are no experimental data for the region above about 130 km. Four models of total particle concentration are shown in Table VI for comparison. Values in the second column were computed from Rocket Panel data [44] on density p and mean molecular weight p according to the equation (2.1)

n

=

NOP/P

where N o is Avogadro’s number. In this case, p decreases from 28.97 to 14.55 in the region from 80 to 200 km. Values in the third column were proposed by Havens et al. [73]. For the region between 100 and 130 km, their values are considerably higher than corresponding ones by other investigators. Values in the fourth column by Kallmann et al. [74] are little lower than those based on Rocket Panel data. Values in the last column by Johnson [46] are a factor of 2 to 3 lower than Rocket Panel values for the region from 100 to 180 km, in harmony with x-ray data [40]. For the purpose of calculating absorption of solar ultraviolet radiation, it is necessary to use a model atmosphere; and one must make a selection,

ULTRAVIOLET

ABSORPTION

PROCESSES IN UPPER ATMOSPHERE

167

TABLE VI. Total particle concentration n in ~ r n a- t~ various altitudes. n

Height in km 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 350 400

Rocket Panel (1952) [44] 2.4 X 7.3 x 2.0 x 4.4 x 8.9 x 2.0 x 5.0 X 1.4 X 5.1 X 2.2 x 1.0 x 5.2 X 2.9 X 1.7 X 1.0 x 6.5 x 4.3 x 2.9 x

1Ol8 1015 1016 1014 1013 1013 10l2 10l2 10" 10" 10" 10'O 1Olo 1O1O 10'0 109 109 109

Havens et al. (1955) [73]

2.3 x 1013 7.4 x 1012 2.7 X loL2 1.2 x 1012 7.2 X lo1"

6.3 X 10'

Kallman et al. (1956) [74]

8.57 x 1.74 x 4.28 X 1.28 X 4.53 x 1.85 X 8.45 X 4.13 X 2.16 X 1.19 x 6.89 x 4.18 x

1013 1013 10l2 1012 10" lox1 1O1O 10'0 10'0 10'0 109 109

1.72 X 10'

1.5 X 10'

7.94 x 108 5.50 x 108 4.03 X lo8 2.23 X lo8

4.7 x 108 1.7 X lo8 6.8 X 10'

1.34 X lo8

Johnson (1956) [46] 2.4 X 7.4 x 2.0 x 4.1 x 6.6 X 1.1 x 2.0 x 5.1 X 2.1 x 5.8 X 3.0 X 1.9 x 1.3 X 9.3 x 7.0 x 5.4 x 4.3 x 3.4 x 2.8 x 2.3 x 1.9 x 1.6 x 1.3 x 1.1 x 9.1 x 7.7 x

10l6 1015 1015 1014 lOI3

1013

10'2 1011 10" 1O1O

10'" 10'0 10'0 109 109 109 109 109 109 109 109 109 109 108 108 108

even if it is somewhat arbitrary. In view of the fact that the x-ray absorption method is free from the problem of rocket outgassing, we may give preference to the results of this method. There is another experiment which indirectly supports the lower densities obtained by x-ray absorption. Byram et al. [75], in their observation of solar Lyman-alpha radiation by means of photon counters, reported that the apparent absorption coefficient of air deduced with Rocket Panel pressures is significantly lower than laboratory values by Preston [76] and by Watanabe et al. [77]. This discrepancy may be accounted for by an error of about 30% in the Rocket Panel pressure data. In fact, a smooth extrapolation of the lower density curve in Fig. 2 down to 65 or 70 km will remove this discrepancy.

168

K . WATANABE

TABLEVII. A tentative reference atmosphere. Altitude in km 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 160 170 180 190 200

in gm/cm3

P in mm Hg

3.5 x 1.8 x 8.1 X 3.2 x 1.2 x 4.3 x 1.7 x 6.5 X 2.7 X 1.2 x 5.3 x 2.6 X 1.3 X 6.9 x 3.9 x 2.4 X 1.4 X 9.3 x 6.5 x 3.5 x 2.1 x 1.4 x 9.8 x 6.9 x

1.8 X 7.9 x 3.2 X 1.2 x 4.5 x 1.7 X 6.9 x 2.8 x 1.2 x 5.9 X 3.0 X 1.6 X 9.0 X 5.4 X 3.5 x 2.4 X 1.7 X 1.3 X 9.8 X 6.2 x 4.2 x 3.0 x 2.1 x 1.5 x

P

10-7 10-7

10-8 10-9 10-9

10-lo l0-l0 10-10

10-11

lo-"

10-" 10-l2 10-12 10-l2 10-13 10-13 10-13 10-13

10-13 10-14

10-14

10-1 10-2

10-2

10-3 loV3 10-4 10-4 10-4

lo-&

10-6

lOW 10-7

10-7 10-7 10-7 10-7

n

T

5

in cm-3

in "K

in atmos-cm

7.3 X 3.7 x 1.7 X 6.5 x 2.5 x 8.9 X 3.5 x 1.4 x 5.5 x 2.5 X 1.2 X 6.0 X 3.1 X 1.6 X 9.2 X 5.6 X 3.4 X 2.1 X 1.6 X 8.4 x 5.1 x 3.5 x 2.4 x 1.7 x

10l6 10'6 10l6 1014 1014 1013 1013 1013 1012

10l2 10l2 10" 10" 10" 1Olo 10'0 1O1O 10'0 1O'O 109 109 109 109 109

235 205 185 180 175 185 195 205 215 225 235 255 280 325 355 410 485 550 605 715 800 830 830 830

lo2 10' 10' lo1 10" 1.8 x 10" 7.3 x 10-1 3.0 X 10-I 1.4 X 10-l 6.8 X 3 . 5 x 10-2 1.9 x 10-2 1.1 x 1 0 - 2 6.5 x 10-3 4.2 x 10-3 2.9 x 10-3 2.1 x 10-3 1.6 x 10-3 1.2 x 1 0 - 3 7.8 x 10-4 5.3 x 10-4 3.8 x 10-4 2.6 x 10-4 1.9 x 10-4

1.8 X 8.3 X 3.3 x 1.3 X 4.8 X

Table VII shows a reference atmosphere [78] used by the author. It was obtained from the lower curve in Fig. 2, including the extrapolated region from 65 to 100 km. In the region above 150 km, the curve in Fig. 2 was extrapolated by using a slope lying between the slope of the Rocket Panel curve and that of Horowitz and LaGow [36]. It was assumed that observed densities a t altitudes above 150 km are probably too high. The results of O2 measurements [59] were used as a guide to estimate mean molecular weights, which were made to vary from 29.0 a t 100 km to 24.0 at 200 km. The total particle concentration was obtained by equation (2.1), arid thc equivalent optical path for vertical incidence, x in atmos-cm, was obtained by the equation

where V ois the molar volume. Values of z are shown in the last columii of Table VII.

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

169

AND X-RAYS 3. SOLARULTRAVIOLET

3.1. Extension of Solar Spectrum

I n order to photograph the solar spectrum in the region below 2900 A, it is necessary to send spectrographs up to altitudes exceeding those attained by balloons or, in other words, well above the ozonosphere. I n 1934, Iiegener and Regener 1791 obtained the solar ultraviolet spectrum a t an altitude of 29.3 km with a balloon-borne spectrograph, but the spectrum extended only to about 28758. Other balloon experiments gave similar results. However, by means of rockets a number of spectrographs, photon counters, and other devices have been sent to altitudes exceeding 100 km, and important advances in our knowledge of the solar spectrum have been made. The first major extension of the solar spectrum in the region below 3000 8 was reported in 1946 by Baum et al. [80]. Using a compact spectrograph with a 40-cm grating ruled with 15,000 lines per inch, they succeeded in photographing the solar spectrum a t altitudes up to 88 km and thereby extended the spectrum to about 2100 8. Similarly, Hopfield and Clearman [81] obtained the solar spectrum in the region from 2200 to 3000 8 with grating spectrographs a t altitudes up to 155 km. During the following several years, a number of attempts were made to extend the spectrum to the region below 2000 8, but this task proved to be more difficult than had been anticipated. It appeared that the intensity of the solar spectrum in the region below 2000A is remarkably weak. Furthermore, efficiencies of spectrographs and films are comparatively poor in this spectral region. Thus, long exposures seemed necessary. Progress on rocket spectroscopy up to about 1952 was reported by Tousey [82]. At the close of 1952, Pietenpol, Rense, et al. [83, 841 succeeded in recording the solar Lyman-alpha line a t 1215.6 A in emission. They were able to obtain a 28-sec exposure of the sun’s spectrum a t about 81 km altitude by means of a biaxial sun-follower [85] and a grazing incidence spectrograph. More recently, Johnson et al. [86,87] and Jursa et al. [88] also photographed the solar Lyman-alpha with spectrographs (normal incidence) and biaxial sun-followers. Furthermore, these investigators observed a number of other emission lines in the region between 1000 and 2000 8. The shortest wavelength line observed so far appears to be a CIII line a t 977.0 8 reported by Johnson et al. [86]. The most revealing spectrum by these investigators shows that (a) the solar continuum is very weak in the region below 2000 8, (b) Lyman-beta a t 1025.7 8 penetrates at least down to 117 km, and (c) some of the prominent emission lines are due to SiIV, CIV, and OVI. The study of the solar ultraviolet spectrum is important to solar physics as well as to atmospheric physics. Analyses of the solar spectrum for the

170

K . WATANABE

region between 2000 and 3000 8 have been reported by Durand et al. [89], Clearman [90], and Wilson et al. [91]. Also directly concerned with solar physics is the limb darkening of the solar disk in Lyman-alpha radi at'1011 as reported by Miller et al. [92]. 3.2. Intensity Measurements

I n order t o carry out quantitative calculations of atmospheric photochemical processes, it is necessary to have reliable data on absolute spectral intensities of solar ultraviolet radiation including soft x-rays. Unfortunately, spectral intensity measurements in the vacuum ultraviolet region of the spectrum are somewhat difficult, and it has been necessary to develop new techniques and new detectors. Furthermore, theoretical estimates of solar ultraviolet radiation of wavelengths less than 2000A are not sufficiently quantitative, because our knowledge of the solar atmosphere, which apparently contributes a major portion of this radiation, is still inadequate. A rocket-borne detector for absolute energy measurement must be culibrated against some reliable laboratory standard. The first important step in intensity measurements for the ultraviolet was carried out by Packer and Lock [93] in 1951. They succeeded in measuring intensities of a dispersed Hz emission spectrum in the region from 900 to 2500 8 by means of a calibrated thermocouple. Subsequently, Watanabe and Inn [94] and Wainfan et al. [95] also made similar intensity measurements which included some wavelengths below 900 A. These measurements with thermocouples permitted calibration of secondary detectors such as a phosphorcoated photomultiplier [94, 96, 971, CaS04:Mn phosphor [98, 991, photon counters [lOO], an ion chamber with nitric oxide [lo] and xenon [Ill, photocells [101], and photographic emulsions [84, 921. Byram et al. [40] have calibrated photon counters for soft x-rays. They found that for x-rays of about 40 8 the counting efficiency of a Geiger counter was better than 99 7%. 3.3. Observed Intensities of Solar Ultraviolet and X-Rays The solar spectral intensity outside of the earth's atmosphere for the region from 2000 to 3 4 0 0 8 has been reported by Johnson et al. [102]. According to their curve, the intensity at 3000 corresponds to emission from the sun as a blackbody of 57OO0K, a t 2400 about 5O0O0K, and the extrapolated value a t 2 0 0 0 8 about 4500°K. Thus the solar intensity decreases rapidly with decreasing wavelength. This deficiency of ultraviolet is, however, consistent with observed solar spectra referred to in Section 3.1, which showed many absorption lines due to various elements in the solar atmosphere. The intensity curve for the cited spectral region is valuable in the study of the ozonosphere.

ULTRAVIOLET ABSORPTION PROCESSES I N UPPER ATMOSPHERE

171

The first successful attempts to measure solar radiation in the region below 2000 8 were made in 1948. Burnight [103, 1041 detected x-rays by means of photographic films with filters, and Tousey et at. [99] used strips of thermoluminescent CaSOl: Mn hosphor and detected radiation in the following spectral regions: 0 to 8 1100 to 1340 8, 1230 to 1340 8, and 0 to 1340 8. Friedman, Byram, and Lichtman [loo] successfully measured radiation in the regions: 0 to 8 A, 1100 to 1350 8, 1425 to 1725 A, and 1700 to 2000 8 with Geiger-Muller-type photon counters. All of these experiments measured undispersed radiation by the filter method. Subsequent intensity measurements have been carried out mainly with photon counters [40, 75, 105-1071, while only a few intensity measurements using spectrographs have been reported [84, 1081. Most of the published results on solar intensities in the spectral region below 2000 8 are summarized in Tables VIII and IX. As shown by these tables, observed intensities vary over a wide range. Some of the variation is no doubt real, but apparently part of the variation is due to uncertainties in calibration and interpretation. It is seen from Table VIII that observed intensities for Lyman-alpha range from 0.1 to 10 ergs cm-2 sec-' by the photon counter method and from 0.4 to 3 ergs cm-2 sec-l by other methods. Such large variation is not inconsistent with the phenomenon of radio fadeout, but at present it is not certain whether this variation is as much as a factor of hundred; de Jager [lo91 suggests a factor of two. It appears that the mean intensity of Lyman-alpha is near 6 ergs cmV2sec-l. Although photon counters are efficient detectors of x-rays, there seems to be some uncertainty in the interpretation of data. For example, x-ray flux between 7 and 1 0 8 for September 29, 1949 was originally reported

1,

TABLEVIII. Observed intensities of solar Lyman-alpha in erg cm+ sec-1. ~

Date Observed Yr Mo Day 1950 1949 1952 1952 1955 1955 1955 1955 1956 1956 1956

2 9 5 12 12 10 10 11 7 7 7

17 29 5 12 13 18 22 4 17 20 25

Intensity

Method

0.4 1-10 0.1 0.5 3.0 5.7 4.0 9.2 6.1 6.1 6.7

CaSO1:Mn phosphor Photon counter Photon counter Photographic emulsion Photographic emulsion Ion chamber Ion chamber Ion chamber Ion chamber Ion chamber Ion chamber

Reference

172

K. WATANABE TABLE

Ix. Observed intensities of solar x-rays in erg

Date Observed

Yr

Mo

Day

1948 1949 1950 1949 1952 1952 1952 1952 1953 1953 1953 1953 1953

12 12 11 9 5 5 12 12 11 11 12 12 12

9 17 21 29 1

5 15 15 15 15 1 1 1

Spectra' range

Intensity

Method

0-8 0-8 8-12 6-10

Detected Detected 0.003 0.0001 0.0004 0.00001 0.2-0.6 1.0 0.0015 0.064 0.0004 0.023 0.053

Photog. emulsion CaSO4:Mn phosphor Photon counter Photon counter Photon counter Photon counter Photon counter Photon counter Photon counter Photon counter Photon counter Photon counter Photon counter

6-7 6-7 8-20 8-60 8-20 44-100 8-20 44-60 44-100

sec-l. Reference

[loo] as 10-4 erg cm-2 sec-', but recent interpretation [40] suggests as high as 0.44 erg cm-2 sec-' by assuming a source temperature of lo6 OK and a value of 0.01 erg cm-2 sec-l by assuming 2 X los OK. Forfurther discussion on interpretations of x-ray data, the reader should consult a paper by Byram et aZ. [40]. I n spite of the mentioned difficulty of interpretation, the x-ray data appear to be consistent. I n the region below about 10 8, x-ray intensities are erg cm-2 sec-l) ; however, according to usually low (order of lov3 or Friedman and Chubb [I061 there is evidence tkat, during periods of increased solar activity, x-rays as short as 1 to 2 A are produced and penetrate into the D layer. In thisoconnection, it is interesting to note that x-rays of wavelengths less than 8 A were observed with certainty by means of CaSOI:Mn phosphor only on one occasion [99], and that was during a sudden ionospheric disturbance. According to Friedman [lSO], during :I quiet period of solar activity the entire flux for the region from 8 to 100 1 amounted to 0.05 erg cm-2 sec-l. Assuming a source temperature of 7 X SO5 degrees, Byram et al. [40] have obtained a total x-ray intensity of 0.1 erg cm-2 sec-'. The intensity of solar radiation in the region around 1450 8 as measured by photon counters corresponds to emission of the sun as a blackbody of 4500°K [loo] or 4000°K [59]. These results are consistent with recent spectrograms of the sun [86, 881 which indicate a very weak continuum :~nd show no emission lines in the region between 1400 and 1500 8.

ULTRAVIOLET ABSORPTION PROCESSES I N UPPER ATMOSPHERE

173

3.4. Theoretical Estimates

For many years, it has been inferred from various indirect evidences that photospheric emission of the sun at 5000 or 6000°K does not supply an adequate number of photons in the extreme ultraviolet to account for the behavior of the earth’s ionized layers. Therefore, a number of investigators have speculated on the nature of the “excess” radiation in the extreme ultraviolet and x-ray regions. Recent advances in our knowledge of the structure of the solar atmosphere permit some calculations on the ultraviolet and x-ray intensity of the sun [111-1181. It appears that both the chromosphere and the corona are important sources of emission lines and continua in the spectral region below 2000 A. In 1947, Bondi et al. [ l l l ] calculated the intensity of high-energy photons emitted by the sun, and Hoyle and Bates [118] estimated that this intensity of photons (-40 d) was adequate to account for the E layer ionization. Woolley and Allen [112], after examining the structure of the corona, suggested that the emission of the Lyman continuum ( < 9 1 2 8 ) from the chromosphere is stronger than the coronal emission. Later, they [113] estimated that the chromosphere emits about 7 X 1014 quanta cm-2 sec-l which is capable of ionizing terrestial gases. Their calculations show that line emission from the chromosphere is the most important source of the solar extreme ultraviolet. The line emission includes emission from CII-VI, NII-VII, OII-VII, and SiII-X. It is interesting to note that solar spectrograms by Johnson et al. [86] show a number of emission lines from such multiply ionized atoms in the spectral region from 900 to 1800 8. Elwert [115-1171 in his recent detailed study of the emission from the solar corona deduced that the intensity maximum of soft x-rays occurs a t about 50 A, that line emission is stronger than continuous emission, and that the flux incident on the earth’s atmosphere is about 0.1 erg cm-2 sec-I [117]. Recently, de Jager [log] has made a critical comparison of the various theoretical calculations and results of rocket measurements. This work essentially summarizes our present knowledge of the solar spectrum in the region below 3000 d, although a few recent reports on solar intensities are not included. Figure 3 is modeled after de Jager [lo91 but includes a few modifications such as a higher value for Lyman-alpha based on recent, reports [92, 1071 and positions of other observed emission lines.

4. ABSORPTION CROSS-SECTIONS OF ATMOSPHERIC GASES 4.1. Experimental Method

Exact quantum-mechanical calculation of atomic and molecular absorption cross-section is, in general, an extremely difficult task. Although con-

174

K. WATANABE

0

10

50 100

200

600

lo00

WAVELENGTH

I

I

I

J

Is00

2000

2500

3000

(8)

FIG3. Intensity of vertically incident solar radiation in the spectral region below

3000 A.

siderable progress in such calculations has been made for some atoms, we must rely almost entirely on experimental determinations in the p e of molecules. Measurements in the spectral region below about 2000 A have been made as early as 1901 by Kreusler [119], who determined the absorption cross-section of air at 1860 A. However, most of the results on crosssections of atmospheric gases have been reported during the past several years by several groups of investigators. Available data are quite extensive, but there remain a number of gaps and discrepancies in published results. This situation may be ascribed to the fact that quantitative measurements are more difficult in the vacuum ultraviolet region than a t longer wavelengths. Most investigators have used vacuum spectrographs and photographic photometry. This method requires extreme care in the calibration of photographic plates. If the spectrograph itself is filled with an absorbing gas, as it is usually done, this gas is likely to diffuse into the light source and seriously affect its spectral intensity, thus nullifying the assumption that the incident intensity is constant. If an absorption cell is placed between the light source and the entrance slit of a spectrograph, photochemical decomposition of the sample gas may be appreciable, as noted by Ladenburg and van Voorhis [120]; and furthermore, window materials, such as LiF, tend to form color centers or become coated with sputtered material from the light source. Systematic errors are possible in indirect pressure measurements, particularly in Aow-

ULTRAVIOLET ABSORPTION

PROCESSES IN UPPER ATMOSPHERE

175

type experiments where windows are not utilized. Very often, extremely pure samples of gas are required, and contamination must be avoided. I n view of the fact that for the near ultraviolet and visible regions most investigators use photoelectric techniques and obtain generally consistent results, it seemed worthwhile to develop similar techniques for the vacuum ultraviolet. Preston [76] used an argon-filled Pt photocell with a LiF wind:w to measure cross sections of several gases a t Lyman-alpha (1215.6 A). Recently, Watanabe, Inn, Zelikoff, Marmo, et al. [9-11, 16, 121-1291 used phosphor-coated photomultipliers with one-meter monochromators to measure cross-sections of several atmospheric gases in the region 850 to 2500 8. The experimental arrangement used by them [I221 is shown schematically in Fig. 4. The absorption cell was placed behind the exit slit to avoid photodecomposition of sample gases. For the spectral region doJvn to 1050 8, the cell was provided with LiF windows; and below 1050 A, the exit slit itself served as the cell window [9], thus keeping the light source effectively separated from the sample gas. Photomultipliers coated with sodium salicylate were found [94, 961 to be linear detectors with remarkably constant quantum efficiency in the region from 850 to 2500 8 and to yield reproducible results. The use of such linear detectors eliminated the laborious and difficult task of calibration involved in photographic photometry and thereby greatly simplkfied data analyses and minimized systematic errors. A resolution of 0.1 A has been obtained [130] with a McPherson one-meter monochromator, so that it should be possible to achieve

-n

I-Meter Orating

AMPLIFIER

/”

RECORDER

W

FIG.4. Experimerit:d arrangement for absorption measurements by B photoelectric method. Light source (D)with quartz capillary ( Q ) , pressure gauges ( G I , Gz, MI , and M z ) , phosphor-coated photomultiplier ( P ).

176

K . WATANABE

--

TO GAS FILLING SYSTEM

c-l

- 1

c-2

MONOCHROMATOR

c FIG.5. Ion chamber with parallel electrodes C-1 and C-2 for study of photoionization.

higher resolution with instruments of greater dispersion. Curtis and others [131, 1321 have extended the photoelectric method to the region from 100 to 900 8, and Hinteregger I1331 has developed a double beam type of photoelectric detector. Thus the photoelectric method appears very promising. Photoionization cross-section ui is defined by the equation (4.1)

ui =

(ni/nt)a

where Q is the total absorption cross-section in cm2 as defined in Section 1.3, nt the total number of photons absorbed per second, and ni the number of molecules ionized per second; ni is obtained by measuring ion currents produced in an ion chamber, such as shown in Fig. 5, placed a t the exit slit of a monochromator. The number of photons entering the ion chamber can be determined by thermocouple measurements. In the case of total absorption, this number is equal to nt . Thus it is possible to determine u i from u. Ionization potentials can be measured to within about 0.01 ev with an ion chamber and 0.001 ev by gas-multiplication techniques, using either a counter [130] or a proportional gas-multiplication chamber [ 1341. For intensity measurements, it is desirable to use gratings with high reflectivity. Platinized gratings [135] appear to be better than aluminized gratings; the reflectivity of the latter becomes very poor with time, particularly under exposure to vacuum ultraviolet radiation. This deterioration is most pronounced in the region below 1400 8. 4.2. 0 2

The absorption spectrum of 0 2 in the vacuum ultraviolet is fairly well known from the work of many investigators [136-1441, and its absorption

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

177

cross-sections have been measured more extensively [9, 76, 77, 95, 119-123, 145-1551 than those of any other gas. Figure 1 shows the potential energy diagram of the 0 2 molecule. For convenience of discussion, the 0 2 spectrum is divided into four regions. 4.2.1. Spectral Region 1760 to 2000 d. The Schumann-Runge bands occupy this region. The rotational analyses of these bands have been made by Curry and Herzberg [140] and by Knauss and Ballard [141]. Recently, these bands have been studied under high dispersion by Brix and Herzberg [144] and by Wilkinson and Mulliken [29]. The latter investigators have discussed the predissociation of 0 2 by absorption in the Schumann-Runge bands and concluded that the photochemical dissociation [156, 1571 of oxygen in the region 1750 to 1850 8 consists of predissociation and direct dissociation involving the 311, state (see Fig. 1). The absorption cross-section for this region is not sufficiently quantitative, as the spectrum is too complicated for photometric measurements with low resolution. Figure 6 shows the u values of the various band maxima and minima obtained with a resolution of 1 8 [123]; the maxima are probably too low and minima too high. The curve is consistent with Kreusler's value [119] at 1860 A and the results of Buisson et at. [145] at 1855, 1858, and 1863 8. Furthermore, the k values 0.34 and 0.22 cm-' [123] for 1782

1750

1800

1850

1900

WAVELENGTH (%I FIG.6. Absorption cross-section of 0 2 in the region 1750 t o 1950 A, showing the band heads and minima of the Schumann-Runge bands. 0 by Kreusler [119] and by Buisson et al [145].

+

178

K. WATANABE

and 1795 8, respectively, are remarkably close to the values 0.44 and 0.28 cm-1 obtained by Wilkinson and Mulliken [29] under high dispersion. 4.2.2. Spectral Region 1250 to 1750 b. This region shows essentially continuous absorption, but recently Tanaka [143] and Watanabe et al. [la11 reported three diffuse bands or narrow continua a t about 1293, 1332, and 1352 A. These bands escaped photographic detection for many years but were readily observed by the photoelectric method even using the manylined spectrum of hydrogen as the source. Tanaka observed them with a Lyman continuum and a 3-meter grazing incidence spectrograph. He suggested that the band a t 1293 8 leads to the dissociation product 3P and the other two to ID lD or 3P 4- ‘S. Absorption cross-sections for the Schumann-Runge continuum have been measured by several investigators [120, 123, 146, 1511. The Taximum of the continuum is rather flat and appears to be closer to 1425 A than to the wavelength 1450 8 which is usually quoted. Furthermore, the intensity distribution is rather asymmetric, the short-wavelength side being steep [123]. There is a disagreement of about 25% in the reported u values for the maximum of the continuum. Although Ditchburn and Heddle [151], using results obtained by the photographic method, have claimed the most accurate value, 1.80 X lo-’’ cm2, repeated photoelectric measurements by Watanabe and Marmo [9] do not support their conclusion. Furthermore, photon counter measurements by Byram et al. [59] support the lower value 1.45 X lo-’’ cm2 [9]. The u value in this region is important to the problem of the dissociation layer of 0 2 in the upper atmosphere. Values obtained by the photoelectric method [9, 1231 are shown in Fig. 7 and tabulated in Table X. The dashed-line curve in Fig. 7 was obtained by Ladenburg and van Voorhis [120]. 4.2.3. Spectral Region 1050 to 1250 8. Molecular oxygen has a complex absorption spectrum in this region, and the observed bands have not yet been definitely classified. Most of the prominent bands are expected to be members of Rydberg series converging to the first ionization potential, but such series have defied identification. The first ionization potential has, however, been determined by the photoionization method [Y] which gives 12.08 f 0.01 ev or 1026.5 A. Most bands in this region are very diffuse, and Price and Collins [I391 have ascribed the diffuseness to predissociation. Tanaka [143], who studied the spectrum in greater detail, confirmed their work, and in addition, reported two new progressions, one of which was suggested to be a member of a possible Rydberg series. This region is of importance to the study of the ionosphere, because solar radiation can penetrate down to the D layer through seven deep windows [123]. Solar Lyman-alpha reaches the D layer through one of the deepest

+

+

179

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

N

I

I

-

I

I

I

I

I

-

-

-

I

I 1300

1400

-

I 1500

1600

1700

WAVELENGTH (8) FIG.7. Absorption cross-section of 0 2 in the region 1250 to 1750 b. Broken curve Ladenburg and van Voorhis [120].Solid curve by Watanabe, Inn, and Zelikoff [123].

windows, and its enhancement during chromospheric eruption may very well provide the necessary energy to account for radio fadeouts. Furthermore, the diffuseness of the spectrum in this region suggests the possibility of a significant amount of metastable oxygen atoms in the upper atmosphere supplied by predissociation. The absorption cross-sections in this region have been measured by a number of investigators. Preston [76] and Williams [147] determined the CT value a t Lyman-alpha. Later Weissler and Lee [150] reported u values for about fifteen lines. More recently Watanabe et al. [123, 91 and Lee [155] measured u values for a number of wavelengths, and their results are on the whole in good agreement. Very recently Watanabe et al. 11581 remeasured this region photoelectrically with resolutions of 0.2 and 0.1 A in order to obtain more quantitative data. Their results for 0.2 A resolution are shown in Figs. 8 and 9. The fact that these curves are very similar to the curve obtained with 1 A resolution by Watanabe et al. [123] is consistent with the diffuseness of the bands. Thus a resolution of 0.1 A is probably adequate to give quantitative data for most of the wavelengths in this region. However, it appears that most of the bands in this region undergo considerable pressure broadening a t comparatively low pressures. In Table XI are shown minimum u values a t seven of the deepest windows obtained by Lee [155] and Watanabe et al. [158]. The latter found

180

K. WATANABE

TABLE X. Absorption cross-section of 02 in 1CW cma for the region 1250 to 1750 A

x 1252 54.5 56 57 59.5 60.5 62 63.5 64.5 66 69 71 74 77 79.5 83.5 87 90.5 93 96.5 99 1302 06 09 12.5 17 21.5 25 29

33.5 36.5 39.5 43 45 49 51 55 61 66 69 75 78 84 91.5

a

1.04 0.855 0.706 0.594 0.465 0.401 0.442 0.342 0.242 0.182 0.119 0.067 0.093 0.153 0.249 0.364 0.472 0.542 0.584 0.550 0.520 0.444 0.357 0.516 0.721 1.10 1.60 2.05 2.31 2.29 2.20 2.24 2.79 3.46 5.76 7.10 7.10 8.18 9.62 11.3 12.4 12.7 13.2 13.4

x 1394 1400.5 02.1 04.3 07.4 08.6 10.5 12.9 14.8 16.3 20.2 23.0 27.7 30.0 32.9 36.2 37.8 40.9 43.5 46.0

50.3 52.1 55 57.5 60 63 67.5 73.5

79 86 89 91.5 95 99 1504

10.5 17 22.5 32 37.5 41.5 44.5 47 51

U

13.6 13.8 14.0 14.1 14.5 14.4 14.6 14.6 14.5 14.5 14.6 14.6 14.6 14.6 14.6 14.6 14.6 14.6 14.5 14.4 14.3 14.2 13.7 13.6 13.4 13.2 13.2 13.0 12.6 12.1 11.8 11.9 11.4 11.3 10.9 10.6 10.2 9.89 9.26 8.66 8.44 8.14 8.07 7.76

x 1555.5 62.5 69.5 72 77.5 81 85.5 89 91 96 1602 08 13 20.5 23.5 28.5 33.5 36.5 38.5 44 48 54 58.5 63 67 71 77.5 82 87 89 97 1702 05 12 17 22 27 32 37 42 47 49 51

U

7.58 7.21 6.50 6.46 6.14 5.87 5.76 5.35 5.09 4.94 4.65 4.16 3.87 3.42 3.31 2.98 2.79 2.68 2.57 2.34 2.23 2.01 1.82 1.71 1.56 1.45 1.30 1.19 1.12 0.985 0.892 0.810 0.754 0.676 0.610 0.558 0.506 0.442 0.394 0.353 0.309 0.268 0.242

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

FIG.8. Absorption cross-section of 0.2 d resolution.

0 2

181

in the region 1160 to 1280 d, obtained with

lower u values at three of these windows. It is very probable that there is in this region a weak continuum with a maximum u value of about 10-20 cm2,which is less than an earlier estimate by Weissler and Lee [150]. Several investigators have given special emphasis to the u value of O2 at the wavelength of the Lyman-alpha line because of its importance to cm2 atmospheric physics. Preston [76] obtained the value 1.04 X and also reported a considerable pressure effect. Watanabe et al. [123] obtained the value 1.00 X cm2and confirmed the pressure effect. More em2 recent reports by Ditchburn et al. [153] and Lee [155] give 8.4 X and 8.5 X cm2,respectively, both based on photographic photometry. Furthermore, they apparently observed no pressure effect. Thus there is some discrepancy between the results obtained by the photoelectric and photographic methods. As mentioned in Section 2.4, the higher value of Preston [76] is consistent with recent data on upper atmosphere densities measured by Byram et al. [75]. The shape of the window a t Lyman-alpha obtained with 0.2 A resolution [158] is shown by the curve in Fig. 10. It is similar to those by Ditchburn et al. [153] and Lee [155]. The pressure effect

182

K. WATANABE

1060

I080

1100

1120

II40

1160

WAVELENGTH (8) FIG.9. Absorption cross-section of 02 in the region 1060 t o 1160 A, obtained wit,h 0.2 A resolution.

on the u value a t Lyman-alpha, also obtained with a resolution of 0.2 A, is shown in Fig. 11. This result agrees almost exactly with earlier photoelectric results [76, 1231. 4.2.4.Spectral Region below 1060 b. This region of the absorption spectrum of O2 includes the ionization continuum as well as a number of bands. Several investigators [137-139, 1421 have examined the spectrum, and many of the prominent bands have been identified as members of Rydberg series. Price and Collins [139] and Tanaka and Takamine [142] have reported four Rydberg series converging to three electronically excited states of 0 2 , the '% ,411u, and " Z , in Fig. 1. As seen from photographs [139,0142], the absorption spectrum is fairly well known down to about 600 A ; an absorption spectrum below this wavelength is difficult to obtain, due to the lack of a suitable continuous source. Absorption cross-sections in this region have been reportoed by a number of investigators [9, 132, 149, 150, 1551. From 850 to 1050 A there are several strong bands known as the Hopfield bands [137] and some unidentified bands. Clark [149] and Weissler and Lee [150] reported a number of u values; but more recently Lee [155] and Watanabe and Marmo [9] studied this region in greater detail, using the many-lined spectrum of Hz as light

ULTRAVIOLET ABSORPTION PROCESSES I N UPPER ATMOSPHERE

183

TABLE XI. Absorption coefficients of 0 2 a t seven windows. The M indicates approximate position of minimum. Photographkt

Photoelectric* in ; i

k in cm-1

1217,3 1216.5 1215.7 M 1215.0 1214.8 1188.9 1188.3 1187.8 1187.1 M 1186.6 1167.2 1166.8 M 1166.1 1157.4 1157.0 M 1145.3 1144.3 1143.0 1142.8 M 1126.9 M 1110.5 1109.9 1108.9 1108.3 M 1107.8

0.60 0.40 0.27 0.50 0.70 0.64 0.39 0.25 0.18 0.35 0.35 0.27 0.52 0.60 0.51 0.70 0.65 0.33 0.26 0.53 0.48 0.35 0.25 0.11 0.32

h

~~

h

in

B

1217.2 1216.2 1215.7 1215.0 1214.6 1188.8 1188.6 1187.5 1187.0 1186.6 1167.3 1167.0 1166.3 1157.5

k

in em-*

-

0.55 0.43 0.23 0.64 0.40 0.48 0.38 0.43 0.35 0.38 0.30 0.28 0.55 0.50 0.56 0.30 0.43 -

1110.2 1110.0 1109.2 1108.5

0.31 0.43 0.32 0.44

-

1145.5 1143.8 1143.0

-

-

-

~~

* Watanabe et al. [1581. t Lee [1551.

source. The absorption spectrum and u values obtained by the latter for the region down to 850 A are shown in Fig. 12 and Table XII. Watanabe and Marmo also showed that band absorption in this region leads to preionization. The region below 850 A appears to consist mainly of continuous absorption according to the work of Weissler and Lee [150, 1551 and Aboud et al. [132]. The maximum of the Zontinuous absorption seems to be in the region between 400 and 600 A, belowowhich the D value decreases rather rapidly to low values a t around 100 A [132]. The u value at the maximum is about 2.1 X lo-'' em2 according to Lee [155], and 3.5 X lo-'' cm2 according to Aboud et al. [132], a considerable discrepancy. Photoioniaation cross-sections ui of 0 2 have been measured by Wainfan

184

K . WATANABE

1214

1216

1218

WAVELENGTH

(A)

1220

FIG.10. Absorption cross-section of O2 in the neighborhood of Lymnn-dph:i (1215. 7

A).

10

20

30

40

50

Absorption cell pressure in cm hg FIG.11. Pressure dependence of the absorption coefficient (k value) of 1215. 7 A.

0 2

at

et al. [154] for about 20 wavelengths in the region from 980 to 470 A, and by Watanabe and Marmo [9] for about a hundred wavelengths in the region from 1030 t o 850 & Their results appear to be consistent in the overlapping region and are shown in Table XII. Of particular interest to atmospheric physics are the (J; values a t Lyman; beta (1025.7 8) and for the Hopfield bands. Total cross-sections a t 1025.7 A

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

U

l - . L t . A u 10-19

-

M'

185

H'

LYMAN B 850

I

I

900

950

I 1000

I

1050

II I

WAVELENGTH ( X I

FIG.12. Absorption cross-section of

0 2

in the region 850 to 1100 A.

by Clark [149] and Weissler and Lee [150] are respectively 1.96 X 10-l8 and 1.59 x 10-l8 em2. The latter is very close to the value given in Table XII. According to Watanabe and Marmo [9], ui is 0.90 X 10-l' cm2 a t Lyman-beta, as the ionization yield was found to be 5 8 % ; furthermore, they obtained ui values for the pre-ionized Hopfield bands. 4.9. Nz A number of investigators [138, 159-1681 have studied the uItravioIet absorption spectrum of Nz which begins at 1450 A with the 0-0 band of the Lyman-Birge-Hopfield (L-B-H) bands [160, 1671. Figure 13 shows the electronic states associated with most of the known absorption bands. I n this figure, the dissociation energy of Nz is taken as 9.76 ev. Many states, Some of which are not shown, have been identified through numerous studies of emission spectra both in the vacuum ultraviolet and in the longer wavelength regions. Our knowledge of the electronic states of N2 is more advanced than that of 0 2 . Although the absorption cross-section of Nz has been measured by many workers [9, 76, 122, 131, 153-155, 169, 1701, the available results are, on the whole, less satisfactory than those for 0 2 . This situation may be explained by the fact that the N2 absorption spectrum down to 800 A consists mostly of sharp hands which demand high-resolution photometry. 4.3.1. Spectral region 1000 to I450 $. The absorption spectrum in this

186

K. WATANABE

TABLE XII. TotaI absorption cross-section u and photoionization cross-section ui of 0 2 in 10-18 cm2.

x in A 1050.7 1049.1 1047.2 1045.3 1044.2 1040.0 1038.4 1036.6 1035.5 1034.3 1032.2 1030.8 1029.4 1027.6 1025.7 1023.7 1020.7 1019.2 1017.8 1016.8 1015.8 1013.6 1011.4 1010.0 1008.9 1007.7 1005.5 1003.8 1002.0 1001.o 998.9 997.5 995.0 993.4 991.1 989.3 988.0 986.2 984.2 983.6 982.5 981.6 980.8 978.7 976.9

0

0.52 2.3 1.71 0.335 (0.149) 1.71

1.15 0.71 0.63 1.26 1.12 1.04 1.51 1.16 1.55 1.51 1.50 1.26 0.99 1.07 1.30 1.14 1.12 1.04 1.43 1.43 2.45 5.57 1.34 1.35 1.61 1.44 2.45 20.1 2.01 1.67 2.23 5.79 4.90 19.0 27.5 12.6 3.42 3.12 3.27

0.007 0.033 0.22 0.90 1.13 1.12 1.04 0.76 0.80 0.93 0.71 0.86 0.89 1.20 1.37 1.92 4.1 1.30 1.33 1.48 1.35 2.03 10.2 1.57 1.44 1.9 4.1 3.6 9.1 10.5 5.6 2.4 2.5 2.4

x in A

U

Bi

975.5 974.1 973.0 971.6 969.6 968.1 967.0 964.7 962.9 961.9 960.5 959.0 957.4 955.5 954.7 953.9 952.7 951.3 949.9 948.1 947.0 944.2 943.1 942.0 940.7 939.9 938.8 938.0 936.5 933.5 931 .O 930.0 927.9 925.4 924.6 922.7 921 .O 919.9 918 .o 916.0 915.1 913.4 911.1 909.9 907.0

17.5 8.55 23.4 21.6 2.45 3.90 8.55 36.4 8.84 5.87 10.2 3.35 6.43 35.3 42.0 4.68 3.49 3.31 3.31 11.2 54.3 3.23 3.35 3.79 4.31 7.80 30.5 29.4 3.86 4.57 25.7 17.5 4.16 6.24 16.7 11.2 4.57 4.01 5.79 17.5 5.01 5.05 5.83 14.5 4.54

8.0 4.2 12.0 14.2 2.4 3.2 6.8 28 7.3 4.6 5.9 2.7 4.6 22 36 4.1 3.5 3.1 2.9 8.1 38 3.0 3.0 3.0 3.7 6.6 18.6 23 4.0 3.2 16.2 12.4 3.4 5.6 14.0 9.7 4.5 3.2 4.2 13.3 3.8 3.8 4.1 9.7 4.2

ULTRAVIOLET

ABSORPTION

PROCESSES IN UPPER ATMOSPHERE

187

TABLEXII. Total absorption cross-section u and photoionization cross-section ui of 02 in 10-18 om2. Continued

x in R 904.9 903.6 901 .o 900.3 897.7 896.4 894.1 892.9 890.9 888.2 886.3 883.3 880.8 878.9 877.1 874.8 873.1 872.0 869.1 867.0 865.4 862.2 859.3

U

4.72 8.40 12.3 12.3 6.10 6.47 10.3 8.74 9.0 5.24 8.88 5.24 5.54 9.85 8.25 5.91 8.88 10.4 8.29 7.29 8.66 7.55 7.43

Ui

4.1 6.8 7.9 9.0 5.7 5.2 8.6 7.5 6.8 3.7 5.5 3.8 4.4 6.3 6.9 4.2 5.0 7.5 5.3 5.0 4.4 3.9 3.9

x in R 850 843 833 825 797 787 770 762 728 720 716 704 687 662 644 625 598 575 550 530 508 473

U

Ui

11.4 14.3 14.5 16.5 35.2 31.0 22.7 24.9 31 .O 34.6 32.5 33.4 21.9 29.0 31 .O 31.5 24.0 31.2 29.3 29.0 27.7 26.0

4.5 4.3 4.7 6.3 8.4 13.1 10.9 13.6 20.8 25.5 19.0 26.5 19.8 28.5 29.4 28.6 23.9 25.6 24.2 26.2 24.9 23.1

region is comprised primarily of the L-B-H bands. These na:row and sharp bands have been identified up to the 13-0 band at 1130.4 A and possibly the 14-0 band a t 1114.2 A [167].As these bands correspond to the forbidden transition, a lug + X'Z;, they are very weak, and to photograph them Birge and Hopfield [160] used an equivalent path of about 40 atmos-em, Tanaka [167] up to 6 atmos-em, and Wilkinson [168] about 2 atmos-ern with a 21-ft spectrograph. These equivalent paths are one to two orders of magnitude greater than that required to photograph the SchumannRunge bands of 0 2 . In addition to the L-B-H bands, Tanaka [167] observed new bands a t 1123.4,1098.9,and 1075.6 A, and suggested these may be the first three bands of the transition C3n,+- XlZ; (see Fig. 13). Published u values of Nz for this region are probably semiquantitative, as they have been obtained under comparatively low resolutions. According to Watanabe et al. [122], the maximum u value in the L-B-H bands is about 4 X em2,which is no doubt too low. Recent photoelectric measurements [158]with a somewhat higher resolution (0.1 A) and an equivalent path of 4 atmos-cm gave 7 X cm2 as the maximum cross-section. As

188

K. WATANABE

t

u

I

FIG.13. Energy level diagram of N z showing most of the transitions observed in absorption.

this equivalent path is comparable to that used in the high-resolution study of Wilkinson [168], it seems reasonable to take cm2as the maximuni u value for the strongest bands and cm2 for bands lying above 9.8 ev. Between the L-B-H bands, the u values are expected to be much lower; cm2 by Watsnabe et al. [la21 and 6 X lopz3cm‘ upper limits are 3 X by Ditchburn et al. [153], who used a long equivalent path length. These cross-sections as well as Tanaka’s work [167] do not support the high u values for the region 1000 to 1300 reported by Weissler et al. [l69]. Of particular interest to the study of the upper atmosphere are the u values of N2 at Lyman-alpha, Lyman-beta, and a t other lines in this region observed in the solar spectrum. Ditchburn et al. [153] have obtained u = 6 X cm2for the upper limit a t Lyman-alpha by using an equivnlent path length of 500 atmos-cm. This value is no doubt better than the upper limits given by Preston [76] and by Watanabe et al. [77]. At Lymaiibeta the reported upper limits are 1.1 X cm2 by Lee [155] and 3.7 X lopz1cm2by Watanabe and Marmo [9]. These low cross-sections

ULTIZAVIOLET ABSORPTION PROCESSES I N UPPER ATMOSPHERE

189

I GROUP 0'

lo-=

-. I

GROUP p

I

I

U GROUPrn

I

I )O

WAVELENGTH C i ,

FIG. 14. Semiquantitative absorption cross-section of Nz in the region 850 to

1000 A.

are consistent with the fact that no absorption continua of Nz have been identified in the spectral region above 1000 A. 4.3.2. Spectral region 800 to 1000 8. The absorption spectrum of Nz in this region is extremely complicated, but most of the prominent bands have been identified by several investigators [162-1651 as members, of Rydberg series. An accurate value of the first ionization potential (15.580 ev) was determined by Worley and Jenkins [162], and many of the non-Rydberg bands were classified by Worley [165]. Photographs of the N2 absorption spectrum are given by Worley [165] and by Tanaka and Takamine [164]. Absorption cross-sections for a number of wavelengths in this region have been reported by Clark [149], Weissler et al. [169], Lee [155], and Watanabe and Marmo [9]. I n view of the complexity of the spectrum, the u values for this region are probably mostly semiquantitative; however, the results are useful in showing the upper limits of minima and lower limits of band maxima. Figure 14 shows the range of values obtained by Watanabe and hlarmo [9]. The gross features and relative intensities of the bands are consistent with Worley's photograph and band assignments [165]. Although Hopfield [159] stated that continuous absorption sets in below 990 8, and Weissler et a,l. [I691 have suggested a possible continuum in the region 800 to 1300 A with u values of about 3 X 10-ls cm2,recent studies [9, 1551 failed to reveal a continuum. The upper limit of the N2 windows in this

190

K. WATANABE

% h-

I 30-

z 0

-

I

c

ASTOIN CURTIS WSSLER

I

I

----

I

I

1

N2

-

a

-wc

@i WAINFAN

0

-

120v)

-

I 200

I

I

I

400

WAVELENGTH

I 600

I

I 800

(8)

FIG.15. Absorption cross-section of Ns in the region 150 to 850 A.

region appears to be less than 3 X om2 [9, 1551. Further studies, particularly with higher resolutions, should be carried out. 4.3.3. Spectral region below 800 b. This region consists mainly of the ionization continuum; however, a number of Rydberg bands [138] are superimposed down to about 650 A, as can be seen in photographs by Takamine et al. [163, 1641. Recently, Worley [166] identified a new Rydberg series converging to a 211 state of Nz+ a t 16.94 ev above the ground state of Nz (see Fig. 13). The absorption cross-sections in this region have been measured by Weissler et al. [169], Curtis [131], and Astoin and Granier [170]. Furthermore, Wainfan et al. [154] have obtained photoionization cross-sections. Their results are summarized in Fig. 15. The observed u values differ by as much as a factor two. At the helium resonance line a t 584 b, the u value of Astoin and Granier [170] is in good agreement with the value 1.8 X 10-17 cm2 determined by Clark [149] and the value 1.9 X lo-’’ om2 by Marmo [171]. These results suggest that both the total and the photoionization cross-sections of Nz by Wainfan et al. [154] are somewhat too high.

4.4. NO The vacuum ultraviolet absorption spectrum of NO has been studied by a number of investigators [136, 172-1801, mainly in the region from 1600 to 2300 A. Their results and those obtained from various emission spectra of NO have provided most of our knowledge of the electronic states of this molecule. These states are summarized in Fig. 16. The dissociation energy

ULTRAVIOLET ABSORPTION PROCESSES I N UPPER ATMOSPHERE

191

FIG.16. Energy level diagram of NO and NO+

6.49 ev is consistent with that of Nz in Fig. 13. Three excited electronic states of NO have been determined by Tanaka [173] from Rydberg series observed in the region below 900 8; however, all attempts to identify extensive Rydberg series converging to the first ionization potential have failed because of the complexity of the spectrum in the region 1300 to 1700 A. According to Watanabe [lo], the first ionization potential of NO is 9.25 ev by the photoionization method and 9.24 ev by a cycle using the fourth ionization potential [ 1731 and assuming that the Miescher-Baer emission bands [181, 1821 correspond to the transition shown in Fig. 16. Although the suggestion by Sutcliffe and Walsh [176] that the A and E states represent the first two members of a Rydberg series appears to be consistent with the ionization limit given above, further identification of other bands is necesspy. Superimposed on the ionization continuum in the region 1000 to 1340 A are a number of diffuse bands. None of these bands has been identified. The total absorption and photoionization cross-sections of NO have been reported by several workers [lo, 77, 128, 129, 183-1871. The results cover the entire spectral region from 150 to 2300 A with comparatively few ma-

- K.

192

WATANABE

jor gaps; however, most of the u values for the region above 1400 A, where the absorption bands show numerous rotational lines, are no doubt semiquantitative. Absorption cross-sections in the region above 1400 8 were measured by Mayence [I831 using a photographic method and by Marmo [lag, 184.1 using a photoelectric method. Mayence observed a weak continuum with a maximum a t 1470 A and interpreted i t as a transition B211e-A22 leading to the dissociation products N(2D) and O(3P)at 8.87 ev. Marmo, however, questioned this interpretation on the basis that (a) u values for this region must be measured with high resolution, and (b) the conclusion is not consistent with the dissociation energies of NO. As described by Marmo [129], the values in this region showed a pronounced pressure effect apparently due to limited resolution, and the results were considered to be semiquantitative. Even with an improved resolution of 0.2 8 Marmo [184] found a large apparent pressure effect, and hence attempted to obtain quantitative oscillator strengths by using pressure broadening with foreign gases. Further investigations in this region are desirable, since quantitative u values may be valuable for atmospheric physics. Absorption cross-sections for the region 1050 to 1400 8 were determined by Marmo [129] and confirmed by Watanabe [lo].Furthermore, the photoionization cross-section was first measured by Watanabe et al. [128], and Inter their results were revised by Watanabe [lo] after improved thermocouple measurements. The revised results are shown in Fig. 17. The high ionization yield of nitric oxide was confirmed by Zelikoff and Aschenbrand (T

I 3

,-*.:,:.:.:.

!

I

0

2

I

I

I

’..”,.*..: .* 1 I

% .

1

1200 1250 1300 1350 WAVELENGTH (%I FIa. 17. Total absoption cross-section (solid curve) and photoionization crosssection (dots) of nitric oxide in the region 1100 t o 1350 A. 1150

193

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

TABLEXIII. The ‘J and ui (in lo-’* em2) of nitric oxide in the region 1060 t o 1340 A.

x 1065 66 80.2 81.7 84.7 87.1 88.7 90.0 92.7 95.3 97.9 1100.5 02.1 04.3 05.8 07.1 11.0 12 14.9 16.3 19.0 21.2 24.0 26.3 27.5 31.4 33.2 35.3 37.5 39.7 41.5 44.2 45.9 46.9 48.5 50.8 54 57 59.7 61.2 63.7 66.0 69.1 72.1 74.3 75.8

9.3 8.6 7.6 8.9 6.5 7.8 6.0 5.6 4.8 4.7 4.5 4.3 4.1 4.1 4.3 5.6 6.7 6.0 4.7 4.5 3.7 4.1 3.7 3.7 4.3 3 .O 3.0 3.2 3.5 3.4 3.5 3.4 3.2 3.2 2.98 2.98 2.79 2.79 2.79 2.61 2.57 2.57 2.53 2.68 2.61 2.57

‘Ji

x

6.6 6.5 6.3 7.5 5.2 6.5 5.5 4.9 4.4 4.0 3.8 3.6 3.6 3.9 3.9 5.0 5.9 5.4 3.9 3.9 3.3 3.7 3.1 3.4 4.1 3.0 3.0 2.9 2.8 2.9 3.1 2.8 2.9 2.8 2.77 2.86 2.76 2.62 2.68 2.53 2.47 2.36 2.38 2.32 2.36 2.33

1178.2 80.4 82.5 84.7 87.8 89.3 91.5 93.2 95.6 98.0 1200.3 02.1 04.9 06.6 09.2 11.3 15.6 17.4 19.2 20.9 23.4 25.5 28.2 30.0 31.9 33.8 35.4 39.5 41.3 43.2 45.9 47.4 51.3 53.6 55.0 57.2 60.3 61.8 64.2 66.4 68.6 70.6 74.0 77.1 78 79.5

0

2.42 2.27 2.27 2.23 2.34 2.34 2.34 2.31 2.31 2.23 2.31 2.20 2.16 2.20 2.12 2.16 2.42 2.42 2.38 2.31 2.20 2.05 1.90 1.97 2.09 2.05 2.01 2.01 2.09 2.01 1.97 2.01 2.42 1.90 1.94 2.12 2.09 2.01 1.86 2.01 2.16 2.12 2.42 1.90 1.83 1.75

‘Ji

2.30 2.20 2.27 2.23 2.20 2.29 2.25 2.22 2.22 2.14 2.15 2.11 2.08 2.07 2.02 2.01 2.02 2.09 2.08 2.00 2.04 1.89 1.69 1.75 1.67 1.71 1.68 1.76 1.77 1.74 1.64 1.66 1.68 1.57 1.62 1.61 1.54 1.38 1.18 1.12 1.10 1.10 1.19 1.04 1.05 1.03

104

K . WATANABE

TABLE XIII. The u and

~i (in 10-18 cmz) of nitric oxide in the region 1060 to 1340 d. Continued

x

U

Ui

1281.1 83.1 84.3 86.4 87.5 90.4 93.3 95.3 97.2 99.7 1302.3 04.1 07.2 10.9 12.8

1.97 2.23 2.83 2.79 2.68 1.79 2.01 2.35 2.27 1.94 1.71 2.16 1.94 2.20 2.12

0.99 1.08 1.05 1.04 1.16 1.02 0.99 1.11 1.07 0.84 0.54

0.50 0.44 0.45 0.45

x 1315.4 16.8 19.0 23.3 25.1 27.6 29.3 33.9 35.9 38.6 41.3 42.3 43.7 45.5

U

1.75 1.83 1.97 3.13 3.24 3.28 1.79 2.91 3.46 2.20 1.60 1.53 1.60 1.90

Ui

0.44 0.46 0.48 0.48 0.45 0.54 0.44 0.41) 0.52 0.38 0.205 0.087 0.075 0.019

[188] through photochemical studies a t 1236 8. As seen from the lower curve in Fig. 17, the ionization continuum exhibits vibrational fine structure, which was recently confirmed by Hurzeler et al. [189]. The steps in the ionization continuum correspond to the vibrational levels of NO+ ground state, and the A d s agree with those of the lower state of the MiescherBaer emission bands [181]. By subtracting the ionization continuum and the bands from the u curve, Marmo [184] h a t obtained a smooth dissociation continuum extending from 1200 to 1400 A and suggested that the dissociation products N(20) and O(3P)are associated with this continuum. This result is consistent with his earlier observation I1291 that the minimum u values in the region below 1380 A were jndependent of pressure. Recently, u values for the region 1050 to 1350 A were measured with ii resolution of 0.2 b [158] and were found to be independent of pressure. Some u and ui values are tabulated in Table XIII. In particular, the U~ value at Lyman-alpha is important to the theory of the D layer formation; the value obtained by Watanabe [lo] is 2.02 X 10-l8 cm2. In the region below 1000 b, Sun and Weissler [185] obtained u values down to 374 A; and Granier and Astoin [186] down to 150 A. The former observed two continua, and the latter, a t least three. These continua, which they obtained by tracing the envelope of the minimum u values, are sketched in Fig. 18. The u values, including maxima, range from about 10-17 to 4 x lo-’’ cm2. The photoionization cross-sections for the region down to 687 A were reported by Walker and Weissler [187].

ULTRAVIOLET I

ABSORPTION PROCESSES IN UPPER ATMOSPHERE l

I

I

l

1

1

1

195

1

c

“E

P3

g

-

-------

GRANIER-ASTOIN SUN-WEISSLER

-

NO

6F

0

y 2

-

-.*

-

,-

v)

E? n

~,

u

:

z ;I

n

-

‘\

-

I I

\

0

.

..--.‘ *I

,

\

’\

v)

m

a

I 0

I 200

I

I 400

I

I 600

I

I 800

I

I 1000

I 1200

4.5. 03

The absorption spectrum of ozone has been studied by many investigators in the near ultraviolet and at longer wavelengths, but only by Price and Simpson [17] in the vacuum ultravioletb The latter found no absorption bands in the region from 1600 to 2300 A and were unable to observe absorption at shorter wavelengths due to oxygen absorption. It seems worthwhile to make further attempts to photograph the ozone spectrum in the region below 1600 8. The absorption cross-sections of ozone in the region from 1050 to 2200 A were determined by Tanaka et al. [126] with a photoelectric method. They used ozone samples of purity from 90 to 95% in an absorption cell and applied corrections for impurities such as ,OZand COZ. The absorption cross-sections for the region 1050 to 2200 A are shown in Fig. 19. There appears to be continuous absorption throughout this region with several diffuse bands in the region below 1500 8. These bands are probably members of Rydberg series as they lie in the region where such series are expected. The ionization potential of 12.80 ev has been determined by Herron and Schiff [18] with the electron impact method. Since ozone has been found [57] in the D layer, it may compete with nitric oxide in absorbing solar Lyman-alpha. The r value of ozone a t Lyman-alpha is 2.4 x 10-17 cm2 oroten times that for nitric oxide. I n the region from 2020 to 2200 A, the results shown in Fig. 19 are from 10 to 40% lower than those reported by Ny and Choong [190] and Vassy [191], but are in good agreement with those by Inn and Tanaka [192].

196

K. WATANABE

FIG. 19. Absorption cross-section of ozone in the region 1050 to 2200 A.

The absorption cross-sections of ozone in the near ultraviolet and visible regions have been determined by many workers; however, emphasis should be given to a recent extensive work by Vigroux [193] as well as that by Inn and Tanaka [192]. 4.6. Water Vapor

At present very little is known regarding the importance of water vapor in the upper atmosphere. The existence of this vapor at high altitudes up to at least 80 or 90 km is inferred from observations of noctilucent clouds. In 1950, Hopfield [194] pointed out that water vapor may be important in the absorption of ultraviolet radiation from the sun. About the same time, Meinel [195] identified the OH radical in the emission spectrum of the night sky, and Bates and Nicolet [196] made a theoretical study of the photochemistry of atmospheric water vapor for the altitude range from 60 to 100 km. The absorption spectrum of water vapor in the vacuum ultraviolet is fairly well known from the work of several investigators [15, 136, 194, 197, 1981. Leifson [136]observed a broad continuum in the region 1610 to 1780 8 with a maximum a t about 1700 8 and another continuum beginning a t about 1392 8.Henning [197] observed two continua and a number of diffuse bands in the region 600 to 1100 8. Rathenau [198] reported continua be-

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

197

ginning at 17.8 and 24.5 ev as well as a number of bands in the region 500 to 1780 8. By identifying two Rydberg series in the region 1000 to 1250 8, Price [15] established the first ionization potential (12.59 ev), which was later confirmed by the photoionization method. The results of these investigators are fairly consistent, but, because of the diffuseness of the observed spectrum, many of the band progressions have not been classified. The absorption cross-sections of water vapor have been recently studied by several investigators [127, 199-2041. However, there seem to be considerable discrepancies in some of the results. Perhaps the chief source of error lies in the determination of vapor pressure, since water vapor tends to adsorb strongly on the walls of the absorption chamber. For convenience of discussion, the spectrum is divided into three regions. 4.6.1. Spectral region 1.450 to 1900 8. This region consists mainly of a continuum beginning at about 1860 8 and having a maximum a t about 1650 8. The absorption cross-sections of this region were first measured by Wilkinson and Johnston [199] by a photographic method. Similar methods were also used by Harrison et a'. [200] and Johannin-Gilles [201], while Watanabe and Zelikoff [127] used a photoelectric method. Although there is fair agreement in the approximate wavelength of the maximum absorption, considerable difference appears in the shape of the absorption curve. Wilkinson and Johnston observed three broad bands a t 1608, 1648, and 1718 8, while Johannin-Gilles observed six bands in the same region. On the other hand, Harrison et al. and Watanabe and Zelikoff found no such bands. Furthermore, Johannin-Gilles observed a large pressure dependence in the u values while the others did not. There is, however, a fairly good agreement in the maximum u value. Curves by Wilkinson and Johnston [199], Harrison et al. [ZOO], and Watanabe and Zelikoff [127] are shown in Fig. 20. Around 1650 8, the curve by the latter lies between those obtained by the other investigators. The oscillator strength of the continuum is about 0.041. 4.6.2. Spectral region 1050 to 1450 8. The absorption cross-sections for this region were measured by Watanabe and Zelikoff [127] and are shown in Figs. 20 and 21. The wavelengths of the bands in the region 1250 to 1450 A with a spacing of about 800 cm-l are in good agreement with those reported by Rathenau [198]. The continuum underlying these bands extends to about 1150 8 as shown in Fig. 21, and its oscillator strength is about 0.05. Both Rathenau [198] and Price [15] investigated the broad diffuse bands in the region 1050 to 1250 8. The latter was able to resolve some rotational structure and to identify most of the prominent peaks as members of Rydberg series. Watanabe and Zelikoff [127] reported that most of the u values

198

K. WATANABE

10-17

67

-z5 0

tw ln

& 10'" ln 0

a

0

z

0 I-

n K

0

ln

m a 10-lo

1200

1300

1400

1500

WAVELENGTH

1600

1700

I800

1900

(&I

FIG.20. Absorption cross-section of water vapor in the region 1200 t o 1850 1. x- -x by Wilkinson and Johnston, [199] 0--0 by Harrison et al., [200] solid curve by Watanabe and Zelikoff [127].

FIG.21. Absorption cross-section of water vapor in the region 1050 t o 1250 A.

in this region were independent of pressure, but bands atl 1101, 1112, 1120, 1152, and 1240 showed about 20% decrease in u value with n tenfold increase in pressure, probably due to insufficient resolution. At Lyman-alpha, the following u values have been reported: 1.45 X lo-''

ULTRAVIOLET ABSORPTION PROCESSES I N UPPER ATMOSPHERE

I

I

I

199

I

b

3

z

2 $2-

0 LL

0

z

p0:

0 v,

m

U

0

I

I

I

I

I

200

400

600

800

1000

,

(A)

WAVELENGTH

FIG.22. Absorption cross-section of water vapor in the region 150 t o 1100 A.

c

E

3

z

?

-

-

-

-

J\

0

w

v)

4\ (I, lo-“ -

-

v)

1

-

0

a

0

IP

2

0 b

a

a

0 v)

m

a

10-18

~

900

1000

1100

cm2by Preston [76], 1.34 X lo-’’ by Ditchburn et al. [153],and 1.43 X 1O-l’ by Watanabe et al. [77]. 4.6.3. Spectral region below 1050 d. The most extensive measurement of cr values for this region was made by Astoin et al. [202, 2031, who covered the region from 160 to 1100 A. The continuous absorption curve which they obtained by tracing minimum u values is shown in Fig. 22. Absorption cross-sections for the region 850 to 1100 A obtained by

200

K . WATANABE

Watanabe and Jursa [204] are summarized in Fig. 23. They also measured the ionization yield from 850 to 988 8 and found the yield to steadily increase from about 34% at 981 8 to 75% a t 850 8. These yields are consistent with those obtained by Wainfan et al. [154], who measured down to 473 8.

4.7. NzO I n the past, NzO has been suggested as the source of atmospheric NO. Laboratory photochemical studies by Zelikoff and Aschenbrand [205, 2061 show that NO is produced by the reaction (4.2)

0

+ N20 + 2N0

However, the amount of N2O in the atmosphere is only about 0.4 atmos-cm according to McMath et al. [207];thus it is not apparent that this reaction contributes a significant amount of atmospheric nitric oxide. The photochemistry of NzO in the upper atmosphere has been discussed by Bates and Witherspoon [208]. The absorption spectrum of NzO in the vacuum ultraviolet has been investigated by several workers. Leifson [ 1361 and Sen-Gupta [209] observed two continua, one extending from 2000 to 1680 8 and the other from 1550 to the transmission limit of fluorite. A very detailed study of the region 800 to 2200 8 was carried out by Duncan [210]. He reported a Rydberg series, but the series limit (12.72 ev) is somewhat lower than the photoionization value (12.90 ev) shown in Table I. Recently, Tanaka et al. [all] examined the region 600 to 900 8 and found five Rydberg series converging to three limits, 16.39, 16.55, and 20.10 ev. Absorption cross-sections of NzO have been measured throughout the region from 100 to 2200 8. Romand and Mayence [212] covered the region 1390 to 2200 8, and Zelikoff et al. [124], the region 1050 to 2100 A. The u values reported by these two groups differ by about 20 % for the two continua with maxima at 1450 and 1820 8. Zelikoff et al. measured u values for two other continua (1275 and 1080 8) and discussed the dissociation products associatedowith these and other known continua of N2O. In the region below 1000 A, Astoin and Granier [213] found three continua with maximum u values of about 3 x cm2. 4.8. COz

It appears that COZdoes not play an important role in the photochemistry of our atmosphere in spite of the fact that there is as much as 320 atmos-cm of this gas. However, we may not be justified in neglecting this constituent, since CO and atomic oxygen are known to be produced from COz by the action of ultraviolet radiation. Bates and Witherspoon [208] have discussed this problem.

ULTRAVIOLET ABSORPTION PROCESSES I N UPPER ATMOSPHERE

201

Although the absorption spectrum of COZ in the vacuum ultraviolet has been studied by several investigators [14, 136, 197, 1981, most of the bands have not been identified. Furthermore, very few measurements of (T values for COZhave been reported. Wilkinson and Johnston [199] found contituous absorption in the region 1440 to 1670 8 with a maximum at 1495 A and three weak bands. Inn et al. [125] carried out a more extensive and detailed study and reported many bands in the region 1050 to 1750 b. Measurements in the region below 1000 8 made by Sun and Weissler [214] show a continuum beginning a t 860 8 and also some bands. 4.9. 0 and N Atoms

There is no doubt that atomic oxygen occupies a very important place in the photochemistry of the upper atmosphere, but the role of atomic nitrogen is not yet clearly established. Unlike the case of oxygen, there appears to be no strong continuum or diffuse band in the ultraviolet absorption spectrum of Nz up to its ionization limit (796 A) to provide a significant amount of atoms by dissociation. However, atomic nitrogen may be produced in the region above about 150 km by the dissociative recombination process (4.3)

N2+4e -+ N' 4-N"

which was proposed by Bates [215] to account for the disappearance of Nz+. Absorption cross-sections of oxygen and nitrogen atoms are based almost entirely on quantum mechanical calculations. Bates and Seaton [216] cm2for N at their respective ern2for 0 and 9 X obtained 2.6 X spectral heads (13.61 and 14.54 ev). The maximum value for 0 is 13 X cm2 at about 23 ev; and for N, 11 X 10-l8 em2 at 19 ev. By studying the absorption of N atoms in a Philips ionization gauge, Ehler and Weissler [217] have estimated the value 12.8 X 10-l8 cm2. Inasmuch as atomic oxygen is expected to be a major constituent in the region above 100 km, it is necessary to consider its absorption at wavelengths corresponding to its principal absorption lines. In fact 01 emission lines have already been observed in solar ultraviolet spectra; and, therefore, (T values at these lines would probably be very helpful. In this connection, Kato [218] has pointed out that the wavelength of the transition 3d 3D-2p4 3P of the 0 atom coincides with that of Lyman-beta so that the latter in the solar spectrum may be filtered out by atmospheric oxygen atoms a t high altitudes. 4.10. Other Minor Constituents

Among the other minor atmospheric constituents are CHI , N H 3 , CO. Hz , rare gases, and Na. In this section, only brief references to their absorption cross-sections are given.

202

K. WATANABE

.

4.10.1. CH, The ultraviolet absorption spectrum of methane begins at about 1450 8 and consists essentially of continua with very slight indications of band structure. The absorption cross-section was first measured by Wilkinson and Johnston [199] for the region down to 1370 8. Later, Moe and Duncan [219] and Watanabe et al. [122] extended their measurements down to about 1050 A. More recently, Ditchburn [220], and Sun and Weissler [221] made measurements down to 400 8. Watanabe [Ill found the ionization potential to be 12.99 f 0.01 ev, and Wainfan et al. [151] have measured some photoionization cross-sections. .&lo.$. N H 3 . The absorption spectrum of ammonia in the region below 2200 8 consists of many bands superimposed on several continua. Duncan [222, 2231 made a detailed study of the spectrum down to 850 8. The ionization potential which he obtained from a probable Rydberg series appears to be too high according to electron impact measurements. The photoionization value, 10.15 ev [l6], is consistent with the latter. Absorption cross-sections have been published by Tannenbaum et al. [224] for the region 1700 to 2200 A, by Watanabe [lo] for 1050 to 2200 8, and by Sun and Weissler [221] for 370 to 1300 8. Photoionization cross-sections have been reported by Watanabe [lo] and Walker and Weissler [225]. 4.10.3. CO. This molecule is one of the most thoroughly studied diatomic molecules in emission spectra. However, comparatively little work has been carried out in absorption. A preliminary study of absorption intensities in the region 1050 to 1600 8 was reporte by Watanabe et al. [122]. Sun and Weissler [214] obtained u values in the region 370 to 930 8. 4.10.4. H z and Rare Gases. Tanaka [226] has studied the absorption spectrum of H2 in great detail and also reported on earlier work. Lee and Weissler [227-2291 have measured the u values for Hz and the rare gases in t,he region below 900 8. 4.10.5. Na. The presence of sodium in the upper atmosphere is well established by the appearance of the D lines in airglow spectra, but the amount is estimated to be very small. By means of rocket-borne photometers, Koomen et al. [230] have found the height of maximum intensity of the sodium airglow to be about 85 km. The photoionization cross-section of N a has been measured by Ditchburn et al. [231] in the region 1600 to 2400 A.

5. SOMEATMOSPHERIC ABSORPTION PROCESSES 5.1. General I n order to explain the production of the various atmospheric layers, many hypotheses have been proposed. For example, the processes suggested for the formation of the D layer include x-ray ionization of air, and photoionization of molecular oxygen, of atomic oxygen, of nitric oxide, and of

203

ULTRAVIOLET ABSORPTION PROCESSES I N UPPER ATMOSPHERE

sodium. With the recent advances made in our knowledge of the solar spectrum, the absorption cross-section of gases, and the physical structure of our atmosphere, some of the hypotheses have been shown to be untenable. Theoretical calculations, particularly by several investigators who have kept in close touch with experimental work, have been most helpful in this process of elimination and in defining problems. A comparison of early publications by Mitra [3], Bates, Massey, and Seaton [232-2341, and more recent reviews by Bates [235] and Nicolet [236] shows that a steady progress has been made. These references also provide essential background information on the photochemistry of the upper atmosphere. We should also refer to papers on recombination processes, such as those by Massey [237] and Bates [238], but unfortunately reliable rate coefficients are still very scarce. I n the present review, no attempt is made to present a comprehensive survey of the photochemistry of our atmosphere; only a few examples of absorption processes are outlined. 5.2. Penetration of Solar Ultraviolet Radiation

Our present knowledge of the physical structure of the upper atmosphere (Section 2) and absorption cross-sections of gases (Section 4) permits us to estimate the penetration of solar radiation with considerable accuracy. Figure 24 shows the altitude a t which the intensity of solar radiation a t vertical incidence is reduced by a factor of e. The reference atmosphere given in Table VII was used for computing optical densities.

03-

w n 1003 II J

a

-

--

-

50

-

-

-

-

-

-

-

0

500

1000

1500

WAVELENGTH

(8)

2000

2500

FIG.24. Penetration of solar ultraviolet radiation.

3000.

204

K . WATANABE

I n the region 2000 to 3000 A, the absorption is due chiefly to ozone. A “typical” ozone distribution with its center of gravity a t 25 km and with total layer thickness of 0.23 atmos-cm was employed as a model. Although the Rayleigh scattering and the weak, 0 2 absorption were included, these contribute little down to about 2200 A. The absorption in the region 800 to 2000 8 may be ascribed mainly $0 0 2 .The Schumann-Runge bands appear in the region 1750 to 2000 A; some of them are indicated by jagged curves, and others by crosses (maxima) and dots (minima). The Schuma2n-Runge continuum and a few bands account for the region 1300 to 1750 A. The solid curve is obtained if O2 is not dissociated, while the broken curve is based on the 0 2 densities a t nltitudes 110 to 130 km obtained by Byram et al. [59]. Thus, according to Fig. 24, it is necessary to exceed an altitude of about 120 km to see this region of the solar spectrum. In the region 1000 to 1300 A, there are several atmospheric windows; seven of them reach the region below 80 km. Someof the absorption maxim: are indicated by crosses, and minima by dots. The region 800 to 1000 A is due to the 0 2 ionization continuum (dots), pre-ionized Hopfield bands (crosses), and Worley bands of N2 (squares). Several of the many bands are indicated. The broken curve in this region takes into account the dissociation of 0 2 . The contribution from the ionization continuum of atomic oxygen is probably small in the region 800 to 900 A, since its cross-section is only 2.6 X cm2 [216]. The region 200 to 800 A is somewhat uncertain because the atmospheric composition for altitudes above 140 km is not accurately known. I n particular, the extent of N2 dissociation is not established. Nevertheless, it appears that most of the absorption may be ascribed to Nz and 0 atoms. Absorption cross-sections for this region are not sufficiently consistent and complete to give us a detailed picture. Some absorption peaks due to strong bands of Nz may reach 170 km but not much higher. Figure 24 confirms the main features of a curve published in 1951 by Tousey el al. [99] which was based on rather meager data available a t that time. Figure 24 is also consistent with the curve published more recently by Ditchburn [239]. These results show that the F layers cannot be explained by the maximum rate of primary absorption leading directly to photoionization. 5.3. Ozone Layer

The present photochemical theory of the ozone layer is basically the same as that proposed by Chapman [1, 21 in 1930. The following reactions are involved :

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

205

where the J’s are the rates of dissociation per particle and the k’s are the collision coefficients. Wavelengths mainly between 1700 and 2400 A are involved in reaction (5.1), while the visible spectrum as well as the ultraviolet produces the reaction (5.2). The rates of production of 0 and O3 are given by

+

+

- 1c2n1n2nM- k3nln3 kmns (5.8) dnl/dt = 2J2n2 J3n3- 21c1n12nM (5.9)

dnaldt

=

-Jan3

+ kzn1n2nM- Ic3n1n3 - 2k4n32- kbn2n3

where nl , n2, 1 2 3 , and n M are particle concentration in C M - ~ of 0, 0 2 , O3 , and the third body M , respectively. Terms involving Ic4 and k6 are negligible, so for photochemical equilibrium, equations (5.8) and (5.9) become (5.10) (5.11)

+

2J~n2 J3n3- 2k1n?nM- k2nln2nM- k3nlna= 0

+

Jan3 - lcznln2n~ lcsnlns

=

0

Some investigators [53, 551 have neglected equation (5.3) involving k l in addition to equations (5.6) and (5.7). However, as pointed out by Johnson et al. [57], this is not justified for altitudes above about 60 km where the three-body recombination process becomes increasingly important. Recent calculations have been reported by Bates and Nicolet [196] for the region 60 to 90 km and by Johnson et al. [57j for the region 30 to 90 km. In addition, Watanabe and Zelikoff [240] made revised calculations using new data on Jz and J 3 . Figure 25 shows the ozone concentration as a function of altitude obtained by these investigators. It can be seen that their results are in good agreement. Thus the ozone layer appears to be in photochemical equilibrium from about 40 km to a t least 70 km. At 50 km the concentration of atomic oxygen is about the same as that of ozone; a t 70 km the former is about a hundred times greater than the latter; and a t higher altitudes ozone becomes negligible compared to atomic oxygen. 5.4. O2Dissociation Layer Following Chapman’s [2] early prediction of the dissociation layer of O2 in our atmosphere, various investigators have calculated the distribution of

206

K. WATANABE

X

W

5

6

7

BATES 8 NICOLET NRL EXPT. WATANABE 8 ZELIKOFF

-

8

9

10

II

12

13

OZONE CONCENTRATION (loglo particles/cm')

FIG.25. Observed and calculated ozone concentrations versus altitude.

atomic and molecular oxygen in the region near 100 km. The primary absorption process is given by equation (5.1),oand wavelengths in the region 1300 to 1800 A and probably 1000 to 1300 A are effective for this altitude. Recently, Penndorf [241], Moses and Wu [242], and Nicolet and Mange [243, 2441 have reconsidered the problem under different assumptions. One of the major differences is in the assumed photon flux of solar ultraviolet. For example, Nicolet and Mange used a radiation temperature of 4500°K for the sun, while others used higher temperatures up to 6000". In view of the fact that rocket measurements give a relatively low photon flux, corresponding to about 4000" blackbody radiation [59], the analysis by Nicolet and Mange appears to be the most realistic. They showed that mixing and diffusion must be included in a photochemical theory of 0 2 dissociation, because the time of photochemical equilibrium is of the order of days. Thus, their results show that the transition layer of 0 2 dissociation is rather broad (-50 km), while previous calculations suggested layers of about 10 km. Photon counter measurements [59] in the region 110 to 130 km seem also to indicate a rather gradual change in the transition layer; however, further data are needed to give a more quantitative picture of the change in composition. 5.5. D Layer Although various hypotheses have been proposed for the formation of the normal D layer (-60 to 90 km), there is a t present a fairly general

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

207

agreement that this layer is produced by the reaction (5.12)

NO

+ hv(9.25-12 ev)

--f

NO+

+e

which was proposed by Nicolet [4] in 1945. Bates and Seaton [234] have examined all other hypotheses and found them untenable. Their conclusions regarding 0 2 and NO were confirmed by Watanabe et al. [77, 2451 by means of experimental u values of these molecules. The necessary radiation can be ascribed chiefly to the solar Lyman-alpha line. The incident flux for this radiation is about 6 ergs cmV2sec-l (-3.7 X 10" quanta cm-2 sec-l) according to a very recent report by Chubb et al. [ 1071 who used nitric-oxide ion chambers for measurements. The absorption cross-section of 0 2 a t the wavelength of Lyman-alpha is cmW2[76, 1231, and this line in the solar spectrum can pene1.04 X trate to 70 km according to Fig. 24. I n fact, Byram et al. [75] have observed it at about 74 km with photon counters. The case for nitric oxide would be most conclusive, if its concentration could be determined. Negative results of nitric-oxide absorption in the infra-red set an upper limit of concentration to about 10" molecules ~ m - ~ . Some investigators [235, 236, 2461 have shown that nitric oxide must be formed photochemically, but quantitative estimates of concentrations are still very uncertain. Mitra [246] estimated as high as 10l2 molecules cmP3, while Nicolet [247] considered this value as unacceptable and estimated the order of lo8 or 5 X lo* ~ m a- t ~80 km. These lower values are consistent with the rocket measurement [75] that the attenuation of Lymanalpha is mainly controlled by 0 2 . Using equation (1.16) and letting ui = 2.0 X lo-'* cm2 [lo], n = lo8,and I = 2.9 X loll quanta at cm-2 sec-I a t 80 km, we obtain about 60 ion pairs ~ m sec-I - ~ which is sufficient to account for the D layer. Marmo et al. [248] have recently observed the formation of an artificial ion cloud by releasing NO gas at an altitude of 95 km and suggested that the ionization was caused by solar Lyman-alpha. Furthermore, by a similar experiment a t night they [249] observed a chemiluminescence by the action of NO on probably atomic oxygen. The increased ionization in the D layer during radio fadeout has been ascribed to the enhancement of solar Lyman-alpha and to solar x-rays of wavelengths less than 10 A. The latter appears to be the primary cause according to evidences obtained by Friedman et al. [106, 1071. 5.6. E Layer

The lower boundary of the E layer is at about 90 km, but the upper boundary is not clearly defined. Many investigators have taken the latter to be about 200 km or more in accordance with estimates based on reflec-

208

K . WATANABE

tion of radio waves. However, rocket measurements by Seddori et al. [67] seem to indicate an upper boundary of about 150 km. At present there is no general agreement on the process producing the E layer. As seen from Fig. 24, the region 90 to 120 km is controlled by wavelengths in the region 800 to 1300 A and the region below about 100 A. Unlike the case of the D layer, there are several photoionization processes which can reasonably compete for the available photons in these spectral regions. During the past several years, various investigators [232, 234, 245, 250-2571 have made more or less detailed studies to establish the dominating process. The production of the E layer by the action of solar x-rays was proposed in 1938 by Hulburt [258] and Vegard [259]. Later, Hoyle and Bates [118] examined this proposal in considerable detail and concluded that x-rays of about 40 A from the solar corona would give a maximum ionization in the E layer. This study was further investigated by Nicolet [251], Choudhury [252], and Rawer and Argence [253]. The case for the x-rays has been greatly strengthened by rocket measurements carried out by Byram et al. [105], who claimed that the sun supplies enough soft x-rays to account for the entire E layer. According to their recent report [40] the total flux of soft x-rays is about 0.1 erg cm-2 sec-l which is approximately 2 X lo8 photons cm-2 sec-'. In 1938, Wulf and Deming [260] suggested that the E layer is produced by the photoionization of molecular oxygen a t its first ionization potential or (5.13)

02(X3&) 4- hv(-800-1027

A) + 02+(X2n,)+ e

However, an objection was raised that the relevant cross-section appeared to be very small ( ~ 1 0 - ~to0 cm2). Therefore, Nicolet [4] suggested that the strong Hopfield bands in the same spectral region may be preionized and that the E layer may be due to the process

(5.14)

O2

+ hv (at Hopfield bands) +

02*

+e

+ OZ+

Later, several workers [252-2551 made further studies of this problem, particularly in connection with the dissociation layer of oxygen. More recently Watanabe et al. [9, 2451 obtained quantitative data on cross-sections to show that both processes (5.13) and (5.14) have maximum rates of absorption in the E layer and suggested that it is not possible yet to discard the oxygen theory in favor of the x-ray theory of the E layer. In particular, a t the wavelength of Lyman-beta, 0 2 has CT value of 1.55 x 10-'8 cm2 and u i value of 0.90 X 10-l8 cm2 (see Table XII), and Johnson et al. [86] have identified the Lyman-beta line in a spectrum obtained a t 115 km. As the flux of Lyman-alpha is about 3.7 X loll quanta cmW2sec-' (see Section

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

209

5.5), the incident flux for Lyman-beta may be about 1O'O quanta cm-l sec-l. This flux may be significantly attenuated by the absorption of atomic oxygen as pointed out by Kato [218]. Nevertheless, as long as Lyman-beta penetrates down to 115 km or lower, it must contribute to the ionization of the E layer. Furthermore, a CIII emission line at 977.0 A was recorded in the same spectrogram [86], so that the radiation in this spectral region does not appear to be negligible as extrapolated by Byram et al. [105]. Very recently, Inoue [257] has criticized the two major theories described above and has made extensive calculations to show that atomic oxygen is first excited to the 3 3D and 3 states and then is ionized by the absorption of visible light, thus producing the E layer. He suggested that the 01 lines a t 1302 to 1306 A recorded on the rocket spectrogram [86] came from the ionosphere and not directly from the sun; however, ionospheric radiation of such enormous intensities which were derived would have been easily detected by the nitric-oxide ion chambers used by Chubb et al. [107]. 5.7. F Layers

From the data on absorption cross-sections and atmospheric densities alone, it is difficult to discuss the F layers. As shown in Fig. 24, practically all wavelengths of ultraviolet radiation reach the altitude of 140 km; but there are a number of Nz bands in the region 650 to 900 which can produce maximum absorption a t about 150 km or possibly a little higher. Some of these bands are probably pre-ionized, as suggested by Tanaka and Takamine [164]. If the FI layer is as low as 150 km, which was suggested by Nicolet and Mange [a611 and Bates [72], it may be possible to associate this layer with some absorption process. Otherwise, the F 1 layer as well as the F z layer must be explained mainly by other atmospheric processes such as recombination. Thus the problem appears to be quite difficult. Recently, Havens et al. [73] have suggested that the F layers result from the general ionization of the atmosphere by resonance lines 584 A (HeI), 304 A (HeII), and radiations at shorter wavelengths. However, Bates [256] emphasized that the reaction (5.15)

0

+ hv(>13.6 ev) -+

Of

+e

may contribute to the F layers. Furthermore, the role of atomic nitrogen is rather vague, but may not be negligible. Although several investigators [246, 262-2641 calculated the distribution of atomic nitrogen, some of the assumptions involved are very uncertain. ACKNOWLEDGMENTS The author is indebted to many investigators for assistance and stimulating discussions, particularly his former associates a t the Air Force Cambridge Research Center and the Naval Research Laboratory.

210

K . WATANABE

LIST OF SYMBOLS

A Rngstriim unit ev electron volt g acceleration of gravity hv quantum of light H scale height I light intensity J rate of dissociation k absorption coefficient in cm-’ K Boltzmann’s constant L absorption path length m molecular mass n particle concentration no Loschmidt’s number No Avogardro’s number P pressure T temperature in degrees Kelvin z layer thickness in atmos-cm z altitude x wavelength in Angstrom p molecular weight p density in gm u absorption cross-section in cm2 u i photoionization cross-section in cm2 REFERENCES 1. Chapman, S. (1930). A theory of upper atmosphere ozone. Mem. R o y . Meteorol. SOC.3, 103-125. 2. Chapman, S . (1930). On ozone and atomic oxygen in the upper atmosphere. Phil. Mag. [7] 10, 369-383. 3. Mitra, S. K. (1952). “The Upper Atmosphere,” 2nd ed., 713 pp. The Asiatic Society, Calcutta. 4. Nicolet, M. (1945). Contribution t o the study of ionospheric structure. M e n i . I n s t . R o y . Meteorol. (Belgium) 19, 1-162. 5. Newell, H. E. (1953). “High Altitude Rocket Research.” Academic Press, New York. 6. Herzberg, G. (1944). “Atomic Spectra and Atomic Structure.” Dover Publications, New York. 7. Herzberg, G. (1950). “Spectra of Diatomic Molecules,” 2nd ed. Van Nostr:ind, New York. 8. See p. 459 in ref. 7. 9. Watanabe, K., and Marmo, F. F. (1956). Photoionization and total absorption cross section of gases. 11. 02 and Nz in the region 85C-15008. J. Chem. Phys. 26, 965-971. 10. Watanabe, K. (1954). Photoionization and total absorption cross section of gases. I. Ionization potentials of several molecules. Cross sections of NH3 and NO. J. Chem. Phys. 22, 1564-1570. 11. Watanabe, K. (1957). Ionization potentials of some molecules. J . Chem. Phys. 26, 542-547.

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

21 1

12. Foner, S. N., and Hudson, R. L. (1956). Ionization potential of the OH free radical by mass spectrometry. J. Chem. Phys. 26, 602-603. 13. Kandel, R. J. (1954). Appearance potential studies. I. Determination of excess kinetic energy. J. Chem. Phys. 22, 14961499. 14. Price. W. C., and Simpson, D. M. (1938). The absorption spectra of carbon dioxide and carbon oxysulphide in the vacuum ultraviolet. Proc. Roy. SOC. (London) A169, 501-512. 15. Price, W. C. (1936). The far ultraviolet absorption spectra and ionization potentials of HzO and HgS. J . Chem. Phys. 3, 147-153. 16. Watanabe, K., and Mottl, J . R. (1957). Ionization potentials of NH, and some amines. J . Chem. Phys. 26, 1773-1774. 17. Price, W. C., and Simpson, D . M. (1941). The absorption spectra of nitrogen dioxide, ozone, and nitrosyl chloride in the vacuum ultraviolet. Trans. Faraday SOC.37, 106113. 18. Herron, J. T., and Schiff, H . I. (1956). Mass spectrometry of ozone. J . Chem. Phys. 24, 1266-1267. 19. Price, W. C., and Tutte, W. T. (1940). The absorption spectra of ethylene, deutero-ethylene and some alkyl-substituted ethylene in the vacuum ultraviolet. Proc. Roy. SOC.(London) A174, 207-219. 20. Morrison, J . D., and Nicholson, A. J . C. (1952). Studies of ionization efficiency. Part 11. The ionization potentials of some organic molecules. J . Chem. Phys. 20, 1021-1023. 21. Price, W. C., and Walsh, A. D. (1945). The absorption spectra of triple bond molecules in the vacuum ultraviolet. Trans. Faraday SOC.41, 381-388. 22. Moore, C. E . (1949). Atomic energy levels. Nat. Bur. Standards (U.S.) Circ. No. 467. 23. See p. 200 in “Atomic Spectra and Atomic Structure” [6]. 24. DuMond, J. W. M., and Cohen, E. R . (1953). Least-square adjustment of the atomic constants, 1952. Rev. Mod. Phys. 26, 691-708. 25. Gaydon, A. G. (1950). “Dissociation Energies.” Dover Publications, New York. 26. Brix, P., and Herzberg, G. (1953). The dissociation energy of oxygen. J. Chem. Phys. 21, 2240. 27. Watanabe, K., unpublished material. 28. See Table 39 in “Spectra of Diatomic Molecules” [7]. 29. Wilkinson, P. G., and Mulliken, R. S. (1957). Dissociation process in oxygen above 1750A. Astrophys. J. 126, 594-600. 30. Warfield, C. N. (1947). Tentative tables for the properties of the upper atmosphere. Natl. Adv. Com. Aeronaut. Notes No.1200, 1-50. 31. Whipple, F. L. (1952). Exploration of the upper atmosphere by meteoritic techniques. Advances in Geophys. 1, 119-154. 32. Elterman, I,. (1953). A series of stratospheric temperature profile obtained with the search light technique. J. Geophys. Res. 68, 519-530. 33. Best, N. R., Durand, E., Gale, D. I., and Havens, R . J. (1946). Pressure and temperature measurements in the upper atmosphere. Phys. Rev. 70, 985. 34. Havens, R. J., Koll, R. T., and LaGow, H. E. (1952). The pressure, density and temperature of the earth’s upper atmosphere to 160 km. J. Geophys. Res. 67, 59-72. 35. LaGow, H. E., and Ainsworth, J. (1956). Arctic upper-atmosphere pressure and density measurements with rockets. J . Geophys. Res. 61, 77-92. 36. Horowitz, R . , and LaGow, H. E. (1957). Upper air pressure and density measurements from 90 to 220 kilometers with Viking 7 rocket. J . Geophys. Res. 62,57-78.

212

K. WATANABE

37. Ference, M., Stroud, W. G., Walsh, J. R., and Weisner, A. G. (1956). Measurements of temperature at elevations of 30 to 80 kilometers by the rocket-grenade experiment. J. Meteorol. 13, 5-12. 38. Weisner, A. G. (1956). Measurement of winds a t elevation of 30 to 80 kilometers by the rocket-grenade experiment. J. Meteorol. 13, 30-39. 39. Stroud, W. G., Nordberg, W., and Walsh, J. R. (1956). Atmospheric temperature and winds between 30 and 80 km. J. Geophys. Res. 61, 45-56. 40. Byram, E. T., Chubb, T. A., and Friedman, H. (1956). The solar X-ray spectrum and the density of the upper atmosphere. J. Geophys. Res. 61, 251-263. 41. Sicinski, H. S., Spencer, N. W., and Dow, W. G. (1954). Rocket measurement of upper atmosphere ambient temperature and pressure in the 30- to 75-kilometer region. J. A p p l . P h y s . 26, 161-168. 42. Bartman, F. L. (1954). Falling sphere method for upper air density and temperature. In “Rocket Exploration of Upper Atmosphere” (by R. L. E. Boyd and M. J. Seaton, eds.), pp. 98-107. Pergamon Press, London. 43. Liu, V. C. (1956). On a Pitot-tube method of upper atmosphere measurements. J. Geophys. Res. 61, 171-178. 44. Rocket Panel (1952). Pressures, densities and temperatures in the upper atmosphere. P h y s . Rev. 88, 1027-1032. 45. Newell, H. E. (1955). Rocket data on atmospheric pressure, temperature, density and wind. A n n . ghophys. 11, 115-144. 46. Johnson, F. S. (1956). Temperature distribution of the ionosphere under control of thermal conductivity. J. Geophys. Res. 61, 71-76. 47. Paneth, F. A. (1937). The chemical composition of the atmosphere. Quart. J . R o y . Meteorol. SOC.63, 433-438. 48. Gluckauf, E. (1946). A micro-analysis of the helium and neon contents of air. Proc. R o y . SOC.(London) A186, 98-119. 49. Jones, L. M. (1954). The measurement of diffusive separation in the upper atmosphere. See pp. 143-156 in “Rocket Exploration of Upper Atmosphere”

W1. 50. Reasbeck, P., and Wiborg, B. W. (1954). Chemical analysis of upper atmosphere air samples from 50 km t o 93 km height. See pp. 158-161 in ref. 42. 51. Martin, G. R. (1954). The composition of the atmosphere above 60 km. See pp. 161-168 in “Rocket Exploration of Upper Atmosphere” [42]. 52. Goldberg, L. (1954). The absorption spectrum of the atmosphere. In “The Earth as a Planet” (G. P. Kuiper, ed.), pp. 434490. Univ. Chicago Press, Chicago. 53. Dutsch, H. U. (1946). “Photochemische Theorie des atmospharischen Ozons unter Berucksichtigung von Nichtgleichgewicht-Zustanden und Luftbewegungen.” Leeman and Go., Zurich. 54. Fabry, C. (1950). “L’ozone atmosphbrique.” Centre National de la Recherche Scientifique, Paris. 55. Craig, R . A. (1950). The observation and photochemistry of atmospheric ozone and their meteorological significance. Meteorol. Monographs 1 (2), 1-50, 56. Prokof’eva, I. A. (1951). “Atmosfernyi ozon.” Akademiia Nauk, Moscow. 57. Johnson, F. S., Purcell, J. D., Tousey, R., and Watanabe, K. (1952). Direct measurements of the vertical distribution of atmospheric ozone to 70 kilometer altitude. J. Geophys. Res. 67, 157-176. 58. Johnson, F. S., Purcell, J. D., and Tousey, R . (1951). Measurements of the vertical distribution of ozone from rockets. J. Geophys. Res. 66, 583-594. 59. Byram, E. T., Chubb, T. A., and Friedman, H. (1955). Dissociation of oxygen in the upper atmosphere. P h y s . Rev. 98, 1594-1597.

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

213

60. Townsend, J. W., Meadows, E. B., arid Pressly, E . C. (1954). A mass spectrometric study of the upper atmosphere. See pp. 169-188 in “Rocket Exploration of Upper Atmosphere” [42]. 61. Johnson, C. Y . , and Meadows, E. B. (1955). First investigations of ambient positive-ion composition to 219 km by rocket-borne spectrometer. J . Geophys. Res. 60, 193-203. 62. Johnson, C. Y., and Heppner, J. P. (1955). Night-time measurement of positive and negative ion composition to 120 km by rocket-borne spectrometer. J . Geophys. Res. 60,533. 63. Meadows, E. B., and Townsend, J. W. (1956). Neutral gas composition of the upper atmosphere by a rocket-borne mass spectrometer. J . Geophys. Res. 61, 576-577. 64. Johnson, C. Y., and Heppner, J. P. (1956). Daytime measurement of posit,ive and negative ion composition to 131 km by rocket-borne spectrometer. J . Geophys. Res. 61, 575. 65. Seddon, J. C. (1953). Propagation measurements in the ionosphere with the aid of rockets. J . Geophys. Res. 68, 323-335. 66. Jackson, J. E . (1954). Measurements in the E-layer with the Navy Viking rocket. J . Geophys. Res. 69, 377-390. 67. Seddon, J. C., Pickar, A. D., and Jackson, J. E. (1954). Continuous electron density measurements up to 200 km. J . Geophys. Res. 69, 513-524. 68. Jackson, J. E., Kane, J. A., and Seddon, J. C. (1956). Ionospheric electron density measured with the Navy Aerobee-High rocket. J . Geophys. Res. 61, 749-751. 69. Lien, J . R., Marcou, R. J., Ulwick, J. C., McMorrow, D . R., Linsford, L. B., and Haycock, 0. C. (1953). Bifurcation of the E-region. Phys. Rev. 92, 508. 70. Lien, J . R., Marcou, R . J., Ulwick, J. C., Aarons, J., and McMorrow, D. R. (1954). Ionospheric research with rocket-borne instruments. See pp. 223-239 in “Rocket Exploration of Upper Atmosphere” [42]. 71. Hok, G., Spencer, N. W., and Dow, W. G. (1953). Dynamic probe measurements in the ionosphere. J . Geophys. Res. 68, 235-242. 72. Bates, D. R. (1954). Consideration of the results obtained by rockets. See pp. 347-355 in “Rocket Exploration of Upper Atmosphere” [42]. 73. Havens, R. J . , Friedman, H., and Hulburt, E . 0. (1955). The ionospheric Ft region. In “Physics of the Ionosphere,” pp. 237-244. Physical Society, London. 74. Kallmann, H. K., White, W. B., and Newell, H. E. (1956). Physical properties of the atmosphere from 90 to 300 kilometers. J . Geophys. Res. 61, 513-524. 75. Byram, E . T., Chubb, T. A., Friedman, H., and Gailar, N. (1953). Lyman-alpha radiation in the solar spectrum. Phys. Rev. 91, 1278-1279. 76. Preston, W. M. (1940). The origin of radio fade-outs and the absorption coefficient of gases for light of wavelength 1215.78. Phys. Rev. 67, 887-894. 77. Watanabe, K., Marmo, F. F., and Inn, E. C. Y. (1953). Formation of the D layer. Phys. Rev. 90, 155-156. 78. Watanabe, K., unpublished material. 79. Regener, E., and Regener, V. H. (1934). Aufnahmen des ultravioleten Sonnenspektrums in der Stratosphare und vertikale Ozonverteilung. Phys. 2. 36, 788-793. 80. Baum, W. A., Johnson, F. S., Oberly, J. J., Rockwood, C. C . , Strain, C. V., and Tousey, R. (1946). Solar ultraviolet spectrum t o 88 km. Phys. Rev. 70, 781-782.

214

K. WATANABE

81. Hopfield, J . J., and Clearman, H. E. (1948). Ultraviolet spectra of the sun from V-2 rockets. Phys. Rev. 73, 877-884. 82. Tousey, R. (1953). Rocket spectroscopy. J . Opt. SOC.A m . 43, 245-251. 83. Pietenpol, W. B., Rense, W. A., Walz, F. C., Stacey, D. S., and Jackson, J . M . (1953). Lyman-alpha line photographed in the sun’s spectrum. Phps. Rev. 90, 156. 84. Rense, W. A. (1953). Intensity of Lyman-alpha line in the solar spectrum. Phys. Rev. 91, 299-302. 85. Nidey, R. A., and Stacey, D. S. (1956). Photoelectric angular error sensors. Rev. Sci. Instrum. 27, 216-218. 86. Johnson, F . S., Malitson, H. H., Purcell, J. D., and Tousey, It. (1955). Emission lines in the solar ultraviolet spectrum. Paper 101 presented at April Meeting Am. Astronom. SOC..Princeton. 87. Johnson, F. S., Purcell, J. D., andTousey, R. (1954). An extension of the extreme ultraviolet solar spectrum. Phys. Rev. 96, 621. 88. Jursa, A. S., LeBlanc, F. J., and Tanaka, Y. (1955). Results of a recent attempt t o record the solar spectrum in the region of 9W3000A. J . Opt. Sue. Am. 46, 1085-1086. 89. Durand, E., Oberly, J. J., and Tousey, R. (1949). Analysis of the first rocket ultraviolet solar spectra. Astrophys. J . 109, 1-16. 90. Clearman, H . E. (1953). The solar spectrum from A2285 to X3000. Astrophys. J . 117, 29-10. 91. Wilson, N . R., Tousey, R., Purcell, J. D., Johnson, F. S., and Moore, C. E. (1954). A revised analysis of the solar spectrum from 2990 to 2635A. Astrophys. J . 119, 590-612. 92. Miller, S. C., Mercure, R., and Rense, W. A. (1956). Lyman-alpha intensity and solar limb darkening from rocket spectrograms. Astrophys. J . 124, 580-585. 93. Packer, D. M., and Lock, C. (1951). Thermocouple measurements of spectral intensities in the vacuum ultra-violet. J . O p t . SOC.A m . 41, 699-701. 94. Watanabe, K . , and Inn, E. C. Y. (1953). Intensity measurements in the vacuum ultraviolet. J . Opt. SOC.Am. 43, 32-35. 95. Wainfan, N., Walker, W. C., and Weissler, G. L. (1953). Preliminary results on photoelectric yields of Pt and T a and on photoionization in 0 2 and Nn in the vacuum ultraviolet. J . Appl. Phys. 24, 1318-1321. 96. Johnson, F. S., Watanabe, K., and Tousey, R . (1951). Fluorescent sensitized photomultipliers for heterochromatic photometry. J . O p t . SOC.A m . 41, 702-708. 97. Johnson, F. S. (1956). Spectral radiance of the carbon arc from 1900 to 2500 Angstroms. J . Opt. SOC.A m . 46, 101-103. 98. Watanabe, K. (1951). Properties of a CaSOk :Mn phosphor under vacuum ultraviolet excitation. Phys. Rev. 83, 785-791. 99. Tousey, R., Watanabe, K., and Purcell, J. D. (1951). Measurements of solar extreme ultraviolet and X-rays from rockets by means of a CaSO,: Mn phosphor. Phys. Rev. 83, 792-797. 100. Friedman, H., Lichtman, S. W., and Byram, E. T. (1951). Photon counter measurement of solar X-rays and extreme ultraviolet. Phys. Rev. 83, 1025-1030. 101. Hinteregger, H. E., and Watanabe, K. (1953). Photoelectric cells for the vacuum ultraviolet. J . O p t . SOC.A m . 43, 604-608. 102. Johnson, F. S., Purcell, J. D., Tousey, R., and Wilson, N. (1954). The ultraviolet spectrum of the sun. See pp. 279-288 i n “Rocket Exploration of Upper Atmosphere” [42].

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

215

103. Burnight, T. R. (1949). Soft X-radiation in the upper atmosphere. Phys. Rev. 76, 165. 104. Burnight, T. R . (1952). Ultraviolet radiation arid X-rays of solar origin. I n “Physics and Medicine of the Upper Atmosphere” (C. S. White and 0. 0. Benson, Jr., eds.), pp. 22G238. Univ. New Mexico Press, Albuquerque. 105. Byram, E. T., Chubb, T. A., and Friedman, H. (1953). The contributionof solar X-rays to E-layer ionization. Phys. Rev. 92, 1066-1067. 106. Friedman, H., and Chubb, T. A. (1955). Solar X-ray emission and the height of the D layer during radio fade out. See pp. 58-62 in “Physics of the Ionosphere” [731. 107. Chubb, T. A,, Friedman, H., Kreplin, R. W., and Kupperian, J . E. (1957). Lyman alpha and X-ray emission during a small solar flare. J . Geophys. Res. 62, 389-398. 108. Mercure, R., Miller, S. C., Rense, W . A., and Stuart, F. (1956). The sun’s disk in Lyman-alpha radiation. J . Geophys. Res. 61, 571-573. 109. de Jager, C. (1955). The ultra-violet and X-ray spectra of the sun. Ann. ghophys. 11, 330-352. 110. Friedman, H. (1955). The solar spectrum below 2000 Angstroms. Ann. GBophys. 11, 174-180. 111. Bondi, H . , Hoyle, F., and Lyttleton, R . A. (1947). On the structure of the solar corona and chromosphere. Monthly Not. Roy. Ast. SOC.107, 184-210. 112. Woolley, R . v. d. R., and Allen, C. W. (1948). The coronal emission spectrum. Monthly Not. Roy. Ast. SOC.108, 292-305. 113. Woolley, R . v. d. R., and Allen, C. W. (1950). Ultraviolet emission from the chromosphere. Monthly Not. Roy. Ast. Soc. 110, 358-372. 114. Nicolet, M. (1952). Solar physics and the atmosphere of the earth. See pp. 182223 in “Physics and Medicine of the Upper Atmosphere” [1041. 115. Elwert, G. (1952). Uber die Ionisations und Rekombinations-prozesse in einem Plasma und die Ionisationsformel der Sonnenkorona. 2. Naturforsch. 7, 432439. 116. Elwert, G. (1953). Uber die Bedeutung einer weichen Rontgenstrahlung der Sonnenkorona fur die Bildung der normalen ionospharischen E-schicht. J. Atmosph. Terr. Phys. 4, 68-77. 117. Elwert, G. (1954). Die weiche Rontgenstrahlung der ungestorten Sonnenkorona. Z.Naturforsch. 9, 637-653. 118. Hoyle, F., and Bates, D. R. (1948). The production of the E-layer. Terr. Magn. 63, 51-62. 119. Kreusler, H. (1901). Photoelectric current applied to ultraviolet photometry. Ann. d . Phys. 6, 412423. 120. Ladenburg, R., and van Voorhis, C. C . (1933). The continuous absorption of oxygen between 1750 and 1300A and its bearing upon the dispersion. Phys. Rev. 43, 315-321. 121. Watanabe, K., Inn, E. C. Y., and Zelikoff, M. (1952). Absorption coefficients of gases in the vacuum ultraviolet. J . Chem. Phys. 20, 1969-1970. 122. Watanabe, K., Zelikoff, M., and Inn, E. C. Y. (1953). Absorption coefficients of several atmospheric gases. Geophys. Res. Pap. (Geophys. Res. Directorate) Air Force Cambridge Research Center No. 21. 123. Watanabe, K., Inn, E. C. Y . , and Zelikoff, M. (1953). Absorption coefficients of oxygen in the vacuum ultraviolet. J. Chem. Phys. 21, 1026-1030. 124. Zelikoff, M., Watanabe, K., and Inn, E. C. Y. (1953). Absorption coefficients of

216

K. WATANABE

gases in the vacuum ultraviolet. Part 11. Nitrous oxide. J . Chem. Ph?js. 21, 1643-1647. 125. Inn, E. C. Y., Watanabe, K., and Zelikoff, M. (1953). Absorption coefficients of gases in the vacuum ultraviolet. Part 111. COZ. J . Chem. Phys. 21, 1648-1650. 126. Tanaka, Y., Inn, E. C. Y., and Watanabe, K. (1953). Absorption coefficients of gases in the vacuum ultraviolet. Part IV. Ozone. J . Chem. Phys. 21, 1651-1653. 127. Watanabe, K., and Zelikoff, M. (1953). Absorption coefficients of water vapor in the vacuum ultraviolet. J . Opt. SOC.A m . 43, 753-755. 128. Watanabe, K., Marmo, F. F., and Inn, E. C. Y. (1953). Photoionization cross sections of nitric oxide. Phys. Rev. 91, 1155-1158. 129. Marmo, F. F. (1953). Absorption coefficients of nitrogen oxide in the vacuum ultraviolet. J . Opt. SOC.Am. 43, 1186-1190. 130. Sakai, H., Little, J. W., and Watanabe, K. (1957). Photoionization counter for determination of ionization potentials. J . Chem. Phys. 26, 1335-1336. 131. Curtis, J. P. (1954). Absorption coefficients of air and nitrogen for the extreme ultraviolet. Phys. Rev. 94, 908-910. 132. Aboud, A. A., Curtis, J. P., Mercure, R., and Rense, W. A. (1955). Oxygen gas continuous absorption in the extreme ultraviolet. J . Opt. SOC.A m . 46, 767-768. 133. Hinteregger, H . E. (1953). Differential phototubes for the vacuum ultraviolet. J . Opt. SOC.A m . 43, 328. 134. Sakai, H., Watanabe, K., and Griswold, K., unpublished material. 135. Watanabe, K. (1953). Improvement of grating efficiency in the vacuum ultraviolet by platinizing. J . Opt. SOC.A m . 43, 318. 136. Leifson, S. W. (1926). Absorption spectra of some gases and vapors in the Schumann region. Astrophys. J . 63, 73-89. 137. Hopfield, J. J. (1930). New ultraviolet spectrum of helium. Astrophys. J . 7 2 , 133-145. 138. Hopfield, J. J . (1930). New oxygen spectra in the ultraviolet and new spectra in nitrogen. Phys. Rev. 36, 789-790. 139. Price, W. C., and Collins, G. (1935). The far ultraviolet absorption spectrum of oxygen. Phys. Rev. 48, 714-719. 140. Curry, J., and Herzberg, G. (1934). Uber die ultravioletten Absorptionsbanden des Sauerstoffs (Schumann-Runge-Banden). Ann. d . Phys. 19, 8W808. 141. Knauss, H. P., and Ballard, S. S. (1935). Rotational structure of the SchumannRunge bands of oxygen in the vacuum region. Phys. Rev. 48, 796-799. +142. Tanaka, Y., and Takamine, T. (1942). Vibrational structure of the *&- (08) 32,- Rydberg series of 0 2 Sci. Pap. Inst. Phys. Chem. Res. (Tokyo) 39, 436-446. 143. Tanaka, Y. (1952). On the new absorption bands of the oxygen molecule in the far ultraviolet region. J . Chem. Phys. 21, 1728-1733. 144. Brix, P., and Herzberg, G. (1954). Fine structure of the Schumann-Runge bands near the convergence limit and the dissociation energy of the oxygen molecule. Canad. J . Phys. 32, 110-135. 145. Buisson, H., Jausseran, C., and Rouard, P. (1933). Transparency of the lower atmosphere. Rev. opt. 12, 70-80. 146. Schneider, E. G . (1937). The effect of foreign molecules on the absorption coefficient of oxygen in the Schumann region. J . Chem. Phys. 6, 106-107. 147. Williams, S. E. (1940). Absorption of hydrogen Lyman radiation by atmospheric gases. Nature 146, 68. 148. Vassy, A. (1941). Atmospheric absorption in the ultraviolet region of the spectrum. Ann. phys. 16, 145-199.

.

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

217

149. Clark, K. C. (1952). Ionospheric absorption by Nz and 0 2 of certain extreme ultraviolet solar wavelengths. Phys. Rev. 87, 271-276. 150. Weissler, G. L., and Lee, P. (1952). Absorption coefficients of oxygen in the vacuum ultraviolet. J . Opt. SOC.A m . 42, 200-203. 151. Ditchburn, R . W., and Heddle, D. W. 0. (1953). Absorption cross-sections in the vacuum ultraviolet. I. Continuous absorption of oxygen. (1800 to 1300A). Proe. Roy. SOC.(London) A220, 61-70. 152. Ditchburn, R . W., and Heddle, D. W. 0. (1954). Absorption cross-sections in the vacuum ultra-violet. 11. The Schumann-Runge bands of oxygen. (20001750A). Proc. Roy. SOC.(London) A226, 509-521. 153. Ditchburn, R. W., Bradley, J. E. S., Cannon, C. G., and Munday, G. (1954). Absorption cross-sections for Lyman a and neighboring lines. See pp. 327-334 i n “Rocket Exploration of Upper Atmosphere” 1421. 154. Wainfan, N., Walker, W. C., and Weissler, G. L. (1955). Photoionization efficiencies and cross sections in 0 2 , N Z, COZ , A, HzO, HZ, and CHd . Phys. Rev. 99, 542-549. 155. Lee, P. (1955). Photodissociation and photoionization of oxygen as inferred from measured absorption coefficients. J . Opt. Soe. Am. 46, 703-709. 156. Flory, P. J. (1936). Predissociation of the oxygen molecule. J . Chem. Phys. 4, 23-27. 157. Volman, D. H . (1956). Photochemical oxygen-hydrogen reaction a t 18498. J . Chem. Phys. 26, 288-292. 158. Watanabe, K., Mottl, J., and Sakai, H., unpublished material. 159. Hopfield, J. J. (1927). Absorption spectra in the extreme ultra-violet. Phys. Rev. 29, 356. 160. Birge, R. T., and Hopfield, J. J. (1928). The ultra-violet band spectrum of nitrogen. Astrophys. J . 68, 257-278. 161. Sponer, H . (1927). Die Absorptionsbanden des Stickstoffs. 2.Phys. 41,611-618. 162. Worley, R. E., and Jenkins. F. A. (1938). A new Rydberg series in N Z. Phys. Rev. 64, 305. 163. Takamine, T., Suga, T., and Tanaka, Y. (1938). On the band spectra of nitrogen forming a Rydberg series. Sci. Pap. Inst. Phys. Chem. Res. (Tokyo) 34, 854-864. 164. Tanaka, Y., and Takamine, T. (1942). Vibrational structure of the 8: + ‘2: Rydberg series of Na . Sci. P a p . Inst. Phys. Chem. Res. (Tokyo) 39, 427-436. 165. Worley, R. E. (1943). Absorption spectrum of N Zin the extreme ultraviolet. Phys. Rev. 64, 207-224. 166. Worley, R. E. (1953). A third Rydberg series of N Z. Phys. Rev. 89,863-864. 167. Tanaka, Y. (1955). Absorption spectrum of nitrogen in the region from 1075 t o 1650A. J . Opt. Soe. A m . 46, 663-664. 168. Wilkinson, P. G. (1957). High-resolution absorption spectra of nitrogen in the vacuum ultraviolet. Astrophys. J . 126, 1-9. 169. Weissler, G. L., Lee, P., and Mohr., E. I. (1952). Absoluteabsorptioncoefficients of nitrogen in the vacuum ultraviolet. J . Opt. SOC.A m . 42, 84-90. 170. Astoin, N . , and Granier, J . (1957). Sur le spectre d’absorption de l’azote dans l’ultraviolet extrhme. Compt. rend. 244, 1350-1353. 171. Marmo, F. F., private communication. 172. Lambrey, M. (1930). Spectroscopic researches relating to nitric oxide and nitrogen. Ann. phys. 14, 95-183. 173. Tanaka, Y. (1942). The extreme ultraviolet absorption spectra of nitric oxide. Sci. Pap. Inst. Phys. Chem. Res. (Tokyo) 39, 456464.

218

K. WATANABE

174. Tanaka, Y. (1949). On the absorption spectrum of nitric oxide molecule in the extreme ultraviolet region. 11. J . Sci. Research Inst. (Tokyo) 43, 160-167. 175. Tanaka, Y., Seya, M., and Mori, K. (1951). On the absorption spectra of nitrogen oxide in the extreme ultraviolet region. Sci. of Light (Tokyo) 1, 80-84. 176. Sutcliffe, L . H., and Walsh, A. D. (1953). The ultraviolet absorption spectrum of nitric oxide. Proc. Phys. SOC.(London) A66, 209-216. 177. Ueda, M. (1955). The absorption spectrum of nitric oxide in the far ultraviolet region. Sci. of Light (Tokyo) 3, 143-155. 178. Miescher, E. (1956). Rotationsanalyse der p’-Banden des NO-Molekuls. Nclv. Phys. A C ~ 29, U 401-409. 179. Herzberg, G., Lagerqvist, A., and Miescher, E. (1956). Fine structure analysis and mutual perturbation of the 6 and j3 bands of the NO molecule. Canad. J . Phys. 34, 622-624. 180. Goto, K. (1956). On the spectrum of nitrogen oxide in the extreme ultraviolet region. Sci. of Light (Tokyo) 6 , 4-9. 181. Miescher, E., and Baer, P. (1952). Band spectra in the Schumann region of N O and Nit with enriched nitrogen-15. Nature 169, 581. 182. Tanaka, Y. (1953). On the Miescher-Baer emission bands in the far ultraviolet region. J . Chem. Phys. 21, 562-563. 183. Mayence, J . (1952). Absorption spectra a t low temperature. Study of NzO gas, NO gas and liquid. Ann. phys. 7 , 453-505. 184. Marmo, F. F. (1954). Vacuum ultraviolet absorption studies of nitrogen oxide. Thesis, Department of Chemistry, Harvard University, Cambridge, Massachusetts. 185. Sun, H., and Weissler, G. L. (1955). Absorption coefficients of nitric oxide in the vacuum ultraviolet. J . Chem. Phys. 23, 1372-1373. 186. Granier, J . , and Astoin, N . (1956). Sur le spectre d’absorption de l’oxyde asotique dans I’ultraviolet extr6me. Cornpl. rend. 242, 1431-1433. 187. Walker, W. C . , and Weissler, G. L. (1955). Photoionization efficiencies and cross sections in NzO and NO. J . Chern. Phys. 23, 1962. 188. Zelikoff, M., and Aschenbrand, L. (1956). Vacuum ultra-violet photochemistry. P a r t IV. NO at 12368. J . Chem. Phys. 26, 674476. 189. Hureeler, H., Inghram, M. G., and Morrison, J. D. (1957). Private communication. 190. Ny, T. Z . , and Choong, S. P. (1933). L’absorption de la lumi&-e por I’oeone entre 3050 e t 2150A. Compt. rend. 196, 916-918. 191. Vassy, A. (1941). Atmospheric ultra-violet absorption. Ann. phys. 16, 145-203. 192. Inn, E. C. Y., and Tanaka, Y. (1953). Absorption coefficient of ozone in the ultraviolet and visible regions. J . Opt. SOC.Am. 43, 870-873. 193. Vigroux, E. (1953). Contribution to the experimental study of the absorption on ozone. Ann, phys. 8 , 709-762. 194. Hopfield, J. J. (1950). The absorption spectrum of water vapor between 900 and 2000 Angstroms. Phys. Rev. 77, 560-561. 195. Meinel, A. B. (1950). Hydride emission bands in the spectrum of the night sky. Astrophys. J . 111, 207. 196. Bates, D. R . , and Nicolet, M. (1950). The photochemistry of atmospheric water vapor. J . Geophys. Res. 66, 301-327. 197. Henning, H. J. (1932). Die Absorptionspektren von Kohlendioxyd, Kohlenmonoxyd und Wasserdampf im Gebiet von 600-900AE. Ann. d . Phys. 13,599-620. 198. Rathenau, G. (1933). Untersuchung am Absorptionsspektrum von Wasserdampf und Kohlendioxyd im Gebiet unter 2000h;. Z. Phys. 87, 32-56.

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHER.E

219

199. Wilkinson, P. G., and Johnston, H . L. (1950). The absorption spectra of methane, carbon dioxide, water vapor and ethylene in the vacuum ultraviolet. J. Chem. Phys. 18, 190-193. 200. Harrison, A. J., Cederholm, B. J., and Coffin, E. M. (1951). The ultraviolet absorption spectra of water, ethyl alcohol, diethyl ether, and methyl alcohol. Technical Report, Mount Holyoke College, Holyoke, Massachusetts, pp. 21-24. 201. Johannin-Gilles, A. (1953). Absorption de la vapeur d’eau dans I’ultraviolet de Schumann. Compt. rend. 236, 67c678. 202. Astoin, N., Johannin-Gilles, A., and Vodar, B. (1953). Absorption de la vapeur d’eau dans l’ultraviolet extreme. Compt. rend. 237, 55&560. 203. Astoin, N. (1956). Sur le spectre d’absorption de la vapeur d’eau et d’eau lourde dans l’ultraviolet extreme. Compt. rend. 242, 2327-2329. 204. Watanabe, K., and Jursa, A., unpublished material. 205. Zelikoff, M., and Aschenbrand, L. (1954). Vacuum ultraviolet photochemistry. Part I . Nitrous oxide a t 1470A. J. Chem. Phys. 22, 1680-1684. 206. Zelikoff, M., and Aschenbrand, L. (1954). Vacuum ultraviolet photochemistry, Part 11. Nitrous oxide at 1849A. J. Chem. Phys. 22, 16851687. 207. McMath, R . R., Pierce, A. K., Mohler, 0. C., Goldberg, L., and Donovan, R . A. (1950). NzO bands in the solar spectrum. Phys. Rev. 78, 65. 208. Bates, D. R., and Witherspoon, A. E. (1952). The photochemistry of some minor constituents of the earth’s atmosphere (COZ , CO, CHa , NzO). Monthly Not. ROY.Ast. SOC.112, 101-124. 209. Sen-Gupta, P . K. (1935). Photodissociation of nitrous oxide. Nature 136,513-514. 210. Duncan, A. B. F. (1936). The far ultraviolet absorption spectrum of Nz0. J . Chem. Phys. 4, 638-641. 211. Tanaka, Y., Jursa, A. S., and LeBlanc, F. J. (1957). Higher ionization potential of linear triatomic molecules. Abstr. Symposium Molec. Structure and Spectroscopy, p. 50. Columbus, Ohio. 212. Romand, J., and Mayence, J. (1949). Spectre d’absorption de l’oxyde azoteux gazeux dans la region de Schumann. Compt. rend. 228, 998-1000. 213. Astoin, N., and Granier, J. (1955). Sur le spectre d’absorption de N20 dans l’ultraviolet extreme. Compt. rend. 241, 1736-1738. 214. Sun, H., and Weissler, G. L. (1955). Absorption cross section of carbon dioxide and carbon monoxide in the vacuum ultraviolet. J. Chem. Phys. 23, 1625-1628. 215. Bates, D. R. (1951). The temperature of the upper atmosphere. Proc. Phys. Soc. (London) A64, 805-820. 216. Bates, D. R., and Seaton, M. J. (1949). The quanta1 theory of continuous absorption of radiation by various atoms in their ground states. 11. Further calculations on oxygen, nitrogen, and carbon. Monthly Not. Roy. Ast. Soc. 109, 698-704. 217. Ehler, A. W., and Weissler, G. L. (1955). Ultraviolet absorption of atomic nitrogen in its ionization continuum. J. Opt. SOC.A m . 46, 10351043. 218. Kato, S. (1954). On the solar Lyman-beta radiation and the ionosphere. J. Geomag. Geoelectricity (Japan) 4, 153-156. 219. Moe, G., and Duncan, A. B. F. (1952). Intensity of electronic transitions of methane and carbon tetrafluoride in the vacuum ultra-violet. J. A m . Chem Soe. 74, 3140-3143. 220. Ditchburn, R. W. (1955). Absorption cross-sections in the vacuum ultraviolet. 111. Methane. Proc. Roy. SOC.(London) A229,44-62. 221. Sun, H., and Weissler, G. L. (1955). Absorption cross sections of methane and ammonia in the vacuum ultraviolet. J. Chem. Phys. 23, 1160-1164.

220

K. WATANABE

222. Duncan, A. B. F. (1935). The ultraviolet absorption spectrum of ammonia. Phys. Rev. 47,822-827. 223. Duncan, A. B. F. (1936). The ultraviolet absorption spectra of ammonia. 111. The absorption spectra of the deuteroammonias. A note on Rydberg series in ammonia. Phys. Rev. 60, 700-704. 224. Tannenbaum, E., Coffin, E. M., and Harrison, A. J. (1953). The far ultraviolet absorption spectra of simple alkyl amines. J . Chem. Phys. 21, 311-318. 225. Walker, W. C., and Weissler, G. L. (1955). Photoionization efficiencies and cross sections in NHs . J . Chem. Phys. 23, 1540-1541. 226. Tanaka, Y. (1944). Absorption spectrum of hydrogen molecule in the extreme ultra-violet. Sci. Pap. Inst. Phys. Chem. Res. (Tokyo) 42, 49-86. 227. Lee, P., and Weissler, G. L. (1952). Absolute absorption of the Hs continuum. Astrophys. J . 116,570-571. 228. Lee, P., and Weissler, G. L. (1955). Absorption cross section of helium and argon in the extreme ultraT6olet. Phys. Rev. 99, 540-542. 229. Lee, P., and Weissler, G. L. (1953). The photoionization cross-section of neon. Proc. Roy. SOC.(London) A219, 71-76. 230. Koomen, M. J., Scolnik, R., and Tousey, R. (1956). Distribution of the night airglow (01) 55778 and Na D-layers measured from a rocket. J . Geophys. Res. 61, 304-306. 231. Ditchburn, R. W., Jutsum, P. J., and Marr, G. V. (1953). The continuous absorption of light in alkali-metal vapours. Proc. Roy. SOC.(London) A219, 89-101. 232. Bates, D. R . , and Massey, H. S. W. (1946). The basic reactions in the upper atmosphere. I. Proc. Roy. Soe. (London) A187.261-296. 233. Bates, D. R., and Massey, H. S. W. (1947). The basic reactions in the upper atmosphere. 11. Proc. Roy. SOC.(London) A192, 1-16. 234. Bates, D. R., and Seaton, M. J. (1950). Theoretical considerations regarding the formation of the ionized layer. Proc. Phys. SOC.(London) B63, 129-140. 235. Bates, D. R. (1954). Physics of the upper atmosphere. I n reference 52, pages 57tr-643. 236. Nicolet, M. (1954). Dynamic effects in the high atmosphere. See pp. 644-712 i n “The Earth as a Planet” [521. 237. Massey, H. S. W. (1952). Recombination of gaseous ions. Advances i n Phys., (London) 1,395-426. 238. Bates, D. R. (1956). Recombination in the ionosphere. In “Solar Eclipse and the Ionosphere” pp. 191-197. Pergamon Press, London. 239. Ditchburn, R . W. (1956). Absorption of ultraviolet radiation by the atmospheric gases. Proc. Roy. SOC.(London) A236, 216-226. 240. Watanabe, K., and Zelikoff, M., unpublished material. 241. Penndorf, R . (1949). The vertical distribution of atomic oxygen in the upper atmosphere. J . Geophys. Res. 64, 7-38. 242. Moses, H. E., and Wu, T. Y. (1952). A self-consistent treatment of the oxygen dissociation region in the upper atmosphere. Phys. Rev. 83, 109-121. 243. Nicolet, M., and Mange, P. (1954). The dissociation of oxygen in the high altitude. J . Geophys. Res. 69, 15-45. 244. Nicolet, M. (1954). Aeronomic problem of oxygen dissociation. J . Atmos. Terr. Phys. 6, 132-140. 245. Watanabe, K., Marmo, F. F., and Pressman, J. (1955). Formation of the lower ionosphere. J . Geophys. Res. 60, 513-519. 246. Mitra, A. P. (1954). A tentative model of the equilibrium height distribution of

ULTRAVIOLET ABSORPTION PROCESSES IN UPPER ATMOSPHERE

247. 248.

249. 250. 251. 252. 253. 254. 255. 256. 257. 258. 259. 260. 261.

262. 263. 264.

221

nitric oxide in the high atmosphere and the resulting D-layer. J . Atmos. Terr. Phys. 6 , 28-43. Nicolet, M. (1955). The aeronomic problem of nitrogen oxides. J . Atmos. Terr. Phys. 7,152-169. Marmo, F. F., Pressman, J . , Aschenbrand, L. M., Jursa, A., and Zelikoff, M. (1956). Formation of an artificial ion cloud; photoionization of NO by solar Lyman-alpha a t 95 km. J . Chem. Phys. 26, 187. Pressman, J., Aschenbrand, L. M., Marmo, F. F., Jursa, A., and Zelikoff, M. (1956). Synthetic atmospheric chemiluminescence caused by the release of NO at 106 km. J . Chem. Phys. 26, 187. Bates, D. R . (1949). The emission of the negative system of nitrogen from the upper atmosphere and the significance of the twilight flash in the theory of the ionosphere. Proc. Roy. SOC.(London) A196,562-591. Nicolet, M. (1952). Actions du rayonnement solaire dans la haute atmosphere. Ann. ghophys. 8 , 141-193. Choudhury, D. C. (1952). Production of the E-layer in the oxygen dissociation region in the upper atmosphere. Phys. Rev. 80,405408. Rawer, K., and Argence, E. (1954). Origin of the ionospheric E-layer. Phys. Rev. 94, 253-256. Bauer, E., and Wu, T. Y. (1953). Origin of the E-layer of the ionosphere. Phys. Rev. 92, 1101-1105. Sato, T. (1954). Formation of the E region of the ionosphere. Rep. Ionosphere Res . Japan 8 , 4 9 4 3 . Bates, D. R. (1956). Formation of the ionized layers. See pp. 184-188 i n “Solar Eclipse and the Ionosphere” (238). Inoue,,Y. (1957). On the ionization mechanism in the ionosphere. Japan. J . Geophys. 1.21-120. Hulburt, E. 0. (1938). Photoelectric ionization of the ionosphere. Phys. Rev. 63, 344-351. Vegard, L. (1938). Processes and conditions in auroral region. Geofys. Publ. 12. (5). Wulf, 0. R., and Deming, L. S. (1938). On the production of the ionosphere regions E. and F and the lower-altitude ionization causing radio fade-outs. T e n . Magn. 43, 283-298. Nicolet, M., and Mange, P. (1952). An introduction to the study of the physical constitution and chemical composition of the high atmosphere. Zonos. Res. Lab. Penn. State. Coll. Sci. Rep. No. 35. Bates, D . R. (1952). Some reactions occurring in the earth’s upper atmosphere. Ann. ghophys. 8 , 194-204. Deb, S. (1952). Nitrogen in the upper atmosphere. J . Atmos. ?‘err. Phys. 2 , 309323. Sato, T. (1953). On the distribution of nitrogen in the upper atmosphere. J . Geomap. Geoelectricity (Japan) 3, 71-82.

This Page Intentionally Left Blank

THE PHYSICS OF CLOUD MODIFICATION James E. McDonald Institute of Atmospheric Physics, University of Arizona, Tucson, Arizona

Page 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 2. Clouds and the Atmospheric Water Vapor Cycle. . . . . . . . . . . . . . . . . . . . . . . . . . 225 ....... ................ 225 2.1. Terrestrial Clouds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 2.2. Scale Consideratio 2.3. Precipitation Release Rates and Atmospheric Water Vapor Turnover ........................................ . . . . . . . . . . . . . . . 228 s of Cloud and Precipitation 3.1. Historical Remarks. . . . . . . . . . . . . . . . . . . 3.2. The Condensation. Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 3.3. The Precipitation Process. . 3.3.1. The Central Problem 3.3.2. The Ice-Crystal Process.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 3.3.3. The Accretion Process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 3.3.4. Summary.. ... ........ ...... ... 262 4. Recent Developments in Cloud-Modification Techniques . . . . . . . . . . . . . . . . . . . 263 263 4.1. General.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Artificial Nucleation of the Ice-Crystal Process. . . . . . . . . . . . . . . . . . . 264 4.2.1. The Dry-Ice Seeding Technique. . . . . . . . . . . . . . . . . . . . . . . . . . . 264 4.2.2. The Silver Iodide Seeding Technique 4.3. Artificial Stimulation of the Accretion Proc 4.3.1. The Water-Spray Technique. . . . . . . . 4.3.2. The Salt-Seeding Technique.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 289 4.4. Other Types of Cloud-Modification Techniques. . . . . . . . . . . . . . . . . . . . . . . . 5. The Evaluation of Modificat ents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 5.1. Physical Evaluation. . . . . . . .... ....... . . 292 ................................... 293 5.2. Statistical Evaluation.. ................................... 297 6. Concluding Remarks. . . . . . . Acknowledgment. . . . . . . . . . . . . . . . . . . . . . . . . . . . .... ..... . . . . . . 298 References.. ................................................................. 298

1. INTRODUCTION

During the decade that followed immediately upon cessation of World War 11 there occurred remarkably vigorous developments in almost all phases of meteorology. It would seem that no single factor suffices to account for this marked acceleration of meteorological research that began in the mid-forties; rather it appears in retrospect that a peculiar combina223

224

JAMES E. MCDONALD

tion of factors joined to produce this phenomenon. First, a very great number of younger workers who had received meteorological training during their military service entered the field in many countries throughout, the world. Second, the recognition of the many ways in which progress in geophysics relates to matters of military importance operated, by virtue of the circumstances of the decade, to stimulate support of geophysical research in general, and meteorological research in particular, a t a level unparalleled in earlier years. Finally, a number of significant new techniques of observation and research which had been developed either directly or indirectly through the exigencies of wartime activities became available to the meteorologist just a t a time when personnel and research support permitted their optimal exploitation. One of the most exciting of these meteorological developments of the past decade has been the discovery of techniques for modifying, to a certain degree, the physical processes occurring within natural clouds. This development, abetted by all of the auspicious circumstances cited above, has given great impetus to advance in all of those portions of physical meteorology concerned with the physics of clouds and precipitation proeesses. As a result, one is tempted to say that there is an order-of-magnitude difference between what is known today about cloud physics and what was known a mere decade ago; but this statement, even if a fair one, tends to obscure what has become still more evident, namely, that our present knowledge of cloud physics is still an order-of-magnitude short of what d, must be if we are ever to exploit optimally the recently recognized prospects of control of natural precipitation. I n the following discussion, the present position of cloud physics will be reviewed, and the progress that has been made toward the goal of controlling, or at least modifying significantly, the phenomena occurring within natural clouds will be summarized. A peculiar circumstance enters in the form of the severe difficulty of accurately assessing that progress as it may (or may not) be exhibited in actual field experiments. These difficulties, and their origin in the physical complexity of the problem, must be clearly understood by the geophysicist or other reader who wishes to understand the current status of cloud modification and the challenge that the goal of controlled cloud modification has placed before the meteorologist. My main objective will be to discuss the salient points of the physical theories of cloud and precipitation processes which have undergone such rapid evolution in the past few years, and to try to call attention to the principal implications which these theories have for prospects of control of certain links in the chain of events leading to precipitation.

PHYSICS OF CLOUD MODIFICATION

2. CLOUDSAND

THE

225

ATMOSPHERIC WATERVAPORCYCLE

2.1 Terrestrial Clouds

The chief goal of man’s efforts a t cloud modification is simply this: to cause more precipitation to fall upon selected continental areas than would occur by wholly natural processes. Before turning to an examination of microphysical phenomena in the cloud that enter into these precipitation processes, it is pertinent to look rather broadly a t the basic question of how there happen to be clouds and rain processes to modify in the first place and to inspect the magnitudes of certain parameters of the hydrologic cycle that influence the scope of potential modification methods. At the start it must be recognized that the clouds with which we are all so familiar and upon which we must depend as immediate sources of all but a small fraction of our continental water supplies are in many respects quite adventitious features of our planet’s atmosphere. Even without exploring recent insights gained in theoretical studies of the geochemical evolution of planetary atmospheres, we need only turn to our closest neighbors in the solar system to see that water substance cannot be taken for granted as an inevitably abundant material on the surface of a planet of the earth’s size, for neither of these neighbor-planets has spectroscopically observable quantities of water vapor in its atmosphere. But, even given the presence of an extensive layer of liquid water covering most of our planet’s surface and noting that its physical and chemical properties are such as to make it almost uniquely suited to support the kind of plant and animal life in which we have so strong a practical interest, it is not immediately evident that it was inevitable that geophysical processes should unfold in such ways as to insure that significant amounts of this material would be transported to continental areas. As a matter of fact, viewed in percentual terms, only a seemingly insignificant fraction of the oceanic water is thus transported in the hydrologic cycle. Of the total amount of terrestrial water variously estimated a t from about 5 X loz3to about 5 X gm (Hutchinson [l]gives 1.4 X loz4 gm, for example), only about lozogm, or a mere hundredth of one per cent of the total, is found a t any time in the rivers and lakes of the continents. A very much larger fraction, perhaps four or five per cent in all [2], is present as comparatively immobile ground water and glacier ice, but these latter constitute quasi-steady-state storage reservoirs associated with processes having such slow replenishment rates as to be poor indicators of the rate of exchange of water between oceans and continents. Taking Bannon’s recent results on world-wide distribution of atmospheric

226

JAMES E. MCDONALD

water vapor, as summarized by Sutcliffe [3], namely a value of 2.5 cm for the global yearly mean precipitable water-vapor content (integrated vapor content from surface to outer limits of the atmosphere, expressed in equivalent depth to which it would stand if entirely condensed and vertically precipitated), we find that there is only of the order of l O l g gm of H20 present in our atmosphere a t any one time as vapor, or a mere thousandth of one per cent of the terrestrial stock of water substance. That the total atmospheric water vapor content is so small despite presence of large bodies of liquid water is, of course, a consequence of the comparatively small saturation vapor pressure of water a t terrestrial temperatures, which, in turn, results from the unusually high latent heat of vaporization of water. (The latter, finally, is a consequence of the molecular structure of water that permits strong association by hydrogen bonding.) The one-thousandth of one per cent of the total terrestrial water that is, at any one time, found in the form of atmospheric water vapor is advected in the air movements of the general circulation from one part of the world to another and thereby occasionally reaches continental interiors where, if forced to ascend by any of several dynamical processes, it may condense to form a cloud. From that cloud there may, if a fairly complex series of events unfolds rapidly enough, fall some precipitation that reaches the continental surface. I n the two following sections certain scale factors and characteristic magnitudes of these processes will be examined. 2.2 Scale Considerations It is instructive to make the following order-of-magnitude calculations to clarify the quantitative aspects of the role of clouds in the hydrologic cycle. These calculations will serve to reveal the scope of the problem man encounters when he seeks to make any appreciable change in the planetary water budget. The immediate objective will be to attempt an estimate of the fraction of total instantaneous stock of water vapor which might be made to undergo artificially induced precipitation if a hypothetical world-wide program of continental rainmaking were operating a t optimistic efficiencies. Such an estimate is surely of academic interest to the geophysicist and also sheds light on fears that have sometimes been raised concerning tampering with the natural hydrologic cycle. Students of the global heat balance who have had to make climatological estimates of mean global cloudiness for use in evaluating the albedo term of the heat-budget have arrived a t a figure of about 0.50 for the mean cloud cover for the year and the earth as a whole. Cloud depths will, in general, decrease with increasing latitude while the zonal mean cloudiness will tend to increase toward the latitude of the subpolar low pressure belt in such a way that it seems reasonable (though clearly very approximate)

PHYSICS OF CLOUD MODIFICATION

227

to assume for present purposes a mean cloud depth of about 2 km as a global average. Liquid water contents will tend to vary latitudinally in similar fashion, both because of the effect of the latitudinal temperature gradient and because of meridional variations in prevailing cloud types, and with these variations in mind, current data suggest that half a gram of cloud water per cubic meter of cloud space will be reasonable as an over-all average. Combining average cloudiness, mean depth, and mean liquid water content yields an average of a tenth of a gram of cloud liquid water per square centimeter of surface for the world as a whole. Bannon’s data, cited above, show that the mean precipitable water vapor content is about 2.5 ern for the global annual mean. Hence, cloud liquid water content accounts for about 0.10/2.5, or only one twenty-fifth, of total atmospheric water vapor content, on an annual global average basis. Of this amount of water, roughly one-fourth will occur in clouds lying over continental areas, so only about x o o t h of the instantaneous store of atmospheric HzO is in a form and location even remotely amenable to useful manipulation by cloud-modification techniques. The next step in this estimate of the geophysical scale of our potential cloud-modification problem should consist in cutting down our last fraction, >iooth, by some factor representing the portion of all continental cloud types that are brought by natural processes sufficiently near to the stage of precipitating so that one can begin to discuss manipulating their release of water. I know of no source of climatic data to assist in making a close estimate of this factor, for there is a glaring lack of good data on worldwide distribution of clouds reported in a manner that could be termed cloud-physically meaningful. I n view of the nature of the mean cloudiness figure of 0.50 used above (applying as it does to all clouds without regard to their being of a type associated with precipitation), I should guess that not more than about one-tenth of the global mean cloud cover is modifiable in the sense of being near enough to undergoing natural release of precipitation that manipulation would hold any promise a t all. It scarcely seems possible that such an estimate of one-tenth could be low by even afactor of two; it is much more likely to be unduly large. Hence, let us say, conservatively, that perhaps one-tenth of >iooth or one-thousandth of the total instantaneous atmospheric HzO is actually subject a t any one time to modification techniques over continental areas. Finally, uncertain as is the present state of evaluation of recent modification experiments, most meteorologists would now probably agree that prospective techniques cannot be expected to augment natural precipitation by much more than about ten per cent, as an optimistic upper limit. This means that about a tenth of a thousandth, or of the order of only one ten-thousandth of the instantaneous world-wide stock of atmospheric water

228

JAMES E. MCDONALD

TABLE I. Estimated magnitudes of quantities controlling effects of world-wide cloud seeding.

Estimated fraction of total atmospheric H20 condensed, a t any one instant, into cloud liquid water (based on mean cloudiness of 0.5 and mean cloud depth of 2 k m ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Same fraction for continental areas only. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Estimated upper limit of fraction of all continental cloud cover amenable t o treatment t o induce artificial precipitation. . . . . . . . . . . . . . . . . . . . . . . . . . Generally accepted (1958) order of upper limiting magnitude of obtainable increase in precipitation by cloud modification methods. . . . . . . . . . . . . . . . Consequent upper limit t o estimated fraction of total atmospheric H20 which, a t any instant, might be undergoing artificially stimulated precipitation over continents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1/25 1/100

1/10

1/10

1/10,000

substance might be undergoing artifically induced precipitation from favorable cloud types over continental areas i f all such clouds throughout the world were continually being treated in a manner yielding ten per cent increases. The quantities leading to this conclusion are assembled in Table I. It bears repeating that the third and fourth fractions listed in Table I are believed to be too high but are used here to render the final conclusions conservative with respect to relative magnitude of the geophysical effects of cloud seeding at a maximum conceivable rate. Two main conclusions may be drawn from the above estimate. First, it shows us that, as in so many other geophysical problems, man’s efforts to alter the over-all course of geophysical events constitute a minuscule alteration of natural processes. Second, it shows fairly clearly that, notwithstanding the mounting evidence for the comparatively large role that latent heat exchanges play in the energetics of the general circulation of our atmosphere, even a very ambitious world-wide cloud-modification program can scarcely be a source of serious interference with the energy balance of the whole atmosphere-and the figures seem to speak directly for such a program being incapable of sensibly altering the prevailing water vapor content of the atmosphere as a whole. The above discussion glosses over a number of points wherein locally important disturbances might possibly occur, but serves to establish perspective concerning what has been the sometimes misconstrued geophysical scale of potential cloudmodification effects. 2.3. Precipitation Rebase Rates and Atmospheric Water Vapor Turnover Rates

In spite of the great practical importance of precipitation, there exists a surprising dearth of reliable figures on the efficiency with which cloud processes release water from our atmosphere. Braham [4]found in a de-

PHYSICS OF CLOUD MODIFICATION

229

tailed analysis of mid latitude thunderstorm clouds, that only about one-tenth of all water molecules entering such clouds as vapor ever reach the ground as precipitation; the remainder is evaporated in the downdraft or is expelled by the upper divergent portions of the cloud circulation. No comparable figure for other cloud types has come to my attention, but pertinent data of a somewhat different type have been presented by Huff and Stout [5] who found that about only 5 % of all of the water flowing as vapor into the space above the state of Illinois over a threeyear period was precipitated upon that area, the remaining 95% simply flowing out of the space in vapor (or cloud) form. The Illinois release rate exhibited a range of from slightly over 4% to slightly over 7 % from one seasonal average to another, so the mean is reasonably representative for all seasons. Reitan [6], using a somewhat different approach, found that during the summer rainy season in Arizona, about 5 % of the total precipitable water vapor overhead at any given time had precipitated during the ensuing twenty-four hours, with an observed range of from about three to thirteen per cent in this daily release rate. Inasmuch as the time-period used by Reitan for the Arizona case is close to the time required for air parcels to move across Arizona at normal summer wind speeds, and inasmuch as Illinois and Arizona have linear dimensions of comparable size, these two estimates can be seen to agree rather well. Both Braham’s figure for a specific rain-producing cloud type and the other two figures of more general nature have this important implication for cloud modification studies: The natural efiiencies of removal of water from the atmospheric water vapor stock seem rather low (when defined as above), so low as to seem to offer some hope that artificially induced increases might be achieved if sufficiently complete knowledge of all pertinent processes were at hand. Specifically, these few efficiency indicators suggest that a cloud-modification technique that accomplished no more than a 10% relative increase of the natural precipitation rate in clouds over a given area would actually represent an absolute increase of release rate of rather less than about one precentage unit on an efficiency scale defined in either of the two basic ways used just above (fraction of vapor entering a cloud, or fraction of vapor flowing across a given geographical area), which does not seem an entirely hopelessly large improvement upon nature. Further indication of the relatively small influence of cloud seeding on the hydrologic cycle, as well as further appreciation of the time-scale of the potentially modifiable meteorological processes operating in the hydrologic cycle, can be gained by considering the following argument (see, for example, Sutcliffe [3]): Define the “turnover time” of the atmospheric water vapor as the length of time required, on the average, for world-

230

JAMES E. MCDONALD

wide precipitation processes to extract from our atmosphere an amount of water equal to its mean instantaneous stock of water vapor. Now, as the average world-wide precipitable water vapor content of the atmosphere is close to 3 cm while the world-wide average annual precipitation is about 100 cm, approximately one-thirtieth of a year, or about twelve days, must be required, in the mean, for one complete turnover of the atmospheric water vapor content. If this estimate of the water vapor turnover time is correct (and it is in very good agreement with results of an independent argument based on consideration of how solar radiation dictates the evaporative rate that constitutes the equilibrant of the above-considered precipitation rate), then, because this is from many points of view a quite short time interval, one is tempted to say that atmospheric precipitation processes are not of such low efficiency after all, despite the efficiency estimates made earlier in this section. But this, it will be seen, becomes simply a matter of definition. From the viewpoint of, say, the Arizonan, a mere 5 % removal rate per day seems regrettably low and worth trying to increase (and this Arizona rate is, in fact, only about half the global average rate implied by a ten-day turnover time); but in ten to twenty days the air parcels that passed over Arizona with a daily loss thereto of only about 5 % of their moisture will have suffered sufficient number of encounters with circulation systems of the cloud-generating type that the probability of their having lost all of their original water vapor content approaches unity. Briefly, the fact that the entire quantity of atmospheric water vapor (exclusive of the small quantities circulating above the tropopause) is effectively removed and replenished once every ten or twelve days must surely be admitted to imply a rapid exchange rate for the world as a whole (especially so when compared with exchange rates of all other atmospheric gases, of which COZis one of the most rapidly exchanged yet has a turnover time now estimated a t about ten years). Nevertheless, this inevitably means that, for any given geographic area, the horizontal .flux of vapor overhead averages more than a n order-of-magnitude larger than the vertically downward $ux of precipitation, and this does make the latter seem to be governed by “inefficient” processes. To repeat, “efficiency” is here entirely a matter of definition and viewpoint. From our estimate of the turnover time we may now draw two conclusions relevant to the general prospects of cloud modification. (1) If the natural hydrologic cycle has a characteristic atmospheric turnover time of only about ten or twelve days for its evaporation-precipitation link, then the rate of recovery from the globally small effects of even a very ambitious continental seeding program will be quite rapid. Thus, we seem, on these grounds, still further justified in rejecting fears

PHYSICS OF CLOUD MODIFICATION

231

that widespread seeding a t rates now envisaged by the most ardent enthusiasts could so deplete the natural stock of atmospheric water vapor as to alter appreciably other meteorological phenomena on a world-wide scale. (2) If the turnover time is of the order of ten days, then during each day, about one-tenth of the water vapor overhead must, on the average, be returned t o the earth’s surface by precipitation. But earlier we concluded that an average of only about one twenty-fifth of the water substance in the atmosphere is a t any one time condensed into clouds and, of this, I guessed that only about one-tenth is of a type capable of yielding significant precipitation; so a t any one moment only about 4550th of the total stock of atmospheric water vapor is instantaneously contained in clouds of a type that are a t or near the precipitation stage. During the course of an average day, clouds in various parts of the world continually form and dissipate and a small portion of these release precipitation before dissipating. The 3550th of the total atmospheric water that is at one moment in a certain set of clouds scattered over the world will not be found in the same set of clouds, say, an hour or two later, for these have by then precipitated or evaporated; but what we can say on a statistical basis is that, if each day world-wide precipitation must account for an average removal of about one-tenth of the total stock of atmospheric water vapor, whereas at any one moment only 3650th of that total is locked up in clouds of precipitating type, then the process of formation of, and release of precipitation from, these clouds must repeat itself some twenty-five times per day, if we assume momentarily that all condensed water in these clouds falls out. But, as a matter of fact, two opposing characteristics of clouds actually enter. One must recognize, first, a tendency for clouds to have passing through their boundaries more total vapor than that equivalent to the liquid water content present a t any one instant and, second, an opposite tendency for only a fraction of all of this vapor to be converted into particles capable of actually reaching the surface as precipitation. I n ignorance of the quantitative balance drawn by actual cloud systems with respect to these two opposing tendencies, I shall here simply ignore both and draw only the less precise conclusion that the entire atmosphere must continually be going through the process of creating its precipitating-type clouds, releasing their total liquid water content, creating their replacements elsewhere in the atmosphere, and so on, for an effective total of about twenty-five repetitions per average day. This crude yet physically meaningful conclusion that has been drawn from foregoing estimates of scale factors and turnover rates of the atmospheric water economy is thus found capable of yielding an interesting estimate of the “effective lifetime” of precipitating-type clouds, namely one twenty-

232

JAMES E. MCDONALD

fifth of a day or about one hour. That this effective lifetime comes out fairly close t o actually observed lifetimes, a t least in the case of convectivetype clouds (order of tens of minutes), is indicative once more that all of the magnitudes being employed in the present discussion must be tolerably close to the true values and thus strengthens the claims to order-of-magnitude validity of my deductions concerning scale factors that must govern hydrologic effects and thermodynamic effects of currently envisaged cloud-modification programs. The short lifetimes of clouds of most types (order of a few tens of minutes in agreement with the estimate just made) and the comparative infrequency of occurrence of the precipitating types, combined with the limited areas of direct influence of any known cloud-modification techniques (to be described in the following), can be used to support^ still further argument that no hazardous disturbance of natural events is imminent. If we take the global mean cloudiness fraction used above, namely one-half, and then reduce this by the estimated factor of one-tenth in order to dispose of the numerous nonprecipitating cloud types that are counted into the cloudiness fraction itself, we have a twentieth of the world covered a t any one time by modifiable clouds. Taking as a seemingly generous figure 100 km2 as the area that might be influenced by, say, a single cloudseeding generator of a generally used type operated in an area with potentially modifiable clouds, then no less than 4000 generators would be required to seed the modifiable clouds lying over an area equal to that of the United States a t an average moment, and about twenty times that number, or some 100,000, would be required to insure that modification could be effected a t any and all times and places at which treatable clouds happened to appear. It seems rather unlikely then that, in the near future, man’s efforts a t rainmaking will even approach the rather tiny upper limit of the maximum fraction (one ten-thousandth) of total atmospheric water vapor which he might be drawing upon on a continuing basis by known modification methods. Do all of these considerations of the relatively minute scale of prospective tapping of the atmospheric water vapor reservoir therefore imply that there can be no practical significance in the recent development of cloud-modification techniques? Not a t all. They indicate only that such efforts can scarcely be expected to interfere sensibly with the waterand energy-exchanges that our atmosphere is continually accomplishing on so vast a scale. That which is comparatively insignificant on the scale of geophysical magnitudes is quite often highly significant as judged by our own human standards. There appears, then, to be good reason to Mean drift rates plus photolytic decay rates (Section 4.2.1) combine t o give an effective area of this order of magnitude.

PHYSICS OF CLOUD MODIFICATION

233

pursue research on techniques of cloud modification for, although the complexity of the underlying problem has defied complete solution even after a decade of investigation, the ultimate prospects remain most intriguing. A final order-of-magnitude calculation in this section devoted to the gross features of the atmospheric steps in the hydrologic cycle will help the reader unfamiliar with atmospheric energy-magnitudes to understand why, a t present, there are no very bright prospects for modifying the hydrodynamic motion fields that are prerequisite to the very existence of clouds. The point here is that precipitation presupposes condensation, and condensation on a scale intense enough to form cloud masses of significant depth presupposes strong updrafts in which adiabatic cooling brings the vapor in the rising air to its condensation point. All meteorologists are familiar with one or another layman’s scheme for creating clouds by some kind of forced upward motion, and in every case there appears the same failure to recognize the enormous amounts of energy nature shuffles about in everyday atmospheric processes. One simple means of estimating the magnitude of the energy involved in a particular case is very relevant here-namely, the calculation of the total latent heat released in a typical thunderstorm. It will be reasonable to consider a thunderstorm downdraft a kilometer in radius in which rain is falling at such a rate and for such a time that one centimeter of rain falls over all of the area of the downdraft during the lifetime of the storm (say, 30 min). A total mass of water amounting to 3 X 1O’O gm will have undergone phase change in such a thunderstorm from vapor to liquid state, with about 600 cal released locally into the atmosphere for each gram condensed, or a total of approximately 2 X lOl3 cal of released latent heat. The largest single energy source currently a t man’s disposal is the nuclear bomb, and for comparison, we may note that the energy released in a nominal atomic bomb of the fission type has been set [7] a t 2 X 1013 call or exactly the magnitude of latent heat release calculated for the single, moderate thunderstorm considered above. Noting then that Braham [4]finds that the total condensation occurring in a typical thunderstorm may be about ten times greater than the amount indicated by just the precipitation reaching the ground, and realizing that latent heat release is but one part (though, to be sure, an important part) of the total energy exchange in a thunderstorm, it becomes clear that even small thunderstorms constitute mechanical systems in which energies in excess of the equivalent of ten World War I1 atom bombs are involved. This surprising magnitude points to the seeming futility of hoping to modify weather by methods directed toward the creation of clouds themselves. There are, to be sure, possibilities of still unsuspected trigger mechanisms

234

JAMES E. MCDONALD

whereby expenditures of small amounts of energy might release large amounts of atmospheric potential energy in statically metastable air masses. One of these that has received more than passing consider.dt’1011 concerns alteration of the albedo of the ground over a large enough area to locally heat the atmosphere to the point of inducing convection and thence cloud formation, and another involves controlled fires set on a scale large enough t o initiate updrafts in an unstable air column. At present these schemes appear to have only marginal interest. For the present, the only real hope, then, seems to be that of manipulating certain of the key microphysical processes occurring within clouds wherein relatively tiny expenditures of energy and material can conceivably alter the course of events set in motion by the tremendous natural energy exchanges that are prerequisite to cloud formation itself. A discussion of the physical details of these critically important microphysical processes that may be amenable to modification will be the subject of the next section.

3. PRESENT STATUS OF CLOUDAND PRECIPITATION PHYSICS~ 3.1 Historical Remarks

One aspect of the history of the development of cloud physics which seems to me to be quite significant is its very slow development prior to about ten years ago. When one considers the great importance of precipitation in man’s activities, above all in agricultural activities, when one considers in how many ways the past decade’s research has been obstructed by sheer lack of many kinds of basic observational data on cloud and precipitation processes, and finally, when one considers the almost complete lack of any really exhaustive theoretical analyses of basic cloudphysical hypotheses carried out earlier than a dozen years ago, the historical background is rather disquieting. The question that is of much more than historical interest is as follows. Why this comparative neglect of so fundamental a problem for so long a time? The answer seems to me to be that we face here one more of those many instances in the history of science where far too little research support was given to investigations while they were apparently of only academic interest. When, after 1946, there seemed to exist some prospect of control over a natural phenomenon whose economic value is so high, support of cloud physics research jumped by, what I would estimate must surely have been, a factor of two to three orders of magnitude, and total numbers of workers in the field must have increased by a factor of something like two orders of magnitude. Yet so complex are the phenomena 2 A comprehensive book by B. J. Mason 17al covering many aspects of cloud physics has recently appeared.

PHYSICS OF CLOUD MODIFICATION

235

one encounters in attempting rational modification of precipitation, that even after a decade of investigations a t these unprecedented levels of support, meteorologists still face many very fundamental questions not yet answered. Had fundamental meteorological research been sustained during earlier decades a t a level in keeping with the importance of meteorological phenomena in all man’s affairs, this embarrassing dearth of basic observational and theoretical background would not have so limited rapid progress toward evaluation of, and improvement upon, the discovery in 1946 of means of modifying cloud processes. To illustrate the slow pace of progress in just the theoretical side of cloud physics in earlier years, no example seems more revealing, in my opinion, than the history of the ice-crystal hypothesis of precipitation, which will be examined in some detail later in this article. I n 1911, A. Wegener, called attention to the difference in the vapor tensions (i.e., saturated vapor pressures) of subcooled water and ice a t given subzero temperatures, yet twenty-two years elapsed before this observation was formally recalled by T. Bergeron who, in 1933, suggested that this vaportension difference might be responsible for the release of most, if not all, natural precipitation. And, even more curiously, an additional seventeen years then elapsed before a really adequate theoretical analysis of this hypothesis was given by Houghton [8]; yet the theoretical instruments that finally laid bare the relevant quantitative aspects of the WegenerBergeron hypothesis were, in all essential respects, already quite well developed in Wegener’s day. This forty-year lag may be illuminatingly contrasted with what has often been no more than a lag of a few weeks or months between appearance of a new hypothesis in nuclear physics and its thorough theoretical and experimental testing. The difference almost certainly lies in the difference in total numbers of well-trained persons vigorously seeking to exploit every perceptible clue to the problems of these two fields. Since, even today, the field of cloud physics is not advancing a t a pace that seems commensurate with the importance of its subject, the lesson of the past forty-odd years seems still to be quite timely: There is need for many more capable workers in this field. 3.2. The Condensation Process

The only physical process capable of cooling to the vapor saturation point large enough volumes of air to yield significant amounts of liquid water in droplet form is that of adiabatic expansion of ascending air, if by “significant” we understand “large enough to support natural precipitation processes.” Air may ascend in a wind current impinging on a mountain slope or along a frontal discontinuity, it may be forced to ascend by dynamical convergence in a cyclonic circulation, or it may be caused

236

JAMES E. MCDONALD

to ascend as a result of buoyant forces. During any such ascent to levels of lower pressure, the air parcels perform expansion work against their surroundings, causing their temperature to decrease a t the dry adiabatic rate of 9.8"C/km, and producing what should be regarded as a very odd phenomenon, namely, the tendency for any water vapor contained in the ascending air to approach and (with sufficient lifting) to reach vapor saturation. This is odd in the thermodynamic sense that whereas most vapors become less saturated upon undergoing expansion, water vapor, because of its anomalously high latent heat of vaporization, behaves oppositely; so we find clouds of drops of condensed water in our atmosphere occupying the upper limits of updrafts, rather than the lower limits of downdrafts. However, our familiar clouds must be regarded as curiosities not only on that score, but more so on the very fact that they occur a t all, for phase transitions from vapor to liquid are always powerfully inhibited by the activation-energy barrier imposed by the appearance of surface free energy during formation of droplets of the liquid phase within an initially homogeneous vapor phase. Only with meteorologically unheard of supersaturations (relative humidities in excess of several hundred per cent) would droplets appear in an adiabatically cooling updraft if they had to be formed by homogenous nucleation, that is by passing through stages where polymers of two, three, four water molecules, and so on, grew by random collision processes. The classical problem of atmospheric condensation theory might well be said to be that of accounting for the improbable but observed behavior of terrestrial clouds which do invariably form as soon as rising air is cooled to just its nominal saturation state (i.e., to a state characterized by temperature and vapor pressure values which would insure molecular equilibrium with a plane surface of pure liquid water if such a surface were present). It has been known for a long time that the clue to this improbable behavior is that our atmosphere contains small but very significant concentrations of particles of such chemical composition as to be hygroscopic and of such size that, though submicroscopic, they are nevertheless large enough and have sufficient affinity for water that they can overcome the free-energy barrier that blocks the path of homogeneous nucleation of pure vapor. These particles are collectively termed condensatim nuclei. The basic thermodynamic principles of the meteorological condensation processes have been fairly well understood for over seventy years, since the days of Hertz and Neuhoff; the existence of the free-energy barrier has been known since 1870 when Kelvin derived its mathematical expression; and the inference that ordinary air must contain some kind of nuclei of condensation dates from work of Coulier in 1875. But the full elucidation of the true physical and chemical nature of the effective nuclei

PHYSICS OF CLOUD MODIFICATION

237

of condensation upon which cloud formation depends cannot yet be said to be achieved. Since a good summary of the status of the problem of the atmospheric condensation nuclei has been given recently by Junge [D] (who has also been the chief contributor to recent advances in our understanding of the condensation nuclei), this facet of cloud physics will be passed over here rather briefly, with only a few comments on those aspects of the condensation problem that influence precipitation and cloud-modification processes. First, it is very important to stress, for the benefit of the nonspecialist, that all measurements of the populations of atmospheric nuclei are in essential agreement on one critical point: There appear a t all times and in all parts of the world to be sufficient numbers of hygroscopic condensation nuclei that cloud growth begins a t only an immeasurably higher altitude than the level of nominal saturation (i.e., a t the “100 % relative humidity” level) in an adiabatically cooling updraft of air. That this circumstance is not simply to be taken for granted is clearly shown by the equally wellestablished observation, to be discussed below, that the atmosphere invariably displays a quite significant deficiency of another class of nuclei, the so-called ice nuclei. These two very important observational facts can be restated, for emphasis, in the following way. The water substance in our atmosphere (at least in the troposphere) never exhibits marked vapor supersaturation, yet it characteristically displays very marked liquid subcooling. Cloud-modification possibilities, a t least as far as concerns any techniques now seriously considered, are profoundly influenced by these two observational facts. The first precludes control of precipitation through any step that might be termed artificial cloud formation, inasmuch as wholly natural processes are already entirely competent to produce clouds whenever adiabatic ascent occurs. But the second fact implies, as will be elaborated below, that nature might occasionally profit from assistance in the form of modifications artificially made in the ice nuclei populations that play, in certain precipitation processes, a quite critical role. The allimportant distinction here is the distinction between condensation nuclei and ice nuclei, a distinction which is too often overlooked by nonmeteorologists, especially in discussions of cloud-modification techniques. Inasmuch as condensation from the vapor phase may involve growth of either liquid drops or solid crystals, it would be logically appropriate to discuss in this section the growth of ice particles by direct condensation onto types of nuclei called sublimation nuclei (and more fully discussed later in this article). Although the point is not yet fully settled, it appears likely that such a growth process may often account for production of cirrus clouds at high altitudes. It is also likely that such processes may be indirectly related to precipitation phenomena when those ice particles

238

J A M E S E. MCDONALD

fall into subcooled clouds a t lower altitudes. Since the growth of cirrus clouds themselves is beyond the scope of this paper and because the genernl role of ice crystals in precipitation processes will be discussed later iii Section 3.3, no discussion of vapor-to-solid condensation will be given here. A significant clarification of the details of cloud droplet growth in adiabatically cooling updrafts was accomplished by Howell [lo], who assembled into a single system of differential equations a description of the iiumerous physical processes that govern particle growth from hygroscopic nuclei to full cloud-drop sizes. Prior to Howell’s work, all of the contributing processes were individually understood reasonably well; his contribution was the mathematical synthesis of this information and the numerical integration of a small number of special cases which displayed quantitatively several highly important features of the condensational growth of drops in the lowest few hundred meters of a cloud. First, his results, even though they cannot be regarded as final and have been shown to contain a few small errors, show beyond doubt that the amount of vapor supersaturation that can develop near a cloud base, with normal numbers of hygroscopic nuclei present in the rising air, is of the order of only a few tenths of a per cent in all but the strongest of convective updrafts; and for the latter cases his results suggest, but do not show with finality, that supersaturations of perhaps no more than one or two per cent may be expected as upper limits. This low upper limit was anticipated by Houghton in 1938 [Ill but is much more fully documented by Howell’s results. It must not be thought that SO low a peak supersaturation is also clearly deducible from direct cloud observation, for even a one per cent supersaturation represents an elevation increment of only a few meters in level of appearance3 of cloud drops in a typical case, and so small a discrepancy can scarcely be detected observationally with presently available methods. Second, and more important for the theoretical clarification of cloud processes leading to precipitation, Howell’s results indicate that only a surprisingly narrow range of cloud-drop sizes can be formed by solely condensational processes, acting upon commonly observed nuclei populations, for the diffusional growth law is of such functional form as to cause the smaller nuclei to catch up with the growth of larger nuclei once the former are activated for growth by the rising supersaturation, as can be seen in Fig. 1. This inherent tendency toward monodispersity of clouds produced solely by condensation processes, recognized and briefly discussed as early as 1933 by Houghton as a general characteristic of the growth of spheres by diffusion, was shown by Howell’s analysis to be so strong as to demand greatly increased attention to the effects of all those collisional processes not taken into account in his growth equa3

Or, more accurately, the level of sudden increase in drop growth-rate.

239

PHYSICS OF CLOUD MODIFICATION

I

10

100 10

I

-x 7

0 .+

e

43 -

1

m

0.I

I

10

0.01 100

Trne of rise (sec)

Fro. 1. Condensational growth curves for a weak updraft (0.6 m sec-I) containing few nuclei (total 500 cm-s). Abscissa is time since the air parcel passed the nominal saturation level. Values of yare numbers of moles of salt per nucleus. Note that nuclei of l0-l’ M are barely activated by the peak supersaturation, 0.35%, while those of 10-’8 M or less are not activated a t all in this updraft. The small range of drop sizes finally produced by purely condensational processes is clearly evident. From Howell [lo], by courtesy of the American Meteorological Society.

tions-a case of a very significant insight being gained by a somewhat negative result, since observed drop-size distributions even for lower layers of clouds exhibit much broader ranges than seem obtainable by any purely condensational process, judging now on the basis of Howell’s 1949 analysis. Others have added to present knowledge of condensation physics. Kraus and Smith [12] carried out, almost simultaneously with Howell, a number of analyses of drop-growth, but their integral equation formulation omitted the quite important effects of the warming of growing cloud drops by release of latent heat of condensation, effects which Howell incorporated into his differential equation quite cleverly. Omission of this heating term might easily pass for a negligible error, since examination reveals that it accounts for a drop-warming effect of the order of only

240

JAMES E. MCDONALD

0.01”C; yet so small are the vapor pressure gradients driving the diffusional growth of cloud drops that this tiny temperature increase raises the growing drop’s vapor tension enough to roughly cut in half the mean vapor pressure difference from drop to ambient air! In another study, Squires [13] carried through a number of illuminating computations of factors involved in condensation growth, but unlike Howell, he did not integrate a complete system of equations depicting the interplay of processes actually found in the cloud. There remains great need for extension of Howell’s work, using automatic computer methods to examine many more combinations of updraft speed, condensation level, and nuclei populations. There are also residual uncertainties in the adequacy of the type of diffusion law that underlies Howell’s analysis, so this cannot yet be regarded a closed problem. A final point to be considered under condensation physics as it bears on cloud modifications concerns the still somewhat controversial role of very large hygroscopic nuclei. It is necessary to explain here what is to be understood as a very large nucleus. The terminology, which has acquired standard usage is as follows. Particles with radii below about 0.1 p are termed Aitken nuclei (after the developer of a simple expansionchamber nuclei counter), those with radii between about 0.1 and 1.0 p are called large nuclei, and those with radii in excess of about 1.0 p are referred to as giant nuclei. Junge has gathered considerable evidence (see, for example, [14]) that the number concentration of the atmospheric aerosol rises very rapidly with decreasing size toward a maximum number in the Aitken range (the still smaller particles having such high mobility as to go over rather quickly by Brownian coagulation into Aitken nuclei). His evidence suggests that most of the so-called large nuclei are ammonium sulfate (or a t least are rich in the ions of that salt), though the true nature of the source of such particles is still obscure. The C1- ion found abundantly in sea water is notably absent from the class of large nuclei, but makes up a substantial fraction of the giant nuclei. The Aitken nuclei are so small that they are not activated by the relatively small supersaturations now known to occur in cloud updrafts, while, on the other hand, the giant nuclei are less numerous by several orders of magnitude than the large nuclei; so there now remains little doubt that the bulk of cloud condensation takes place on the large nuclei, those with radii of from 0.1 to 1.0 p . However, despite the small numerical importance of the giant nuclei, considerable interest in this class of nuclei has recently developed as a result of the past few years’ findings relative to the importance of collision and coalescence processes (accretion processes are discussed in Section 3.3.3) in producing precipitation. As will be elaborated below, collisions

PHYSICS OF CLOUD MODIFICATION

241

tend preferentially to occur between drops of dissimilar size. But Howell’s theoretical results show that condensation on populations of large nuclei yield cloud drop-size distributions that are too narrow to yield appreciable collisional growth directly, so a number of investigators have stressed the potential importance of the comparatively few giant nuclei in starting the coalescence process. The work of Woodcock and his collaborators (see, for example, [15] and [IS]) has been particularly important in the past ten years’ developments in our knowledge of the distribution of these giant sea-salt nuclei which may play a critical role in at least some kinds of precipitation processes. Details must be passed over and the single point made that herein we find the one way in which cloud modification of a sort capable of influencing precipitation has been considered possible through manipulation of the population of condensation nuclei. For existing evidence suggests that continental air masses will frequently have too few giant nuclei of the size necessary (order of 10-20 p in radius on entry into the cloud base) to start the accretion process immediately at a rate that is significant. Hence, the argument proceeds, one might artificially stimulate the accretion processes by adding to updraft air a rather modest number of very large salt particles. More will be said of this approach later (see Section 4.3), but before leaving this brief discussion of condensation as it bears on cloud modification, the reader must be cautioned to note that any such addition of a relatively small number of giant nuclei to existing natural populations of the much more numerous large nuclei cannot at all be construed as artificial cloud formation: the generalization still stands that no promising means are yet at hand for influencing the hydrologic cycle through anything that can properly be called artificial production of clouds. Nature, for the present, must supply the clouds. 3.3. T h e Precipitation Process

Condensation is necessary but not sufficient to account for precipitation on the scale characteristic of the earth’s atmosphere, where some lox9 gm falls upon the entire world on an average day. But, despite its great importance, the mechanism by which the atmosphere accomplishes precipitation of portions of its total stock of water from time to time has long resisted satisfactory explanation. In the next four sections, the nature of the problem and some recent progress toward its solution will be briefly examined in order to delineate the background against which cloudmodification studies must be viewed. 33.1. The Central Problem of Precipitation Theory. Numerous observations, especially during recent years, from mountain observatories and from research aircraft, stand in good agreement with respect to the order of magnitude of the number-concentration of cloud drops: some hundreds

242

JAMES E. MCDONALD

per cubic centimeter. This concentration becomes higher in clouds of the convective type where strong updraft speeds lead to the higher peak supersaturations (order of a few per cent, as noted earlier) that permit activation of the more numerous smaller nuclei, and becomes lower in cloud types associated with slowly ascending air, where only the larger - ~ seem to represent nuclei are activated; but 100 cmP3 to 1000 ~ m would reasonably well the lower and upper limits, respectively, to the commonly occurring range of cloud-drop concentrations. It is necessary to distinguish, at least in a gross way, between cloud drops and raindrops. There is, of course, no absolute criterion, since there exists L: continuous distribution of water drop-sizes from embryonic cloud drops of less than a micron in radius to the largest raindrops that are mechanically stable in normally turbulent air, namely, those with radii in the neighborhood of five millimeters. Nevertheless, the fact that drop radii are commonly of the order of 10 p in clouds of nonprecipitating types whereas modal raindrop radii in most heavy showers are of the order of 1 mm, puts two orders-of-magnitude separation in radial dimensions between clearly cloud and clearly raindrops. Just one order of magnitude from each is the drop of 100 p in radius, which makes this size a nicely symmetric dividing point between the two classes of particles. About the same division happens to be indicated by another, more physical, criterion: Whereas a 10-p drop has a terminal falling speed of about 1 cm sec-', too low to permit it to fall through even very gentle cloud updrafts, and whereas an unstably large raindrop of 5 mm radius falls with a terminal velocity of about 9 m sec-l, rather larger than most cloud updrafts, a drop of 100 p radius falls a t about 70 cm sec-l, about equal to updraft speeds in weak convective clouds. For these and other reasons, it is common to take the 100-p radius as the arbitrary point of separation, with smaller particles being regarded as large cloud drops, and larger particles being considered small drizzle drops. Now, condensation processes alone will yield their largest cloud drops when acting with the fewest possible nuclei in a cloud whose base is as low in the atmosphere and at as high a temperature as is meteorologically attainable, and when the rising air parcels are carried by convection to the greatest possible altitudes. Hence, to place an upper limit on clouddrop sizes attainable solely by condensation, we might imagine a tropical cumulus with a base a t only about 500 meters altitude and base temperature of 30°C, which extends to the tropopause a t about 16 km altitude, and we may take the drop concentration to be as low as 100 C M - ~near the base. By a simple calculation based on the thermodynamics of the saturated adiabatic process we find that by the time a given air parcel has been transported adiabatically to the cloud-top level in such an extremely

PHYSICS OF CLOUD MODIFICATION

243

favorable case, each of its drops will have grown, considering only the effects of condensation processes and assuming as a crude approximation to Howell's picture that all drops grew a t equal rates, to a radius of merely 40 p. A drop of this small size, falling back through the cloud at its terminal velocity of about 20 cm sec-' would require almost exactly one day to reach the cloud base even if all updrafts could somehow be avoided, and it would then completely evaporate after falling only about a hundred meters in the nonsaturated air just under the cloud base. Thus, we find that such a process is quite incapable of producing precipitation in the ordinary sense of the term. In contrast to the above hypothetical case of the most favored combination of circumstances dependent solely on condensational growth, one observes that actual clouds that yield heavy precipitation often have drop concentrations substantially larger, have bases much higher and colder, have tops much lower, have strong updrafts through which precipitation particles must descend; and yet release their precipitation, in the form of drops with radii as much as two orders-of-magnitude larger than in the above case, in times of the order of only an hour after fist formation of the cloud. Closer scrutiny of these two cases quickly reveals that the essential difference between actual precipitating clouds and our hypothetical mostfavored cloud, supporting only purely condensational drop growth, is a difference in rapidity of development of very large drops (raindrops) whose terminal fall velocity is great enough to enable them to descend gravitationally through the often strong updrafts within the cloud and to pass quickly through the nonsaturated clear air between cloud and ground without wholly evaporating. This difference, quantitatively put, is the difference between condensationally produced cloud drops with radii of the order of 10 p and observed raindrops with radii of the order of 1 mm. In these two magnitudes is implied the central problem of precipitation theory: to account for the conversion of some million cloud drops into a single precipitation particle in a time of the order of only a n hour after condensation starts at the base of a n incipient cloud. The comparative particle sizes, concentrations, and terminal falling speeds of the several particle types encountered in cloud physics are drawn to scale in Fig. 2 to display the important disparities in these magnitudes for cloud and precipitation particles. Only within the past decade have meteorologists acquired what might justifiably be called a general understanding of the physics of these aggregation processes. Clearly, rational application of any prospective cloudmodification techniques aimed at augmenting precipitation will demand very detailed knowledge of these processes, and indeed it has been the recent discovery of conditionally effective modification techniques that

244

JAMES E. MCDONALD

r rodm in mcrons n number per llter V - terminal velocity in ~

~

v.

I

-

cmtimfers per

coI roindrop

r=

1000.

n.1.

'4.650

FIG.2. Comparative sizes, concentrations, and terminal fall velocities of some particles involved in condensation and precipitation processes. Note particularly the great difference in radius of a typical cloud drop and of a typical raindrop.

has stimulated most of the research that underlies our present tentative answers to the central problem of precipitation theory. It now appears that there are just two essentially different aggregation processes, or more accurately, just two essentially different ways in which the aggregation process can be initiated. One of these is called the ice-crystal process, the other is the accretion process. Many other processes have been postulated in the past, but all other hypothetical processes have been winnowed out by the stringent rate requirements imposed by the surprisingly short lifetimes of individual clouds. 3.3.2. The Ice-Crystal Process. The ice-crystal process, also referred to frequently as the Bergeron process or the Bergeron-Findeisen process, hinges upon the coexistence of ice particles and subcooled liquid water drops within a cloud a t temperatures below 0°C. That liquid water is quite frequently found in the subcooled state in clouds is not, I believe, common knowledge among nonmeteorologists. This fact certainly seems to contradict every-day observations of freezing phenomena one sees, for example, in winter in high latitudes. However, meteorologists have known for a t least half a century that it is much more typical to find clouds a t a temperature of, say, - 5 to -lO"C, containing only subcooled liquid drops than to find clouds in that temperature range comprised solely of ice crystals. Furthermore, about twenty years ago, difficulties with icing of aircraft (which can only occur in the presence of subcooled

PHYSICS OF CLOUD MODIFICATION

245

water and never in penetrating clouds comprised wholly of ice particles) had revealed that subcooling to a t least as low a temperature as about -35°C occasionally occurs in clouds. (It deserves passing note here that the term “subcooling” is preferable to the commonly used “supercooling,” and the latter should be abandoned.) The reason why clouds are so susceptible to subcooling was not clarified until quite recently. Textbooks on meteorology written as little as half a dozen years ago still contained references to lack of mechanical vibration in clouds, or obscure references to suggested effects of surface tension on the freezing temperature, as possible explanations, and the general impression with which the student was left was that this common tendency toward marked subcooling in clouds was an anomaly not clearly explainable. Actually, subcooling in cloud drops is not anomalous at all and, in fact, the situation would only be truly thermodynamically anomalous if the reverse case were true, that is, if all clouds at subzero temperatures were pure ice clouds. It is now known as a result of recent theoretical work (see, for example, Krastanow [l?],Turnbull and Fisher [18], Mason [19], and McDonald [20], of which the last reference contains a general review of meteorological implications) that pure water (and any other pure liquid) undergoes subcooling for reasons associated with the kinetics and thermodynamics of phase change. Freezing in a mass of pure liquid water can only begin as a result of random collision processes that form an embryonic ice crystal one molecule a t a time, and such a process is astonishingly improbable without appreciable subcooling. Thermodynamic analysis of this process reveals that there is a critical embryo size, at any given temperature, below which the embryo is iinstdde and tends to dissociate but above which spontaneous and rapid growth to a macroscopic ice crystal takes place. Throughout a mass of subcooled water, chance collisions tend continually to build up fractions of such embryos and thermal disgregation tends continually to dissociate them; but with increasing degree of subcooling, the later tendency is suppressed in such a way that the probability of molecular fluctuations forming an embryo of critical size in a given mass of subcooled water in a given time interval rises toward unity. Theoretical prediction of the temperature at which a cloud drop of given size should undergo this kind of homogeneous nucleation, as the above process is called, is at present blocked by ignorance of the exact value of the surface free energy of an ice-water interface, for this elusive parameter (whose value must be close to 15-20 ergs per square centimeter) plays a numerically decisive role (its cube appears in an exponent!) in the theoretical expression for the nucleation probabilities. Nevertheless, the theory has been quite enlightening in that it accounts for subcooling of pure water drops

246

JAMES E. MCDONALD

on general statistical and thermodynamic grounds and shows that whatever the actual ice-water surface energy is, the consequent nucle.'1t'1011 rates will be extremely sensitive to temperature in such a way as to cause a cloud of the usual drop-size distribution to undergo homogeneous nucleation almost e n masse once a certain critical temperature is approached. There is now considerable evidence (see, for example, Schaefer ['Ll],Bigg [22], Jacobi [23], and Pound et al. [24]) for believing that with drops of the order of 10 1.1 in radius, the critical temperature required for homogeneous nucleation to occur within times of the order of seconds is very close to -40°C (reported values range from about -38" to -41°C). The chief point to be stressed in explaining cloud subcooling is that a cloud is an unusual system in that its mass is distributed over a very large number of very small particles, each one of which must undergo a nucleation event somewhere within its boundaries before the cloud can become wholly frozen, and this is vastly different from the process required to freeze, say, a gallon of water in a single container. I n the latter case, a single nucleation event taking place anywhere within the container suffices to cause the entire mass to freeze, hence the familiar resistance to subcooling in ordinary quantities of water. Now, it is tempting to conclude at this point that subcooling to as much as about -35 to -40°C in clouds is solely due to the effects of drop size on the probability of homogeneous nucleation in such tiny masses of water. But this view cannot be correct. So extremely rapidly does the homogeneous nucleation rate vary with temperature, that temperature effects completely dominate over volume effects. Thus, if it is indeed true (as one may now strongly suspect) that drops of about 10 1.1 radius will undergo homogenous nucleation in times of the order of seconds or minutes near -4O"C, then raindrops of 1 mm radius will undergo homogeneous nucleation at a temperature only a few degrees warmer (somewhere near -36 to -38"C, the exact value depending on the still undetermined specific surface free energy of the ice-water interface). The lower degrees of subcooling :wtually observed in most laboratory studies with drops intermediate in size between cloud and raindrops must be due to some still undetermined effect of size on probability of containing or capturing a freezing nucleus, that is, a foreign particle that can induce so-called heterogeneous nucleation of t,he freezing process at only a modest degree of subcooling. The concept of the freezing nucleus is so important to both the basic theory of the icecrystal process and the theory and practice of many types of cloud-modification techniques that it, and closely related concepts, must be examined in more detail here before returning to the elaboration of the ice-crystal process itself. Terminology has a t times, in the past years, been regrettably confused

PHYSICS OF CLOUD MODIFICATION

247

with respect to various types of nuclei, but the following nomenclature, first formally proposed by Fournier d’Albe [25],has acquired fairly general usage. A freezing nucleus is any small foreign particle whose size and crystalline structure permit it to serve as a growth center for an ice crystal when it is in contact with liquid water a t subzero temperatures. A sublimation nucleus is any small foreign particle whose size and crystalline structure enable i t t o serve as a growth center for an ice crystal that is built up, not by solidification from the liquid phase, but by sublimation (the meteorologist’s term for direct condensation to the solid state) from the vapor phase at subzero temperatures. A generic term embracing both of these two classes of nuclei is often convenient, and ice nucleus is the noncommittal term that subsumes both. The term sublimation was first introduced into meteorological terminology in 1911 by Wegener and has acquired widespread usage in the sense employed in the term sublimation nucleus, yet this is a very misleading usage. Physicists always restrict the meaning of the term to the phase change from solid to vapor; so to speak of a sublimation nucleus is to the physicist a direct contradiction in terms. As a desirable substitute, the term deposition will be used here to denote the phase change from vapor to solid, so here deposition nucleus will be used in preference to “sublimation nucleus.” I n both types of ice nuclei, size is important because the Kelvin effect inhibits growth of too-small crystallites in exactly the way it inhibits growth of too-small liquid drops (Section 3.2), and in general, ice nuclei should be larger than about 0.1 p in radius to be effective, though even those an order-of-magnitude smaller can provide a substantial improvement over truly homogeneous nucleation. Crystal habit and lattice dimensions of the nucleant are now known to be very important, since a surface layer of water molecules will only be electrostatically bonded to the ice nucleus in a configuration compatible with the true ice lattice if the nucleus has a crystalline structure closely similar to that of ice. A mismatch of only a few per cent in any of the relevant lattice distances may prevent effective action as an ice nucleus except a t appreciable degrees of subcooling, and this law of crystal physics has fundamental implications in the problem a t hand. I t cuts down to virtually zero the number of naturally occurring crystalline atmospheric dusts with structural characteristics that permit heterogeneous nucleation of either the freezing or deposition processes at, or within a few degrees below, 0°C. That is, it appears to be one of the geophysical facts of life that, though our atmosphere contains abundant numbers of foreign particles sufficiently hygroscopic to function very efficiently as condensation nuclei, it does not happen to satisfy nearly so well the very much more stringent demands of the crystallization process. As a result, we observe

248

JAMES E. MCDONALD

(as has been noted earlier here) that vapor supersaturation is almost a nonexistent state in our atmosphere whereas liquid subcooling in clouds is extremely common. It would be logically next in order to discuss the exact chemical and physical nature of the principal freezing and deposition nuclei found in the atmosphere and their effective nucleation temperatures, but knowledge has not advanced far enough to permit this. There is not even agreement as to whether it is chiefly freezing nuclei or chiefly deposition nuclei that limit the commonly observed degrees of subcooling. Schaefer [26] has expressed doubt that significant numbers of freezing nuclei are operative in initiating ice-crystal growth in subcooled clouds, basing his view on the rarity with which microscopic examination of snowflakes discloses a frozen mass of cloud-drop size at the center, which is a reasonable, though as he points out, not decisive criterion. By testing in an experimental cold chamber a variety of natural desert and volcanic dusts of types that might become windborne in arid and semiarid regions, Schaefer [26] has found that there are many commonly occurring dusts that become fairly good deposition nuclei in clouds with subcooling to about -15 to -220°C. His results are reproduced in Fig. 3, which shows that at least one type of soil from the Northern Plains area exhibits a threshold of activity not far below that of the favored artificial cloud-seeding agent, silver iodide (which is discussed in Section 4.22). Isono [27] examined electron diffraction patterns of Formvar replicas of natural snowflakes and found evidence that many contained aluminum silicate materials of the clay type, tending to support Schaefer’s suggestion concerning the roIe of silicaceous dusts. However, the technique is difficult and the identification somewhat indirect, so it is by no means decisive. Our knowledge of the chemical and physical characteristics of natural ice nuclei is, today, only about as well developed as mas our knowledge of the nature of condensation nuclei some thirty years ago. It is certainly to be hoped that, just as surprising innovations in detection methods have recently been made in studies of condensation nuclei, so also will more delicate techniques soon be discovered for ascertaining the types and sizes of natural ice nuclei, since this kind of information would be extremely valuable in understanding how nature nucleates the ice-crystal processes which man now seeks to manipulate. Observations such as those displayed in Fig. 3 axe not in themselves sufficient to settle the question of the nature of the effective ice nuclei in the atmosphere, for one must also know the concentrations in which any of these dusts actually occur at cloud altitudes. Natural abrasion processes do not yield dusts with particles much under a few microns in diameter, and dust particles of that size will fall about a kilometer in a time of the order of ten days. Too little is known of the climatoIogy of dust to enabIe

249

PHYSICS OF CLOUD MODIFICATION

1

SILVER IODIDE LOAM - R U G B Y , N.DAK.

IMILAR EFFECl

-

\

/=-

-

LOAM- OAKLE I

KAOLIN-GA.

I Z

l

~

~

ACTIV

~

~

i

~

~

FIG.3. Temperature range of nucleating activity of crystalline dusts from desert areas and volcanic deposits. From Schaefer [26], by courtesy of the American Meteorological Society.

one to decide how the dust addition rate would counteract that fallout rate, but since the fallout rate is not negligible, and since the altitude that the dusts of Fig. 3 must attain to become of cloud-physical importance is of the order of five kilometers, one is prepared to believe that the number concentration of ice nuclei at cloud altitudes will be rather low. Table I1 summarizes the results of a month of daily counts of ice nuclei made in the winter of 1955 at aircraft altitudes up to 15,000 ft in the free atmosphere over Tucson, Arizona, in prevailingly westerly flow [28]. These data for Arizona are sufficiently like corresponding data obtained elsewhere, as for example, the data for eastern Australia obtained by Smith and Heffernan 1291 and still more recent nuclei counts taken over England by Murgatroyd and Garrod [30], to suggest that they may represent world averages tolerably well. Such magnitudes have close bearing on the principles of the icecrystal process of precipitation and its artificial modification, and we are now able to return to the specific discussion of that process having exam-

~

250

JAMES E. MCDONALD

TABLE11. Summary of the average temperatures ("C) at which the ice-crystal concentrations rose t o specified values a t three altitudes over southern Arizona during January 3-31, 1955. (From [28].) Ice-crystal concentrations Altitude (feet) Sfc. 5000 15,000

O.l/liter

l/liter

lO/liter

1OO/lit,er

-25 -26 -27

-27 -29 -29

-28 -31 -31

-30 -33 -33

ined sufficiently closely, for present purposes, the phenomenon of cloud subcooling and the phenomena of homogeneous and heterogeneous nuclea tion. Given the existence of widespread tendency toward cloud subcooling, :L question arises. What will occur when, in a cloud of metastable liquid water drops a t subzero temperatures, a few ice crystals suddenly appear, formed by any conceivable process (homogeneous nucleation, natural heterogeneous nucleation, or artificial heterogeneous nucleation as in silver iodide cloud seeding)? It was this general question that Wegener posed and answered qualitatively in 1911, which Bergeron took up again, still qualitatively, in 1933, and which Houghton [S] treated quantitatively in 1950. Wegener observed that the vapor tension of subcooled water is slightly greater than that of ice a t the same temperature, with a maximum excess of about 0.27 mb a t -12'C, becoming zero a t O'C, where ice and water are in vapor equilibrium, and also approaching zero asymptotically for very large degrees of subcooling. In a cloud containing only subcooled drops, the actual vapor pressure will be, unless the cloud is growing very rapidly, essentially equal to the vapor tension of subcooled water a t thc cloud temperature. Hence, as soon as a few ice particles appear, they constitute vapor sinks and grow rapidly a t the expense of the vapor which is supersaturated with respect to the ice particles by the initial amount of 0.27 mb. (This excess of 0.27 mb, it should be made clear, is a very large supersaturation by diffusional standards. For example, it can be shown to be about ten times greater than the vapor pressure difference from vapor to drops existing in the region of peak supersaturation a t the base of a cloud in a strong updraft.) As the growing ice crystals consume water molecules from the vapor phase, the ambient vapor pressure is rather quickly reduced to a value less than that required for saturation with respect to the subcooled liquid water drops themselves, so the drops begin to suffer net, evaporation. But the latter process merely serves temporarily to replenish the ambient vapor supply being heavily drawn upon by the ice crystals, and what was shortly before a metastable state (prior to appearance of any

PHYSICS O F CLOUD MODIFICATION

251

crystals) is rather quickly stabilized by distillation of all or most of the cloud water from liquid to solid state by diffusion processes. This is the ice-crystal process, and it was essentially the qualitative points of this paragraph that were made by Wegener in 1911. Under what conditions, we must next ask, will the ice-crystal process provide a solution to the central problem of precipitation theory? That is, under what conditions will it produce a million-into-one type of transition in a time period of the order of observed cloud lifetimes, namely, in only tens of minutes? Clearly, only in circumstances where nucleation processes produce approximately one ice nucleus for every million subcooled water drops a t temperatures where diffusion kinetics satisfy the rate requirements. Cloud-drop concentrations in the upper portions of clouds are typically of the order of 100 CM-~,so we must demand about one ice nucleus per 10 liters of cloud space if drops of large enough size to precipitate are to grow by the ice-crystal diffusion process. If a cloud contains too few nuclei, each of these few will quickly grow to such a large crystal (snow crystal) that d l will tend to fall out of the subcooled region before they can extract much of the available water, which leads to a loss of precipitation efficiency. If, on the other hand, nucleation is too profuse, so many ice crystals may nppear that competition among them for the total available water will prevent any of them from growing large enough to fall through the cloud updrafts, and then the requirements for precipitation are not satisfied a t all. (It is this latter possibility that is referred to, in discussions of cloud modification, as “overseeding.”) The ice nuclei counts shown in Table 11, though they cannot yet be assumed t o be broadly representative, show in a general way how and why the ice-crystal process operates under natural conditions. I n a cloud updraft over Arizona in January of 1955, subcooling would have persisted from an altitude of about 3 km, the 0°C level, to about 7 km, where the local degree of subcooling (to about -27°C) would reach a value great enough t o “activate” ice nuclei in total concentrations of about one-tenth per liter. These would grow rapidly and would thus begin to lag behind the rising air, ultimately would cease rising and begin to fall, with respect to the ground, still growing by diffusion as they fell. Upon falling through the 0°C level, the snowflakes would melt and become raindrops under conditions typical of Arizona in winter, while under other climatic conditions they might reach the ground as snow. Houghton’s 1950 analysis [8] of the kinetics of this process brought out a number of interesting quantitative aspects of which only two will be mentioned here. First, his results constitute the first really adequate demonstration that Wegener’s hypothesis is quantitatively important in cloud particle

252

JAMES E. MCDONALD

Elapsed t&e (minutes)

FIG.4. Diffusional growth of ice crystals (solid curves) and accretional growth of water drops (dashed curves). A : plane-dendritic ice crystal, water saturation at, -15°C; B : hexagonal plate, water saturation a t -5°C; C: accretion in a cloud with 5 0 p median diameter drops; D : accretion in a cloud with mean drop diameter of 24 p ; E : accretion in a cloud with median drop diameter of lop. After Houghton [8].

growth. For example, plane-dendritic ice crystals, growing in a cloud region saturated with respect to water a t -15"C, were found by Houghton to increase in mass by a factor of about lo4in only about 20 mill of diffusional growth (corresponding t o a flake-diameter change from a few tens of microns to about one millimeter). Second, the same analysis, which covered certain aspects of the accretion process as well, showed (see Fig. 4) that the ice-crystal process is likely to be significant chiefly in initiating growth of precipitation particles. For, by the time a snow crystal has attnirted by diffusion a mass of about 10 pgm (equivalent to a spherical diameter about 275 p), accretion of cloud droplets and smaller crystals swept out by the falling flake has become equal in importance to diffusion, and shortly thereafter this accretion process completely dominates the growth process. That is, Houghton's results were particularly important in that they r e v d e d that even in clouds of subcooled water whose precipitation history may start through the ice-crystal process, the later stages, wherein the bulk of' the total liquid water is extracted from the cloud, are dominantly influenced by the second of the two main precipitation processes, that of collision and accretion by large particles falling through a cloud of smaller particles. Snowflakes may grow by accretion of other snowflakes and also by accretion of subcooled water drops (riming). Different physical factors influence these two distinct accretion processes, but no discussion of the cornparatively limited fund of present knowledge of such accretion details will be given here. This omission must not be construed as reflecting small im-

PHYSICS OF CLOUD MODIFICATION

253

portance of such details, for we need to know a great deal more about them than we now do. It is interesting to note, as a final point in this summary of the physics of the ice-crystal process, that if our atmosphere happened to contain ice nuclei in number-concentrations and with nucleating efficiencies comparable with those of the abundant and efficient condensation nuclei, the icecrystal process would not only be incapable of yielding precipitation in the ordinary sense of the term, but would positively inhibit precipitation processes by quickly converting every cloud drop to a tiny ice crystal as soon as the drop rose above the freezing level in a cloud updraft. Under such hypothetical conditions, subcooling would be unknown and no millioninto-one transition could oDerate bv the above-described diffusion process. It should be added, parenthetically, that this same principle yields the inverse implication that precipitation probability would, in at least one way, be enhanced if the terrestrial atmosphere contained only few and inefficient nuclei of condensation, for then large degrees of vapor supersaturation would develop before the liquid phase made its appearance; but once it appeared, the few drops would grow rapidly and could precipitate to the ground. The actually observed state of affairs with many condensation nuclei and relatively few ice nuclei is much the more probable one, simply because there are many more salts that are merely hygroscopic than there are crystalline materials with lattice characteristics closely mimicking those of ice. It is principally this latter deficit that leads to those cloudmodification possibilities which have received so much recent attention. These will be considered in Section 4.2. 3.3.3. The Accretion Process. A review of the meteorological literature, especially the textbook literature, of about fifteen to twenty years ago discloses a rather general, though certainly not unanimous, suspicion that most mid-latitude precipitation is at least initiated if not fully produced by the ice-crystal process. This view had been especially strongly defended by Findeisen (see, for example, [31]), whose name is therefore often linked with that of Bergeron in identifying the process. When, more recently, both theory and observation revealed increasing evidence that collision processes must at least rival the ice-crystal process, and possibly greatly exceed it in importance, there was some tendency to regard this as a distinctively new development in precipitation theory, particularly among those of us who had first come into contact with the field of meteorology in those World War I1 years when qualitative references to the Bergeron process had just become well diffused throughout the textbook literature. However, the basic idea of collision and coalescence of drops as a mechanism of growth of precipitation particles is definitely not so new as that, and even quantitative analyses on a very limited scale can be found in the

254

JAMES E. MCDONALD

earlier literature. Simpson [32] was, in 1941, stressing the observations of heavy rainfall from tropical clouds with tops definitely known to lie below the level of the 0°C isotherm. Still earlier, in 1938, Houghton [S] had reported some simple computations of rates of particle growth by gravitationally induced collision, the numerical values of which justified his conclusion that a coalescence process “is capable of explaining the formation of rain drops once cloud particles of unequal size appear at the same level,’’ though it is clear in retrospect that his results could only have corresponded rather roughly with actual rates inasmuch as collection efficiencies were not then known with any accuracy. Findeisen devoted considerable attention to collision processes but viewed them as only secondary in importance to the ice-crystal process. Possible effects of electrical charges on droplets were considered by Schmauss and Wiegand [33] as early as 1929; an early instance of attention to a factor that is currently receiving closer scrutiny. A short but historically interesting note on collisional growth by Humphreys in 1922 [34] shows that already by that early date calculations essentially similar to those Houghton discussed in 1938 had been made; but Humphreys dismisses their results on the curious ground that a drop, after falling 5000 feet within the cloud, sweeping out every drop in its path, would come out “only one-sixteenth of an inch in diameter.” Theorists have grown easier to satisfy over the intervening years. The same note also shows that at that date Brooks had clearly recognized the crucially important point that in the presence of moderate updrafts, the effective distance of fall of a drop undergoing collisional growth would exceed its actual distance of fall with respect to the earth in such a way that this growth process might be much more efficient than Huniphreys’ calculations suggested. Milham’s once widely used 1912 textbook [35] contains, on page 239, a paragraph on the mechanism of raindrop growth which describes, of course without quantitative defense, a statement almost identical with that which one would make today in explaining in general terms the collisional mode of growth. Whether or not still earlier speculations came equally close to the present point of view would be an interesting subject of consideration for the historian of meteorology. The above examples will suffice to show that the long delay in demonstrating conclusively the importance of collision and accretion processes in precipitation was not at all due to failure to recognize the possibility of these processes, but was due simply to failure to pursue the question through all of its quantitative ramifications. Regrettably, a broad variety of terms has been applied at one time or another to the growth processes involving collision and aggregation, and some choice must be made for the usage to be employed here. Such terms

255

PHYSICS OF CLOUD MODIFICATION

as “coagulation,” “aggregation,” ‘(accretion,” and ‘(coalescence” have been used, but not with uniform connotation. Here the term “accretion process” will be adopted as the term embracing all of those growth processes involving collision of particles (liquid or solid) with consequent aggregation of the two colliding particles into a single larger particle. There is need for precise formulation of definitions and uniform adoption of these definitions in this and many other areas of cloud physics. The transition from the earlier period of rather cursory inspection of the gross magnitudes entering into accretion processes to the more recent period of detailed analysis of all facets of the problem seems to me to be quite clearly identifiable in a 1948 paper by Langmuir 1361. In that paper, Langmuir presented results of computations of aerodynamic collision efficiencies of large drops falling through a cloud of smaller drops, and discussed resulting collisional growth rates. Prior to this work, the rather complex aerodynamicsof the collision process had never been quantitatively taken into account. The concept of collision efficiency, and related concepts of coalescence efficiency, and accretion (collection) efficiency, are fundamentally important in precipitation processes. The collision eficiency of a drop of radius R falling at its terminal velocity through a cloud of smaller drops of radius r is the fraction of all of those smaller drops initially lying within the volume swept out by the larger drop which actually collide with the larger drop. Its upper limit, as so defined, is unity, and its lower limit is zero, the zero values occurring in those cases where a large drop falls through a cloud of such small drops that the aerodynamic pressure field set up by the large drop serves to deflect all of the small drops entirely out of its path as it descends upon them. The Langmuir theory shows that for any given R there is a critical value of r, greater than zero, below which the collision efficiency is zero. Also, for any given r less than about 10 F, there exists a critical value of R such that zero collision efficiency will obtain for that R and all smaller R. In Table 111 is presented a list of critical values of R for specified small r, as found by Langmuir. The important implication of these results is that in clouds wherein condensation growth has yielded TABLE111. Critical radii R, of falling drops below which no collision occurs, for various cloud droplet radii r (radii in microns). (From [36].) r

R,

1.5 ‘ 2 3 4

600 350 140 58

I ,

r

RC

5 6 7

31 20 14

256

JAMES E. MCDONALD

drops of only small radii (where “small” here means in the neighborhood of 10 p ) , gravitational collisional growth cannot become important until a few drops somehow attain a size that marks them off distinctly from the bulk of the cloud-drop population. Much of the recent theoretical and observational work on liquid-phase accretion processes is primarily concerned with the difficult question of understanding how and when cloud processes do succeed in overcoming this principal obstacle to collisional growth. The collision of two drops does not necessarily lead to their coalescence into a single larger drop (see, for example, Swinbank [37], and Blanchard [38]), though experimental cases of complete coalescence for certain pairs of drop radii have been reported (see, for example, Gunn and Hitschfeld [39]). That weak external electrostatic fields and net electric charges borne by the colliding drops certainly play a role has been known since the work of Rayleigh. It is impossible to avoid the conclusion that, a t present, we scarcely understand the barest essentials of the highly complex physical phenomena controlling coalescence of colliding liquid drops. However, despite this ignorance, it is conceptually useful to define a coalescence eficiency for any given type of collision as the fraction of all such collisions which actually lead to coalescence. Finally, the product of the collision efficiency and the coalescence efficiency may be identified as the accretion eflciency. (The term “collection efficiency” would perhaps be preferable to “accretion efficiency,” but it has already been used frequently as synonymous with the above concept of collision efficiency and also has distinct meaning in other problems of meteorological instrumentation.) Accretion efficiencies may, in particular cases, remain low because of very low coalescence efficiencies and despite high collision efficiencies, or may remain low because of low collision efficiencies and despite high coalescence efficiencies. The concept of accretion efficiency may be applied to collisions1 growth involving ice particles as well as liquid drops, but “coalescence efficiency” must then be replaced by “clumping efficiency.” Langmuir’s 1948 work on the theory of collision efficiency represented m important contribution not only by virtue of the results Langmuir himself drew from it, but even more so from the fact that almost all subsequent theoretical analyses of growth of precipitation particles by collision processes have employed his collision efficiencies. Shortly after his results became available there appeared three different papers by Bowen [40], Houghton [8], and Ludlam [41] which, viewed jointly, clearly established for the first time that collision and coalescence processes must be of major importance in the precipitation problem. Bowen’s theoretical results [40] revealed the critical role of the updraft velocity, for he found that thediameter of the raindrops falling out of the cloud base varies linearly with updraft

257

PHYSICS OF CLOUD MODIFICATION

TABLEIV. Dependence of accretional growth on updraft speeds. (From [40].)* w (cm sec-1) 10 25 50 100 200

Max height above base (m)

Final diameter (mm)

Growth time (min)

450

0.2 0.5 0.9 1.5 3.0

116 82 69 62 60

750 1200 2200

4000

* In a cloud of initially uniform radii of 20 p and updraft speed w, two drops coalesce near t h e cloud base and begin accretional growth. Table entries show, for each of five values of w ,the maximum height above the cloud base attained by the ccreting drop, the diameter i t has on falling out of the cloud base, and time required t o fall out of the base. speed. Bowen assumed a uniform cloud of droplets of 20 p radius in which two drops somehow coalesce to one of twice the normal mass and the latter then falls relative to the others (ie., rises less rapidly than the others) and grows by accretion of small drops. In Table IV are shown Bowen’s values of peak altitude above cloud base attained by this growing drop just before starting to return to the cloud base, the diameter it attains on falling out of the cloud base, and the total elapsed time for the process, for five different updraft speeds. Surprisingly, his calculations revealed that the time required for completion of the process varies inverdely as the updraft speed (see Table IV) due chiefly to the extremely rapid final descent of the large drops formed in the stronger updrafts. I do not believe that this paradoxical result had ever been anticipated, even qualitatively. It seems correct to say that these findings of Bowen’s marked the turning point in a redirection of attention to the hydrodynamic, as distinguished from microphysical, factors influencing precipitation processes. The calculated growth times which Bowen obtained were too great by a rather uncomfortable factor of two or three, as were also Houghton’s; but, as will be explained below, this is now understood fairly well. Houghton’s analysis [8] was chiefly aimed at comparing the relative rates of particle growth by ice-crystal and collision processes and gave the result, cited earlier here, that once particles reached a mass equivalent to that of a drop of about 275 p diameter accretional growth dominates over diffusional growth regardless of how the early stages of the precipitation process might have been initiated. Ludlam [41] emphasized the critical nature of cloud depth, which is of course inherently closely related to updraft speed, and devoted considerable attention to the growth of just the rare large particles (giant nuclei) which might enter a cloud base with radii already as large as 20 to 40 p . These large drops are of great interest for they have suffi-

258

JAMES E. MZCDONALD

ciently high terminal fall velocities that they sweep out large volumes of cloud, space and thus grow quickly. He concluded that if these rare large drops could attain radii of about 150 p before reaching the cloud summit they were almost certain to enjoy continual growth to raindrop size, since a drop of such radius has a falling speed of about 1 m sec-' and thus could scarcely be borne out of the top of the cloud by the air currents typically encountered in cloud tops. The main conclusion of these three studies may be qualitatively summarized as follows. Accretional growth is strongly enhanced by high updraft speeds and great cloud depths and by presence (or rapid formation) of at least a few drops with radii several times larger than the average for the condensationally produced drops; accretional growth outstrips diffusional growth when ice particles attain diameters of the order of a few hundred microns. As a consequence of the above-mentioned work of Langmuir, Bowen, Houghton, and Ludlam, it had become clear by 1951 that much closer consideration was going to have to be given to all portions of the problem of accretion. Already by that time, the first rush of effort to exploit the newly found prospects of cloud seeding had begun to encounter difficulties inherent in trying to manipulate natural processes whose details were obviously demonstrating their complexity. Koughton's work showed the inseparability of the ice-crystal and accretion processes; and all of the accretion results combined to arouse suspicion that accretion processes alone might account for a larger fraction of all natural precipitation than had been previously recognized. There was a disturbing factor of about two or three in the excess of theoretical precipitation growth times over observed times in all of the work cited above, and there remained serious uncertainty as to how the indispensable larger-than-average drops might be formed within clouds. Finally, serious interest in the practicality of directly modifying the accretion process itself developed at about this same time. All of these research objectives have influenced the more recent investigations of the collision processes which have commanded steadily growing attention in cloud physics research. Aircraft measurements, such as, for example, those of Weickmsnn and aufm Kampe [42], in convective clouds, were pointing more and more to the important role of collisions. The number of cloud drops per cubic centimeter reported by aufm Kampe and Weickmann fell off by as much as a factor of five from the base of the clouds to altitudes of only about 2000 meters above the base (specifically, from about 300 ~ m to - ~about 50 cm-3), whereas liquid water content and mean drop size increased through the same layer. Each of these observations pointed to highly effective coalescence processes of some kind operative near the cloud bases. Radar meteorological methods continued to shed a great deal of light

PHYSICS OF CLOUD MODIFICATION

259

on all parts of the precipitation problem (see general review in this series by Marshall et al. [43]), and have, in particular, documented the importance of accretional growth. Observations of precipitation radar-returns from nonfreezing clouds were reported from many areas, but in themselves only added weight t o what were already fairly conclusive visual observations in the same category. More significantly, radar evidence for the dominance of accretional processes even in deep convective clouds extending to well above the freezing level in extratropical regions began to appear. Reynolds and Braham [44]pointed out that initial radar echoes appeared in the observations of the 1947 Thunderstorm Project a t an average temperature level of about -2"C, and they gave radar meteorological reasons for believing that these echoes must have been produced by precipitation particles still too small t o be falling with respect to the temperature field, thus implying that the earlier growth of these particles must have occurred a t temperatures too warm to bring the ice-crystal process into consideration a t all. This point was pursued still further by Battan [45], who used the same Thunderstorm Project data but reanalyzed it more completely. He concluded that fully 60 % of all first echoes were in convective cloud regions entirely below the freezing level (i.e., in regions warmer than OOC) and hence must have been due to coalescence effects. Such applications of radar techniques seem historically especially interesting in that they were so illuminating in just those cases where earlier visual observations had given only the familiar chronological order of glaciation of cumuliform cloud tops followed shortly by precipitation, an order long regarded as one of the best arguments in support of dominance of the ice-crystal process, despite its post hoc nature. The time-of-growth discrepancy of a factor of two or three between theory and observation that was apparent in Bowen's [40] calculations based on the Langmuir collision efficiencies now appears to be considerably clarified as a result of a most interesting series of observational, experimental, and theoretical investigations by the Sydney group. I n 1953, Adderley [46], using a balloon-borne telemetering technique to measure cloud drop-size distribution, observed drop-size profiles strongly indicative of accretional growth rates some three to four times greater than could be accounted for on the basis of the Langmuir efficiencies, tending to cast doubt on the adequacy of the latter. Then, in 1955, Telford et al. [47] carried out some very cleverly planned laboratory experiments on collision and coalescence phenomena which were designed to yield collision efficiencies for pairs of colliding drops of nearly the same radii. The latter condition is that which obtains during the early stages of development of a rain cloud when condensational processes have created a rather narrow range of drop sizes, whereas previous experimental tests of the Langmuir efficien-

260

JAMES E. MCDONALD

eies (for example [39]) had dealt with quite dissimilar dropsizes in collision. The average value of their experimentally determined accretion efficiencies was greater than unity, in fact exceeded 12, an astonishing result indeed. Scrutiny of photographic records of the collision kinematics disclosed to these investigators that the existence of collision cross sections over twelve times the geometric cross sections of the drops was due to a very remarkable phenomenon: Drop A can capture drop B (of radius, and hence fall velocity, close to A’s) by drawing B sidewise into its (A’s) low-pressure wake region, whereupon B quickly falls down from above A and coalesces with it! Langmuir’s calculations dealt only with collisions in which the collected drop contacted the collector drop on the latter’s underside, and furthermore Langmuir had assumed a type of flow regime incapable of predicting wake phenomena. The drops used in the experiments of Telford el al., had radii of about 80 p , and fell with Reynolds numbers of 20, so they cannot simply be extrapolated down to the case of cloud drops just formed by condensation (radii of the order of 10 to 20 p at most and hence falling at Reynolds numbers well under unity) ; but clearly these results constitute a highly significant finding and one that helps to explain the over-all rapidity of natural collisional growth. Two closely related studies in the same program of the Sydney group have added further valuable insights. Telford [48] carried out a careful theoretical analysis and numerical calculation of certain purely statistical aspects of the collision problem, somewhat similar to, but more extensive than some earlier work by Hitschfeld and Melzak. In brief, Telford considered the growth of the “fortunate” few drops which, as a result of purely random fluctuations of mean-free-times between collisions, enjoy a faster than average growth. Previous collision calculations had generally been based on the assumption of uniform mean-free-times (for simplicity and not because such fluctuations are absent). Telford found that in the very short time of 5 min, the favored few might grow to radii of over 20 I.( from an initially homogeneous cloud of drops of 10 p radius (as a result of some ten coalescenses, whereas under the standard assumption of continuous and uniform growth of all drops, over 30 min would be required to accomplish this same growth. Since there seems little doubt that Telford’s model is more realistic than those previously used, and since the difference between 5 min and 30 min in any cloud processes is a highly significant difference in view of short cloud lifetimes, and finally, since all earlier collisional calculations have indicated that precipitation growth will accelerate rapidly once a few drops of about double the average size have appeared, one sees the key importance of this analysis. It must be carefully noted that no assumption was made by Telford concerning any turbulence-spectrum characteristics for the cloud; his fluctuations are statistical and not physical in origin.

PHYSICS OF CLOUD MODIFICATION

261

The second of the more recent contributions of the Sydney group to be mentioned here has been a very extensive set of calculations carried out by Pearcey and Hill [49], using high-speed computer methods, to determine theoretically the flow patterns governing collision dynamics of cloud drops. This work constitutes a much more complete analysis of collision aerodynamics than went into Langmuir’s 1948 results, and provides the first solutions (numerical, of necessity) for the pertinent flow equations at Reynolds numbers in the transition region between viscous and aerodynamic flow. The most interesting of many of their results is their theoretical affirmation of the 1955 experimental observations of Telford et al. [47] relative to the existence of effective collision efficiencies in excess of unity: Pearcey and Hill’s calculated efficiencies range even as high as 100, and nicely confirm in theory the odd capture kinematics observed photographically by their colleagues. As of the time of this writing, no synthesis of all of these very recent developments in collision theory has been accomplished, but it would appear that, when undertaken, improved theory can now be expected to account for the observations substantially more closely than it could only a few years ago. A number of other recent advances in collision-process theory can only be briefly mentioned. Efforts to get higher theoretical growth rates out of the Langmuir efficiencies by invoking cloud turbulence have been made by East and Marshall [50],their principal result being a required turbulence intensity rather too large to find support in any existing observational data (though this negation is far from fatal to their hypothesis since there is a serious dearth of relevant cloud turbulence data a t present). It is of historical interest to note that, as early as 1939, Arenberg [51] had suggested, qualitatively, that microturbulence might play a significant role in cloud drop-growth, so one sees that fifteen years elapsed between qualitative suggestion and first quantitative analysis of this cloud-physical problem. An aspect of the interaction between condensation and collision processes that holds promise of accounting for another portion of the time discrepancy between theory and observation was treated by East [52]. He studied theoretically the consequences of continued condensational growth of cloud drops at the lower tail of the cloud drop-size distribution as air parcels are carried to levels of high liquid water content; and using only the Langmuir efficiencies he found interesting improvement in agreement between theory and radar observations of heights and times of first precipitation echoes. Woodcock and Blanchard [16] have recently presented further evidence of indirect nature (rainwater salinity) tending to support Woodcock’s earlier suggestions concerning the importance of giant sea-salt nuclei in starting accretional growth in maritime clouds. This work assumes particular interest in that it concerns natural processes which, if Woodcock’s

262

JAMES E. MCDONALD

hypothesis is correct, might serve as models for artificial stimulation of collision growth of precipitation by addition of giant nuclei to updraft air entering cumuli in continental areas where droplets of sea-salt solution with radii of the order of 20 p are probably present in deficient numbers a t times (see Section 4.3). Numerous other studies have also contributed to the current progress in understanding accretion mechanisms of generating precipitation, but cannot be described in detail here, even though many are intimately related to the physics of precipitation. Study of the accretional process of growth of precipitation particles has drawn increasing attention to the macroscopic dynamics of clouds, and this trend has been reinforced by growing awareness of the critical role of cloud lifetimes in all precipitation processes. Also, the question of entrainment of environmental air into clouds and all of the problems of momentum- , mass-, heat-, and vapor-exchange between cloud and environment have become somewhat more clearly appreciated as factors which place limits on precipitation release rates and which, in turn, must therefore limit cloud-modification possibilities. No attempt will be made here to outline the recent progress toward better understanding of cloud dynamics, for the slightly arbitrary reason that dynamic factors are not obviously amenable t o modification, and here the physical basis of modification techniques is chiefly under discussion. Despite this omission, I must emphasize that there appears to be unanimous agreement among cloud physics workers that much more research on cloud dynamics must be undertaken before truly rational cloud-modification programs can be undertaken. 3.34. Summarv. An attempt has been made in this section to outline the historical development and present status of knowledge of those aspects of cloud physics that have st,rong bearing on cloud-modification possibjlities. Impressive recent gains have been made on almost all fronts. Within a relatively few years, questions that had been before meteorologists for many decades have been answered fairly satisfactorily. A great amount of sorting out of the important from the unimportant has gone on. Above all, the whole problem has been brought into much sharper focus for present and future studies. The stimulus to cloud physics research that has been provided by prospects of cloud modification to increase natural precipitation has been, without doubt, the most important single source of impetus to recently accelerated research in this field. In the following sections, the scientific aspects of these newly developed cloud-modification techniques will be reviewed, and our present stock of fundamental knowledge will be compared with the knowledge required to exploit these techniques intelligently. It will be found that the above-reported progress which appears so dramatic when viewed in comparison with earlier historical growth of cloud physics falls,

PHYSICS OF CLOUD MODIFICATION

203

unfortunately, far short of what is required to modify clouds in a truly intelligent fashion. 4. RECENTDEVELOPMENTS IN CLOUD MODIFICATION TECHNIQUES 4.1. General

Pre-1946 “rainmaking” has a long and mostly dubious history. Putting aside the superstitious propitiations of early man and of contemporary primitive groups, there is still left a lengthy record of relatively recent rainmaking efforts, particularly in this country near the turn of the century, carried out, it would appear, sometimes in good faith, sometimes not, but certainly with little or no real scientific basis. During the serious American drought years of the mid-nineteen-thirties interest was stirred in what was to be an indirect means of rainmaking in the central United States by creating ponds and stock-watering tanks throughout the area, the evaporation from which was to augment natural rain. This idea, which reappears periodically in one form or another, is a measure of the general lack of appreciation of the enormous scale of operation of the atmospheric hydrologic cycle, a scale that is not to be sensibly altered by any such trifling additions of vapor to the huge stock existing in the atmosphere in even the most severe drought periods. The current epoch of rainmaking is not in any way historically connected with those earlier attempts. One cannot, however, correctly say that the current period has exhibited none of the charlatanism of fifty years ago, for there have been, unfortunately, some patently fraudulent claims made by some persons and groups seeking to exploit commercially the recent interest in cloud-modification methods, and there has been a nearly continuous distribution of vigorously announced, but poorly substantiated, claims ranging from the seemingly preposterous to the merely enthusiastic. Personal experience leaves in my own mind no doubt that a most regrettable confusion developed after 1946 in the minds of laymen concerning the true scientific status of cloud-modification techniques. This confusion stemmed, I believe, from the fact that the often extravagant claims of some seeding operators were either made in pseudo-scientific form not readily evaluated by laymen or else were announced quite positively without attempt a t clarification of the many existing scientific uncertainties not common knowledge among potential users of the operators’ services. This confusion, it is important to note, never enjoyed the salutary influence of normal debate in the professional scientific literature; many who made strong claims simply did not publish, a t least not in the regular technical periodicals, any results of their seeding operations that could be independently studied by disinterested meteorologists. And in those numerous instances

264

JAMES E. MCDONALD

where they published in nontechnical media, their reported data were completely inadequate to permit scientific evaluation. Although such confusion has had little reference to the pressing research problems of the field, it has had broader reference to the relations between science and the public which are very much the scientist’s concern. Since geophysics is, for a number of self-evident reasons, somewhat prone to entanglement in such confusions, it seems very much in order to commend the history of the past dozen years’ public relations in the area of weather modification to the general attention of geophysicists, though details are not relevant here. 4.2. ArtiJicial Nucleation of the Ice-Crystal Process Q.2.1. The Dry-Ice Seeding Technique. It is not often that the exact start of a major development in a given research area can be identified; but in the case of current cloud-modification studies, the beginning is quite clearly identifiable with a July afternoon in 1946 when V. J. Schaefer, in the course of some investigations of aircraft icing at the General Electric Research Laboratories, introduced some dry ice (solid COz) into a deep-freeze chamber in which he had previously created a cloud of subcooled water drops. An immediate and striking transformation to a cloud of scintillating ice crystals occurred. Schaefer quickly ascertained that neither the chemical nature of the dry ice nor its sublimation temperature of -78°C was essentially involved, but rather that any object sufficiently cold would, if merely waved through the cloud, induce freezing, the temperature threshold lying very near -40°C [21]. Recognizing the potentialities for inducing the Bergeron ice-crystal process in natural subcooled clouds, Schaefer carried out, on November 16, 1946, a dry-ice seeding trial that was successful in causing streaks of snow to fall from the base of a cloud into which he had scattered crushed dry ice from an aircraft. Although it took some years to clarify the point, if the point may indeed be claimed to be fully clarified yet, Schaefer’s basic discovery was the detection of the temperature a t which homogeneous nucleation of subcooled drops of size typical of natural clouds occurred. (More accurately, he found the temperature a t which the time required for nucleation probability to approach unity in such drops was of the order of seconds, rather than of the order of hours as it is only a few degrees above the critical temperature.) On present theories, dry-ice seeding of a cloud amounts to dropping into a subcooled cloud a lot of pea-sized particles each much colder than the critical temperature for homogeneous nucleation and hence capable of cooling to below -40°C the numerous droplets formed in the cold boundary layer where vapor is chilled to below the condensation point. That is, it seems probable that the cold dry-ice particle functions in two distinct and

PHYSICS OF CLOUD MODIFICATION

265

sequential ways: first, to condense vapor to droplets, and secondly, to induce homogeneous nucleation within those droplets which then leave the boundary layer as tiny ice crystals capable of serving as growth centers. The laboratory efficiency of this particular seeding process was reported to be phenomenal: Schaefer [21] estimated that a single pellet 1 cm in diameter can form, under optimal conditions, some l O I 6 ice crystals which, if each could grow to the size of small snow crystal, would amount to 300,000 tons of snow. Such efficiency is never attained in practice, but it remains true that a large magnification factor is available. Although the recognition and exploitation of the opportunity of inducing the ice-crystal process in natural clouds was due to Schaefer, three other investigators had, within the preceding year or two, carried out experiments which had pointed to, but had not shown so decisively, the existence of a critical threshold near -40°C. These were Findeisen and Weickmann, both in Germany, and Cwilong in England. For general reviews of the work of these investigators see Houghton [53] and Ludlam [54]. There is difficulty in assigning credit for discovery of the critical threshold of homogeneous nucleation, for the results of all of these nearly synchronous studies contain internal contradictions that, in my opinion, have never been fully resolved. Schaefer's experiments, using a cold box rather than an expansion chamber, seem to have been the most clear-cut. It is of interest to note here that Findeisen had, in 1938 [31], predicted that interesting trigger effects might be placed a t man's disposal if efficient ice nuclei could ever be added to metastable subcooled clouds a t will. The actual technique of seeding clouds with dry ice to promote the ice-crystal process is basically simple. Cakes of solid COZare passed through a mechanical crusher in the seeding aircraft and dropped as fragments of about 5 to 15 mm in diameter. Although dry ice will start the ice-crystal process even a t levels just slightly colder than O"C, there is little chance for much of a seeding effect if released below about the -5°C level unless ice crystals will quickly be carried by updrafts to the -10°C to -15°C levels of most rapid diffusional growth; hence the technique requires use of aircraft capable of cruising readily a t altitudes of a t least about 3 km, and in many areas of the world and various seasons, altitudes of greater than 5 km must be reached (over 7 km in the southwestern United States in summer, for example). In view of these altitude requirements and considering space requirements for stowage of the dry-ice supply and operation of a crusher, it becomes clear that one must use multimotored aircraft if any significant seeding is to be performed. What, then, is the present estimate of the efficiency of the dry-ice seeding technique? My own answer must be that I do not believe a really accurate estimate has yet been given. There was, of course, a great deal of quite

266

JAMES E . MCDONALD

nonscientific publicity given this technique some years ago, and commercial operators have a t times made strong claims for the efficacy of the technique (before aircraft seeding with dry ice was abandoned as too expensive for most operators), but there have been too few carefully controlled dryice seeding experiments carried out under wholly satisfactory conditions to warrant any categorical statements. Experiments made in Australia (see Smith [55], Squires and Smith [56], and Bowen [57]) gave favorable indications in the sense that seeding with dry ice was followed by observable streaks of precipitation leaving cloud bases in a number of instances wherein no other clouds for about 20 miles around were precipitating naturally. Since there is always a possibility of unconscious bias in selecting clouds for seeding, such a criterion cannot be accepted as a strong one, so these Australian tests, though interesting, cannot be regarded as giving conclusive evidence. Bowen 1571 estimated that meterological circumstances offer possibilities for dry-ice seeding in southeastern Australia rarely enough that an increase of no more than 5 to lo%, a t the upper limit, could ensue from this type of seeding. A statistically much stronger experimental design was incorporated into dry-ice seeding tests carried out by the staff members of the Department of Meteorology of the University of Chicago [58]. I n these experiments only an unfortunately small total number of treated clouds could be studied. Pairs of clouds were chosen by a flight controller who attempted to pick two cumuli as nearly alike as possible in all relevant respects, both of which were then penetrated, while only one was treated with dry ice. Only the operator of the dry-ice crusher in the rear of the B-17 aircraft knew which cloud was actually treated (this being decided by opening an envelope containing previously randomized instructions that dictated whether a given cloud was to be seeded or not) and the behavior of both members of each pair was observed by radar and visual techniques without conscious or unconscious bias based on knowledge of which member of each chosen pair had been treated. I n this randomized-pair technique, a total of 27 pairs was studied in the summer of 1954 in the central United States. From this sample, the radar behavior gave statistical results such that the hypothesis that treatment had no effect could not be rejected, see Table V. For, of all 27 pairs, 7 cases showed that the treated cloud developed an echo while the untreated one did not, whereas 5 cases showed the treated pair developing no echo while the untreated member did. There was a theoretical probability of 0.39 of such a 7 to 5 split occurring purely by chance in the absence of real treatment effects, which is far too large a probability to warrant claims to artificial stimulation. The total sample was smaller than had been sought in this study, and the aircraft could not be operated high enough to optimize the dry-ice treatment, but the

207

PHYSICS OF CLOUD MODIFICATION

TABLEV. Results of dry-ice seeding of 27 randomized pairs of cumulus congestus clouds in the central United States. Each table entry corresponds t o one pair of cumuli. (From [58].) Treated cloud of pair

Untreated cloud of pair

No echo

Echo

No echo

1 7

14

5

~~

results seem to be chiefly significant in that they indicate that, contrary to the claims of a decade ago, dry-ice seeding is not necessarily a n ej’kient technique for stimulating precipitation. Reports of still other dry-ice seeding projects, also less than conclusive in their finding, will be found in [58]. In the Chicago flight experiments, a notable lack of ice crystals was was observed by the flight controllers on repenetrating a cloud after treatment, so this matter was examined theoretically by Braham and Sievers [59], with special attention being given to the question of possible overseeding, since rates as high as 50 lb/mile had been used. They concluded that earlier estimates of numbers of ice nuclei generated by falling dry-ice pellets had been grossly overestimated, and even more significantly, they rioted that crystal growth times lay so close to ordinary lifetimes of cumulus towers that there is both an inherent time limitation in dry-ice seeding and an inherently strong ambiguity in evaluation of seeding efficacy when individual cumuli are treated. It has to be recognized that the design of these particular experiments dictated that isolated cumuli be treated, and it remains possible that dry-ice seeding of clusters of convective towers or massive banks of such clouds might lead to different results. I n summary of the status of the dry-ice seeding technique, it can be said that, though there is no disagreement on the point that dry ice can indeed produce the Bergeron transition in subcooled clouds both in the laboratory and in nature, a variety of circumstances conspire to make this appear, a t present, to be a less promising means of increasing natural precipitation in economically feasible ways than was hoped a decade ago. 4.2.2. T h e Silver Iodide Seeding Technique. Only a few months after Schaefer had made his first observations of laboratory effects of dry-ice treatment of subcooled clouds, his colleague Vonnegut made a very substantial contribution to cloud modification technology 1601. Reasoning that substances very similar to ice in crystalline structure might serve as ice nuclei (i.e., reasoning that nucleation should be dependent on epitaxy) , he searched the literature of crystallography for materials closely resembling ice with respect to crystal system, space group, and lattice dimensions of the unit cell. Two substances out of a large total number examined appeared

268

JAMES E. MCDONALD

TABLE VI. Comparative crystallography of ice, silver iodide, and lead iodide. (From [60].) Substance Ice AgI

PbIz

System Hexagonal Hexagonal Hexagonal

Space group DSh4 CBV4

D3ds

Lattice constants (Angstrams) __-

a

C

4 * 535

7.41 7.490 8.86

4.585 4.54

promising, silver iodide and lead iodide. In Table VI are shown the comparative data for ice, AgI, and PbIz which led Vonnegut to conduct experiments with the latter two materials. After some initially unsuccessful attempts, Vonnegut found means of generating large numbers of tiny particles of these materials, and both proved highly efficient as ice nuclei. Most of the subsequent consideration of such heterogeneous nucleants has been given to AgI, which Vonnegut showed to be effective a t temperatures as warm as -4°C in subcooled clouds. Table VI shows that both lattice constants for AgI differ from the corresponding ice constants by only about one per cent, permitting growth of an ice lattice upon the AgI particle surfaces with very small mismatch, so that the AgI particle serves as just about as good a growth center as a true ice crystal. Despite the difference in space groups, the ice structure is almost identical with that of AgI, with the oxygen atoms occupying all of the lattice points occupied in AgI by silver and iodine atoms and with tetrahedral hydrogen bonding in the ice structure corresponding to tetrahedral silver-to-iodine bonding in AgI. Up to the present time, no naturally occurring dusts have been found to approach AgI or PbI in nucleating efficiency (cf. Fig. 3), and the observed tendency toward marked subcooling in clouds stands as incontrovertible evidence that even if any exist, they certainly are not swept up into the free atmosphere in significant concentrations. All of the principal techniques subsequently used for generating AgI nuclei are derived from Vonnegut’s early work [60, 611. AgI can be coated on filaments which are then electrically heated to evaporate AgI smoke. It can be impregnated into string which is then fed into an oxyhydrogen flame. Silver electrodes can be sparked in the presence of iodine vapor. A solution of AgI and NaI in acetone can be prepared and charcoal or coke particles soaked in it and subsequently burned. Most commonly, the AgI-NaI-acetone solution (KI often replaces NaI) is sprayed into a hydrogen or hydrocarbon flame to form the nuclei. Solutions containing from 5 to 10 % AgI by weight are typically used in the latter method. All of the above nuclei generating techniques are essentially dependent upon vaporization of AgI followed rapidly by quenching in the ambient

PHYSICS OF CLOUD MODIFICATION

269

air to condense out very small AgI particles and these two processes may vary considerably in their effects from one type of generation to another. Vonnegut [60] found that the impregnated-string method produced particles with diameters of the order of 0.01 p , while vaporization from a hot wire gave diameters close to 1 p . Nucleating efficiency is size-dependent, partly by virtue of the Kelvin effect explained above and partly because of more subtle considerations of crystal nucleation, so it is not surprising that Vonnegut found that the 0.01-p particles became effective nuclei only with subcooling to about -8"C, while the 1-p particles were effective a t -4°C. But, because of the generally small particle sizes for all methods of generation, extremely large absolute numbers of AgI particles are obtained per gram of AgI used, typical figures running to about lo1*to 1OI6 nuclei per gram of AgJ. The actual number of effective nuclei produced per second depends upon the rate at which AgI is consumed by a given type of generator and upon physicochemical details of the generation process. Also, mere particle-generation-rate is not a meaningful criterion, because some particles from a given generator are not effective (presumably for reasons of size and surface structure) until large degrees of subcooling are involved, while others are much more efficient. Smith and Boucher [62] have recently conducted comparison tests of three commonly used types of generators, and Fig. 5 summarizes their results. They give figures interpretable in terms of actual AgI consumption only for the propane-acetone generator. For reference these are given here. A solution of 16 lb of AgI and 4 Ib of KI to 55 gal of acetone is burned in propane in this generator a t a rate of just under 1 qt/hr. This amounts to a consumption of about 0.01 gm of AgI per second. The original reference must be consulted for experimental details and for data on the scatter about the curves reproduced in Fig. 5. The steep slope of the curve for the propane-acetone generator and its falloff a t temperatures warmer than about -10°C stand in contrast to the slopes of the other two generators, and it is evident that very different cloud seeding efficacy could result from use of these different generators a t different cloud temperatures. When i t is desired to introduce AgI into clouds from aircraft flying in or near subcooled clouds, large nuclei generation rates are needed to compensate for aircraft speed, which rules out the impregnated string burner, while safety factors preclude use of coke. Hence some form of burner employing a solution of AgI is usually used in aircraft seeding. Warner and Twomey [63] describe a generator mounted externally on an aircraft to take advantage of ram pressure in sustaining a high combustion rate. An acetone solution of AgI, burned at a rate of about two gallons per hour (equivalent to about 1000 gm AgI per hour), gave a linear seeding rate of about 4

270

JAMES E. MCDONALD

14

-

,,' .

Propane-

- 12 --

-

'8 -6 10l

J

I

-

I

$ 8a 86-

I

-4

J I I

I I

I I

I

" '

-8

"

-12

'

"

-16

"

1

-20 -24

Cold Box Temperature (De4.C) FIG.5. Comparison of nuclei output of three types of silver iodide generators. Solid curve: coke particles impregnated with 40% AgI solution; dashed curve: acetone solution of AgI and K I burned in a propane flame; dotted curve: string impregnated with a 10% solution of AgI, burned in a propane flame. After Smith and Boucher in

WI. gm AgI per mile, a contribution of the order of 10l2to 1013 AgI nuclei per mile. As this line source of high nuclear density drifts downwind, turbulent diffusion expands the line into a cylindrical distribution. Estimates that I have made in conjunction with a similar seeding program conducted by the University of Chicago and the University of Arizona indicate that the diameter of the cylinder is of the order of 1 km after about 15 km downwind drift when released a t an altitude of about 6 km. After such diffusive expansion of the trail of nuclei, the calculated average density is about 100 per liter if no photolytic decay occurs. There is a most serious lack of truly critical studies of the physical processes occurring in generators and of the undoubtedly very complex surface physics and chemistry of the nuclei they produce. Nevertheless, a few very informative studies have been completed, and viewed optimistically, they suggest that there is probably much room for improvement in generator efficiencies. Among the results obtained from the rather small number of careful investigations of nuclei production and of nuclei characteristics that have already been carried out, a few will be summarized here for their bearing on cloud-modification physics. One of the first and most important problems to receive attention w u s that of photolytic decay of AgI particles when released into the sunlit atmosphere, an obvious point of suspicion inasmuch as the silver halides are commonly used photosensitive agents in photographic film. The first

PHYSICS OF CLOUD MODIFICATION

27 1

published report on this problem [64] indicated that as a result of irradiation under bright noon-day summer sun in New Mexico the number of AgI particles possessing nucleating effectiveness fell off by a factor of about 100 in one hour. The nuclei were generated by burning an acetone solution. Since times of the order of an hour are frequently required for nuclei released from generators on the ground to be carried by diffusion and convection to cloud altitudes where they can be effective, this firstreported deactivation rate was quite disturbing. Equally discouraging were the results of Inn [65] who irradiated AgI (nuclei evaporated from a filament) with ultraviolet light of intensity roughly that of sea-level sunlight and found that complete photolytic deactivation occurred with wavelengths of less than 4300 8 units in about 20 min. Inn suggested that irradiation might irreversibly dissociate AgI with liberation of iodine and aggregation of silver atoms into surface clusters that spoiled the lattice compatibility between the AgI and the deposited water molecules. Vonnegut and Neubauer [66] reported, shortly after appearance of both of the above studies in 1951, that a deactivation rate of only about 50 % per hour had been found in their laboratory tests with nuclei formed by burning impregnated charcoal. No completely satisfactory explanation was then given, or has ever been given, for these and other contradictory experimental results, but these authors proposed that trace impurities in the AgI might exert marked influences on deactivation rates and suggested that these impurities might vary from one generation method to another. This report was closely followed by another paper by Reynolds et al. [67] in which it was reported that samples of AgI nuclei whose ice-nucleating effectiveness was reduced by three to four orders of magnitude after irradiation by an ultraviolet source for only a few minutes were restored to high nucleating efficiency by treatment with traces of ammonia gas after deactivation. They were able to take account of Brownian precipitation of nuclei on the walls of their experimental chamber and thereby to provide indication that some of the disagreement between existing results of various investigators was a matter of chamber size. By analogy with the process of ammoniation of photographic plates, they hypolhesized that ammoniation of a sample of nuclei might be chiefly a surface chemical effect acting upon the smaller AgI particles not ordinarily effective as nuclei and also not so susceptible to initial photolytic decay. No detailed treatment of this interesting hypothesis has yet been given, and the practical potentialities of ammoniation in cloud-seeding work would seem to warrant further study. Birstein [68] found that photolytic deactivation times were strongly dependent upon ambient relative humidity. Nuclei in an atmosphere of 100 % relative humidity with the same ultraviolet source previously used by Inn [65] were found to lose completely their nucleation powers after 150 min of

272

JAMES E. MCDONALD

irradiation; at 60% this deactivation time fell to 60 min; and a t zero per cent relative humidity the sample lost all its nucleating effectiveness in a mere 5 min. The normal tendency for relative humidity to increase from surface to cloud-base altitudes would, in view of Birstein’s findings, imply that the most serious deactivation effects must occur while ground-released nuclei are drifting upward through the lowest kilometer or so. This suggests that generator sites should be located at highest available elevations for this reason, as well as for other more obvious meteorological reasons. Two other more recent deactivation studies carried out in Australin seem to show conclusively that photolytic decay is a very real difficulty and that decay rates may depend considerably on generator characteristics in a way still not understood. Smith et al. [69] released a known flow of zinc sulfide powder, as a fluorescent tracer material, into the air from the site of their AgI generator and flew aircraft equipment at varying heights and downwind distances to measure concentrations of both of these particulate materials. Whereas earlier flight measurements of just AgI nuclei concentrations had been subject to uncertainty due to ignorance of exact relative importance of photolytic decay on the one hand, and of turbulent dilution on the other, in this study only the ratio of observed number of AgI particles to observed number of fluorescent ZnS particles was considered. This neatly obviated uncertain speculation as to the role of dilution itself. Smith et al. found that AgI nuclei from a kerosene-burning generator decayed in daylight at a rate of about tenfold per hour, while nuclei from a hydrogen-burning generator decayed very much faster, effective nuclei counts falling to one-tenth the initial value in only about eight minutes. Efforts to detect correlation between decay rates and amount and type of cloud cover were unsuccessful, a result that seems somewhat difficult to understand and that deserves further study. In a later investigation carried out in similar fashion, Smith and Heffernan [70] flying AgI and ZnS counting equipment under both daytime and nighttime conditions, found that whereas their daytime flights agreed closely with those of Smith, Heffernan, and Seely [69], their nighttime counts revealed no discernible decay in nucleating efficiency over times as long as 150 min, proving beyond reasonable doubt that the daytime decay is photolytic in nature. In Fig. 6 are shown their 1956 decay measurements for day and night for the case of just the output of a kerosene-burning generator. As pointed out by Smith and Heffernan, the very small spread of the nighttime counts about the zero-decay line is strong evidence that the inherent accuracy of the AgI-ZnS counting method is high and that the spread in the daytime counts is real and not just due to counting errors, though again efforts to correlate decay anomalies with cloud cover (and humidity) were

PHYSICS OF CLOUD MODIFICATION

273

Exposure time (minutes)

FIG.6. Variation of flux of silver iodide nuclei with time of exposure in the free atmosphere under day vs night conditions. After Smith and Heffernan [70].

inconclusive. No dependence of decay rate on ambient air temperature could be found in this study. The reason for the much higher decay rate of nuclei obtained from a hydrogen generator as compared with a kerosene generator is not yet known. I n consideration of all of these results on photolytic decay of AgI nuclei in sunlight there is now little basis for doubting that decay is a factor much too large to be overlooked in seeding operations and that amounts of AgI required to attain a given theoretical AgI nucleating rate in clouds are certainly considerably greater than had first been estimated. Concomitantly, these findings point to the pressing need for careful physical and chemical studies designed to find ways of suppressing photolytic decay or to find materials less subject to photolysis but equally efficient as nucleants. More exact information is badly needed concerning the crystallographic properties of the surfaces on which the ice grows in the nucleation process and concerning the way in which high-temperature processes in nuclei generators influence these properties. A few significant experimental studies carried out in the past few years, chiefly on AgI, provide some initial information of this sort, To aid in the interpretation of these studies, a glossary of terms and crystal properties of AgI has been drawn up in Table VII. Since various authors have identified the three AgI crystal forms in confusingly different ways, this glossary may have reference value beyond its immediate use here, The information contained in it has been drawn from a variety of sources [71-731. At room temperatures the hexagonal form is stable, but since all generators heat the AgI far above even the 146°C transition temperature, the question of which form will be found in nuclei after quenching is one that cannot be answered without informa-

274

JAMES E. MCDONALD

TABLE VII. Crystallographic terminology and properties of silver iodide.* Crystal system Cubic (body-centered) Cubic (face-centered) Hexagonal

Common name of arrangement

Zincblende (ZnS) Zincite (ZnO) or Wurtzite (ZnS)

Temperature range of stability

Lattice constants (Angstroms)

Above 146°C 135" t o 146°C Below 135°C

a = 5.03 a = 6.47 a = 4.585 c = 7.490

* Remarks: The hexagonal (zincite) modification of AgI is sometimes referred to as 8-AgI, the cubic (zincblende) modification as 7-AgI. AgI melts at 552"C, decomposing upon melting into metallic silver and iodine vapor. tion on the crystal physics of the condensation processes, There is sonic evidence that AgI produced by certain generators is principally in an amorphous form. To determine the effects of generator temperature on AgI crystal form, Manson [72] passed the output of a temperature-controlled source through a thermal precipitator in such a way that a deposit of nuclei could be obtained in a form amenable to powder x-ray diffraction analysis. With the source operated a t 650"C, 73 % by volume were hexagonal crystals, while at both 800" and lOOO", 95 % were hexagonal. The root-mean-cube particle diameter for the 800°C case was about 0.1 p . The trend toward higher percentage of the hexagonal modification with higher source temperature Manson interprets, tentatively, as due to distillation of greater quantities of iodine vapor than of silver vapor from the high-temperature source to the quenching section, combined with a tendency (inferred from solution crystallization phenomena) for excess iodine to promote growth of the hexagonal modification rather than the cubic forms during condensation in the quenching section. Since Vonnegut originally chose AgI as a likely nucleant on grounds of structural similarity between ice and the hexagond (wurtzite or zincite) modification of AgI, the above results seem to indicate that high source temperatures might be preferred. However, this last inference is based on the premise that the hexagonal modification will be a better nucleant than the low-temperature cubic (zincblende) modification, and recent investigations cast some doubt on this premise. Manson [74] has called attention to the fact that low-temperature cubic modification has one plane, the diagonal (111) plane, in which the atomic spacing and pattern is almost identical with the basal (001) plane of ice and of hexagonal AgI. He succeeded in preparing samples with widely differing relative amounts of hexagonal and cubic AgI (the latter having strongly developed faces parallel to the (111) plane), and tested

PHYSICS OF CLOUD MODIFICATION

275

their ice-nucleating abilities. In the temperature range from -9" to - 1l"C, he found no observable difference in nucleating ability of a sample coiltaining 70 % hexagonal crystals as compared with another containing 94 % cubic (r-Agl) . Concluding from this that either nucleation occurs preferentially on (111) planes or else that nucleation is not related to surface atomic structure, he next devised a complementary test of the hypothesis that surface structure is unimportant. Calcite and vaterite are two modifications of C a C 0 3 , which can be prepared in such a way that the well-developed faces of the calcite have no hexagonal symmetry while those of the vaterite do have hexagonal symmetry with lattice spacing only 9 % different from that of ice. If two such samples yielded equal nucleating effects, one might conclude that crystal structure of the nuclei faces is unimportant. However, Manson's tests showed that while the calcite crystals proved ineffective with subcooling to as low as -25OC, the vaterite was very effective at -20°C and gave a few crystals even a t temperatures as warm as - 13°C. Hence, Manson's complementary experiment seems to stand as confirmation of the original premise on which AgI was selected by Vonnegut, namely, that nuclei operate through epitactic phenomena that demand lattice compatibility between nucleus and ice. One would hope that nucleating tests might soon be carried out on samples of the low-temperature cubic form of AgI so prepared as to have minimal development of (111) faces, a further test of the epitactial hypothesis. To my knowledge this sort of test has not yet been attempted, though it would complement Manson's reported study rather better and more conclusively than the calcite-vaterite test. Almost concurrently with Manson's just-cited experiments, related studies were being carried on by Pruppacher and Sanger [75] in Switzerland that also gave the result that the cubic r-AgI is about as good a nucleant as is the hexagonal P-AgI. Pruppacher and Sanger did not discuss the possibility of (111) faces controlling the nucleation in the cubic case, so Manson's work is for the moment the only evidence for (111)-plane action. On the other hand, Pruppacher and Sanger seem to have made a potentially very important discovery in their further work on nucleation. Among a very large number of substances which they tested as nuclei, cupric sulfide (CuS) appeared to rival AgI in nucleating efficiency; and to aggravate the general mystery, CuS, though hexagonal in crystal form, has lattice constants very different from those of ice (for CuS, a = 3.80 8, c = 16.43 8). Pruppacher and Sanger feel that epitaxy, though sufficient, cannot now be accepted as necessary for nucleation, and have as an alternative examined evidence bearing on Weyl's hypothesis that nucleation depends upon polarizability phenomena. On the latter hypothesis, nucleation of subcooled water drops is visualized as ensuing when a silver iodide (or other)

276

JAMES E. MCDONALD

crystal contacts a drop as a result of a random collision and thereupon locally alters the oriented dipole layer a t the surface of the water drop iii such a way as to lower the free energy barrier to freezing. No quantitative support for this hypothesis has yet been given and it implicitly depends upon the existence of essentially dry AgI particles reaching the subcooled region of clouds. Other recent work [76] strongly suggests (see below) that AgI nuclei never can remain dry during intracloud ascent to those levels where they can nucleate, so I have strong doubts that Weyl's hypothesis can be a correct description of the actual function of AgI or other similar nuclei in typical cloud-seeding operations. Other objections to Weyl's polarizability model can be raised, but these cannot put aside the nowpuzzling and important problem of how CuS functions. Many investigators have reported long lists of substances which they had found to serve as ice nuclei, and if all of these were correct, the accounting on theoretical grounds would indeed be difficult. However, many of these results are mutually contradictory, and since contamination is easy, above all in laboratories where AgI has been used without great care being taken to avoid casual contamination of room objects, one cannot take them all a t face value. Mason and Hallet [73], after testing many of the reported nucleants under very carefully controlled conditions and after making various kinds of contamination tests, concluded that silver or iodine contamination has probably accounted for most of the observations of high nucleating efficiencies. They were also disposed to question Pruppacher and Sanger's results on CuS on grounds that CuS crystals themselves yield scintillations similar to those of true ice crystals, but in a subsequent report [77], they confirm the CuS observations, using a very pure and specially prepared sample. They found this sample of CuS capable of nucleating a t - 6°C. Further insight into the critical question of the physicochemical nature of effective ice nuclei has come from studies [78] carried out by Birstein on the role of adsorption in heterogeneous nucleation. Samples of various nuclei loosely deposited in the small platinum bucket of a McBain adsorption balance were allowed to come to equilibrium with each of a series of successively higher vapor pressures, and the thickness (number of molecular layers) of the adsorbed water deposit was determined for each pressure by standard adsorption techniques. Birstein found that unusually large numbers of molecular layers of H2O were adsorbed by both AgI and P b I z . At -2O"C, AgI nuclei of average radii of about 0.2 p adsorbed approximately 200 molecular layers of H2O near ice saturation, and PbIz nuclei were only slightly less extreme in their adsorption. Since adsorbed layers of this depth were built up with the vapor held at or below ice saturation, and since a nucleus with as many as 200 molecular layers is

PHYSICS OF CLOUD MODIFICATION

277

already essentially an ice crystal as far as further growth is concerned, Birstein drew the conclusion that both of these nucleants can undoubtedly act as deposition (sublimation) nuclei. He supported this conclusion by growing ice crystals on AgI nuclei within a cold chamber containing only vapor and no drops at -20°C (see also the 1954 work of Schaefer [76], cited later, for very strong evidence that AgI and PbIz can act as deposition nuclei). Subsequently, Birstein [79], repeated these adsorption experiments with photolyzed AgI nuclei and found a marked reduction in adsorption. For example, at -2O"C, only about 25 molecular layers of HzO were deposited at a vapor pressure corresponding to ice saturation, or only oneeighth of the deposit previously found on unphotolyzed AgI nuclei in the earlier study. This latter result shows so very specific a reduction in nucleating ability of AgI after exposure to ultraviolet light of about sunlight intensity that it plus all of the field and laboratory observations cited earlier here seem to close the discussion of whether AgI nuclei actually do undergo photolytic deactivation. Since PbIz is not susceptible to photolysis in the way that AgI is, and since early cold-chamber experiments revealed that PbIz approaches AgI in nucleating efficacy, it would seem reasonable to look to PbIz as a possible substitute for AgI as a nucleant in field programs of cloud modification. An investigation conducted by Schaefer [76] in 1954 seems to me to imply that this is not an attractive possibility, although Schaefer did not himself draw this inference from his work. Using a continuous cloud chamber of the diffusion type, Schaefer performed experiments which Eeem to demonstrate rather neatly that, whereas both AgI and PbI can function as deposition nuclei at about - 5"C, they are not equally effective as freezing nuclei. To test the latter point, Schaefer allowed all condensation nuclei ordinarily present in the air to rain out of the diffusion chamber and then introduced the AgI or PbIznuclei into the upper part of the chamber where the temperature was about 5°C and where the vapor was supersaturated with respect to liquid water. Both the AgI and the PbIznuclei then immediately functioned as condensation nuclei (a result consistent with Birstein's adsorption findings) and the resultant droplets fell through the diffusion chamber to subzero levels. But, whereas the droplets grown in this way on AgI froze reliably at about the -5°C level, those grown on the PbIz nuclei did not freeze until they fell to about the -20°C level. Schaefer suggested that this marked difference in behavior might be due to the fact that PbIz is some lo6 times more soluble than is AgI. I feel sure that this does offer the clue to the explanation, because if one calculates the amount, of water required just to dissolve a nucleus of PbIz of 0.1 p radius, it proves to be about equal to that in a drop of 1 p radius, while a nucleus of AgI of the same size will not be fully dissolved unless contained in a drop of about 100

278

JAMES E. MCDONALD

radius. Now, in ordinary cloud-seeding operations with ground generators, the nuclei enter the clouds primarily through their bases; therefore, in almost all conceivable cases, the nuclei will then spend times of the order of minutes ascending through regions characterized by saturation with respect to liquid water. Hence, I think it is inevitable that such nuclei will always serve first as condensation nuclei and be enveloped by liquid water before they ever reach subzero levels where they can promote crystallization. But this means that whereas the highly insoluble AgI nuclei can survive as solid particles within their containing cloud drops during the ascent, the much more soluble PbIz nuclei will be wholly dissolved and will thereby have lost their effectiveness as ice nuclei. It seems reasonably safe to generalize this behavior to all potential nucleants on the basis of Birstein’s work. That is, those very crystalline particles whose structure promotes epitactic growth of ice crystals by deposition from the vapor phase should also be tolerably good condensation nuclei, so these nuclei will be inside water drops when they reach subcooled regions after ground release. (Aircraft seeding a t or above the freezing level may present somewhat different conditions.) From this, I would draw three inferences. (1) Action of AgI or other similar nuclei can scarcely involve the sort of external-surface-contact dipole-reorientation effects which Pruppacher and Sanger [75] have suggested on the basis of Weyl’s hypothesis. (2) Laboratory cold-chamber experiments and expansionchamber experiments probably give a much less relevant measure of true cloud-seeding effectiveness of crystalline nuclei than has previously been appreciated; such experiments permit epitactic growth by deposition without competition from prior condensational growth, and thereby give overoptimistic results with moderately soluble nuclei such as P b I z . (3) In the case of the latest addition to the small list of highly effective ice nuclei, cupric sulfide, one finds that its solubility happens to be such that a particle of 0.1 p dry radius will just dissolve in a drop of about 10 p radius, so there is only marginal possibility that CuS would actually work under typic:il ground-generator seeding conditions. This last case is, however, so much a. borderline case that final conclusions as to its prospective effectiveness can only be drawn from much closer examination of the kinetics of emdensational growth upon CUS particles. It should be noted that the Kelvin effect on solubility of small particles works against nucleation effectiveness in the case of submicron nuclei, though this will only become n serious factor for nuclei of radii well under 0.1 p . I n addition to all of the microphysical questions concerning behavior of cloud-seeding nuclei, there have also been raised during the past few years some questions concerning whether ground-released nuclei can always be expected to be carried by diffusion processes to sufficiently high altitudes p

PHYSICS OF CLOUD MODIFICATION

279

to enter clouds in the first place. In experiments in New Mexico [80,81]and later in Australia [29], somewhat discouraging results were obtained, for the reported heights to which fluorescent zinc sulfide tracer particles reached after many tens of miles of downwind drift were rather less than typical cloud-base heights above terrain. This question is still open, though Smith and Heffernan, in a subsequent report [70], give data showing that their earlier results [29] had been obtained under conditions where eight of twelve cases were characterized by inversions low enough in the atmosphere to significantly influence diffusion, and the remaining cases were those in which the tracer plume was not explored out to more than 11 km from the source. Hence it would seem that their 1955 results must be interpreted in a restricted sense. Prior to this clarification, a tracer experiment to complement the experiments discussed in references 29, 80, and 81 was set up in Arizona by Kassander [821 for the chief purpose of examining vertical diffusion under such extremely favorable circumstances that if no particles had reached cloud-base altitudes the then extant pessimism would seem to have been fully confirmed. These favorable circumstances involved highly unstable air, namely, adiabatic lapse rates from terrain to about 3 km and a steep mountain slope rising to about 2.5 km just downwind from the release point. The ZnS tracer particles were actually found a t altitudes as high as 4 to 5 km in this experiment, so the one-sided power of the test was not realized. Instead, the test showed that ground-released particles certainly attained cloud-base altitudes under a t least these very favorable meteorological and topographic circumstances. It was repeatedly observed, during this series of experiments, that a very large degree of meandering of the plume occurred. Slight wind changes upwind of and over the mountainous terrain where this experiment was conducted made it extremely difficult to determine just where the tracer particles were going to drift, even though double-theodolite balloon observations and a n auxiliary smoke plume were employed as aids. This experience tends to cast doubt on the frequently repeated assertion that one can “see” that a given generator is producing precipitation downwind, a t least in rough terrain. From this, and from the few other diffkion experiments carried out to explore the drift of ground-released nuclei, it has become quite evident that much remains to be learned about diffusional ascent of nuclei. From the standpoint of optimizing cloud seeding, one wishes to be able to predict what flux of nuclei should be released a t the generator site in order to insure a specified concentration a t a given level within clouds a t a roughly given distance. An error of much more than one order of magnitude in concentration may, on existing theory, spell the difference between seeding success and failure, yet such a precision places demands on turbulence theory that cannot a t present be met, above all under conditions of airflow

280

JAMES E. MCDONALD

over orographic barriers of complex form. Since seeding of orographic clouds holds, in many ways, better promise than seeding of any other cloud type, this difficulty is not a trivial one. Having now discussed the development of the silver iodide seeding technique and having summarized some of the most important work of the past few years on microphysical and macrophysical problems that have arisen in conjunction with this development, it is next in order to attempt a summary of the present state of the art of silver iodide seeding as a practical measure for increasing precipitation. It is, of course, exactly this kind of summary that the geographer, hydrologist, and ultimate water user all press the meteorologist to present. Readers in that category may not, therefore, be satisfied with what I have to say, for I simply do not believe that any concise statement can be given, unless it be the simple one that the practical efficacy of AgI seeding is still not adequately assessed. In 1956 an international conference on the scientific status of cloud modification [83] was held for the chief purpose of taking stock of existing knowledge concerning cloud seeding of all kinds. I have summarized that conference elsewhere [84]in a form that seemed to represent the consensus of the participants. The essential conclusion of the conferees, as far as concerns AgI ground-generator seeding, was that there have not yet been reported any seeding experiments so designed and so evaluated as to permit either a clear-cut positive or clear-cut negative decision as to seeding efficacy. It seemed generally agreed by participants at this conference that the best prospects for success in silver iodide seeding with ground generators lie not in regions of level plains but in regions where marked orographic barriers block prevailing winds that tend to arrive with large moisture ont tent.^ In such areas, nuclei can be swept relatively rapidly into clouds that tend to be quasi-permanent, and existing evaluations for seeding projects in areas filling these requirements look promising. Above all, it seemed unanimously agreed by conferees that, although early enthusiastic claims now seem evidently to have been excessive, there still remain so many poorly understood factors, which may permit real improvements through future advances,'that a certain optimism is in order. In the short 4 Thom [85] has reported statistical evaluations of a number of commercial cloudseeding programs from which he concludes that nonorographic projects exhibit no statistically significant increases, whereas the orographic projects, particularly on the West Coast of the United States, do indicate, according t o his analyses, statistically significant increases averaging about 15%. Because he had available for analysis only results of nonrandomized seeding schemes, he urged that these findings he taken chiefly as guides t o further theory and experiment. That orographic clouds offer optimum opportunity for augmenting natural precipitation has been stressed by others, for example, Bergeron [86] and Ludlam [87].

PHYSICS OF CLOUD MODIFICATION

28 1

time that has since elapsed, I do not believe this picture has changed in any essential way. This residual indeterminacy may seem odd to the reader who is aware of the very large number of AgI ground-seeding projects that have been carried on during the past decade by commercial operators throughout the United States and other countries, for it might be expected that more than ample field data would by now be available from such projects to permit drawing valid conclusions as to AgI seeding efficacy. This, however, is surprisingly far from being the case. Until quite recently, no such projects had ever been set up with a tightly controlled statistical design of the type demanded by the inherent variability of natural precipitation, so no very conclusive statistical inferences have been deducible from even the best of these many projects. Where such projects have been evaluated by disinterested meteorologists and statisticians, the results have fallen in about equal numbers on the side of positive and negative seeding effects, but it is principally to be stressed that little weight can safely be placed on either the optimistic or the pessimistic findings of analyses of improperly designed experiments. This evaluation problem will be considered again in Section 5, where the severe difficulties that beset evaluation of cloud-modification programs will be briefly discussed. Here it need only be said that it has been most regrettable that despite the very large sums of money expended on commercial silver iodide seeding activities since 1946, very little reliable scientific information has been derived from these sources. One objective measure of the volume of sound information contributed from these sources is to be found in the total number of published papers in the American professional meteorological journals that have come from such seeding projects. Two issues of the American Meteorological Society’s Meteorological Abstracts and Bibliographies [88, 891 have been devoted to cumulative abstracts of articles on cloud seeding appearing up to about mid-1955. Out of a total of 430 abstracts for all countries of the world and for all types of publication (professional and popular), I counted just four which had been contributed to any of the three United States professional journals covering the field of meteorology by workers associated with commercial seeding projects. Research on silver iodide seeding has revealed a great deal that was not known in 1947 when Vonnegut discovered its nucleating capabilities in the laboratory. Properly designed field experiments continuing over a number of years are sorely needed, and much more extensive laboratory work is needed on nucleation physics in general and on generator technology in particular. It is still much too early to conclude that photolysis presents

282

JAMES E. MCDONALD

insuperable difficulties, despite recently established photolytic deactiv.d t’1011 rates. Field trials of cupric sulfide (properly designed in the statistical sense) may now be in order, and certainly a continuing search for still unsuspected ice nucleants must be carried on as a matter of basic research. Finally, since there still remain many fundamental gaps in our knowledge of the actual world-wide importance of the ice-crystal process which we attempt to stimulate by heterogeneous nucleation, it is abundantly clew that progress will be seriously impeded unless basic research on cloud dynamics and on natural precipitation mechanisms is vigorously pursued. All of these efforts derive importance from the widely accepted view that some kind of ice-crystal nucleation probably affords greater probability of practical influence on precipitation processes than any other kind of cloud modification now accessible to man. The fact that early claims and hopes for very large increases in precipitation from AgI seeding have not been substantiated, and the fact that initially unrecognized difficulties have, through the research of the past dozen years, been identified and partially elucidated, simply do not justify pessimism as to long-term possibilities. The prospect of heterogeneous nucleation of the ice-crystal process, antiripated by Findeisen and brought to laboratory reality by Vonnegut, remains a challenge to meteorological and physical research whose importance demands, in my opinion, very much more over-all effort than it has yet received.

4 3. Arti$cial Stimulation of the Accretion Process I n this section, only those accretion processes occurring a t temperatures above freezing, or a t least involving liquid water rather than the ice phase, will be considered. As has been elaborated in Section 3.3.3, such accretion processes depend, in one way or another, on the existence of a broad range of cloud-drop sizes. But inasmuch as condensation alone yields only quite narrow drop-size distributions when operating on typical populations of condensation nuclei (Section 3.2), initiation of the liquid accretion process must depend upon the presence of a few very large nuclei in the air entering the cloud, upon some kind of random collision processes, upon microturbulence effects, or upon electrical effects still poorly understood. It hears repeating that, a t present, there is nothing like definitive evidence as to which of these factors tends to be most significant in promoting accretion under natural conditions. Quite probably the first listed factor (large nuclei) frequently plays a decisive role in convective clouds formed in fresh maritime air masses, especially when abundant production of sea-spray has occurred upwind of the area in question. Woodcock [go] and Ludlam [41] have considered the role of comparatively rare (order of one per liter) giant sea-salt nuclei

PHYSICS OF CLOUD MODIFICATION

283

in initiating accretion, and observation [15, 90, 921 of the concentrations of these giant nuclei (equivalent dry radii of the order of 10 p ) tend to support the view that they often occur in numbers adequate to account for observed rain from nonfreezing clouds in maritime air masses. Too few measurements have yet been made throughout the world to make it clear that maritime air always has ample giant salt nuclei (see [91] for some recent aircraft observations); in fact, existing data make that seem doubtful, since rather strong winds appear to be required to form the larger nuclei in appreciable numbers. Available data indicate that the giant nuclei are only produced a t sea-surface wind speeds in excess of about 10 m sec-1, Turning to the case of continental interiors, one finds the picture even less clear. Woodcock found no appreciable reduction in total numbers of nuclei after passage of about 100 km over land. Twomey [92] found quite variable concentrations of large salt nuclei on flights extending for hundreds of kilometers across southeastern Australia, with low counts downwind of convective clouds and under post-frontal inversions. Mere length of overland air trajectory did not seem to be closely correlated with nuclei counts. Twomey noted that there were occasional situations, chiefly in summer, when high vapor contents occurred simultaneously with low counts of the oversize nuclei needed to start accretion processes. Those situations might, he felt, be susceptible to useful modification by seeding with water drops or large salt particles. Reitan and Braham [93] made measurements of large nuclei a t the ground in Illinois and found surface concentrations ol large particles too low to initiate accretion, but later aircraft measurements in the same area seem to suggest that surface nuclei counts are unrepresentatively low, a point that deserves consideration in evaluating all surface nuclei observations. One other set of salt-nuclei counts will be cited to indicate the type of data a t hand. Measurements made in the Punjab in northwestern India [94], a t a distance of about 1000 km from the Arabian Sea, showed that during the summer monsoon season, only about 5 to 10 particles per cubic meter with masses greater than about gm occured in surface air, rather too few to serve in starting the accretion process. I n general, it must be recognized that far too few observational data on continental, and even maritime, counts of giant sea-salt particles have been gathered, and it is to be hoped that more such data will be obtained soon. Despite lack of positive evidence that significant deficiencies of giant nuclei occur, speculation and investigation concerning the feasibility of stimulating the collision-and-coalescence process have proceeded. Two chief modification methods have been proposed: seeding with a spray of large water drops released from aircraft, or seeding the updraft air of convective clouds with dry NaCl particles large enough (masses of order of gm)

284

JAMES E. MCDONALD

to grow into oversize cloud drops quickly enough to start accret,ion ahead of the sometimes too-slow natural processes. 4.3.1. The Water-Spray Technique.In his 1948 paper on collisions1growth, Langmuir 1361 briefly suggested that useful stimulation of drop growth might be obtained by spraying water from an airplane by use of suitable nozzle arrangements. In a 1950 report [95], Langmuir proposed that water seeding might best be accomplished by dropping from an aircraft a quantity of the order of a gallon of water in a balloon attached to a string some 100 ft in length. As such a balloonful of water fell, it would lose much of its initial forward momentum so that on breaking open, after coming to the end of the string, the atomization due to first contact with the air would not break the water into drops too small to start what Langmuir anticipated might be a chain-reaction process (accretional growth followed by raindrop breakup and then more accretional growth). Later experience has made it virtually certain that a mere gallon of water could not significantly affect cloud precipitation processes but variants of this basic idea have produced observable cloud effects. In 1948, Coons et al. [96] carried out some water-seeding trials on cumuliform clouds, using two spraying methods. A P-61 aircraft with two 165-gal wing tanks emptied its water load in about 2 min through 2-in. diam solenoid-operated valves. At an airspeed held down to 150 mph to minimize impact breakup, and with both valves open, this arrangement gave a seeding rate of about 50 gal per flight mile. An alternative scheme utilized a sprayer, mounted in the tail of a B-17, through which water could be pumped a t a rate of about 1 gal/mi. The investigators estimated that the latter technique gave drops of about 0.5 mm diam, while the former method was thought to yield rather larger drops. This experiment is cited primarily to illustrate water dispersal techniques and because it appears to have been the first serious effort to carry out water seeding. The mode of evaluation of results in this experiment made it difficult to assign definite value to the water seeding, and a total of only eight clouds were seeded a t the 50gal/mi rate during the tests. Bowen [97], in 1952, deduced from his previous calculations [40] concerning the collision and coalescence type of liquid accretion process that if drops with radii of about 25 1.1 could be sprayed into the bases of suitable convective clouds, rain should fall in a few tens of minutes. An aircraft was fitted with a 60-gal water supply tank and two spray-bars to permit release of water drops along a strip of air some 6 mi long, thus yielding a seeding rate of about 10 gal per flight mile. No experimental determination of drop size from the sprayer was made, but Bowen suggested that a mean drop-radius of about 25 1.1 was obtained, the range of sizes being broad. In eleven experimental spraying operations on cumuli, Bowen found

PHYSICS OF CLOUD MODIFICATION

285

four cases where rain or hail appeared shortly after spraying, four where only precipitation streaks (virgae) appeared, and only one of the eleven cases wherein no observable results of any kind could be discerned. These results seemed suggestive of real effects, but it deserves strong emphasis that such seeding designs where the clouds to be treated are selected subjectively without randomization render virtually impossible any quantitative applications of probability arguments in the evaluation of the efficacy of treatment. This difficulty will be discussed somewhat more fully in Section 5. The most extensive experimental work on water seeding thus far performed has been done by Braham, Battan, and Byers [58], in 1954 in the vicinity of Puerto Rico and in the midwestern United States. A 400-gal water tank installed in the bomb-bay of a B-17 was equipped with a dump valve through which the water could be rapidly released through the bottom of the fuselage into the ambient air. When a circular valve 4 in. in diameter was employed, the 400 gal emptied in about 70 sec, giving a seeding rate of about 130 gal per flight mile at an airspeed of about 180 mph. The randomized-pairs method of seeding (described in Section 4.2.1) was used in all of these water-seeding trials. After treating over 25 pairs in the Puerto Rico area with the 4-in. valve, it appeared that no significant effects were being obtained with the seeding rate it gave, 130 gal/mi, and visual observations by flight observers tended to the same conclusion. Hence the small valve was replaced by a large dump valve, approximately 10 in?, which would release the 400 gal in only about 18 sec, thus giving a linear seeding rate of about 450 gal/mi a t cruising speed of the B-17. To determine the drop-size distribution produced by this massive dumping method, releases were made with the B-17 flying only a few tens of feet above an airport runway on which dye-impregnated filter papers had been laid out over a large area. The results showed that about 10% of all drops had radii under 150 p, about 50 % of all had radii under 250 p , and about 90% of all had radii under about 500 p. That is, the bulk of the drops sprayed into the clouds by the large valve were equivalent in size to typical drizzle drops. The question of how the spray output was distributed in a direction crosswise to the flight axis was of interest in understanding the seeding process, and efforts were made to determine this in approximate fashion by low-altitude drops over other arrays of dyed filter paper. In one test with the large valve flown 50 ft above the ground, the results showed that 90 % of all of the water released was confined t o a strip of 50 ft in width. Since downwash and tip-vortex effects are probably chiefly responsible for the lateral dispersion prior to control by purely cloudturbulent effects, and since 50 ft of drop should, for a B-17, have given nearly full opportunity for these aerodynamic effects to act, these measure-

286

JAMES E. MCDONALD

ments are probably typical of initial lateral dispersion in actual cloud treatment. Radar tests were made to determine whether the cloud of drops emanating from the valve could be confused with cloud precipitation echoes themselves. It was not found possible to observe just the water plume itself with the radar set subsequently used in evaluating treatment efficacy. Whereas Bowen [97] introduced his water spray into the bases of treated clouds, the Chicago group concluded, from theoretical considerations involving their measured drop-size distributions, that seeding near the cloud tops should be carried out in their flights. I n the tradewind cumuli near Puerto Rico, where cloud bases lie near 2000 ft and tops typically range from 6000 to 10,000 ft altitude, the seeding runs were flown at from 5000 to 7000 ft altitude. In the midwestern summer cumuli of the United States, whose bases ranged from 5000 to 8000 ft and tops from 12,000 to about 20,000, the seedings were usually done in the layer from 10,000 to 15,000 ft altitude. No observations of precipitation actually reaching the ground could be made under the experimental conditions of the trials in either geographical areas. Criterion of treatment effect was therefore necessarily limited to appearance or nonappearance of a radar echo in the two members of each pair. Using this radar-echo criterion, the Chicago group found that no significant effect was produced with either the small or large valve in seeding the cumuli of the Midwest, but only a small number of treatments mas carried out, so those results are not conclusive for the Midwest. Using the small valve, they found no significant effect in tradewind cumuli near Puerto Rico, as stated earlier. But using the large valve (seeding rate 450 gal/mi) in Puerto Rico, they did find statistically significant evidence that water seeding was increasing the fraction of clouds that reached the stage of precipitation (formation of drops large enough to yield a radar echo). Specifically, statistical tests call for the rejection, at the 0.017-confidence level, of the hypothesis that seeding had no effect on the probability of precipitation when water was seeded at the 450-gal/mi rate into the trade cumuli. The seeding effects are somewhat more simply represented in terms of fractions of treated and untreated clouds that developed echoes, and this information is contained in Table VIII. An average of 48 % of the trade cumuli seeded a t 450 gal/mi gave echoes, while only 23% of untreated members of randomized pairs gave echoes, or less than half as many as in the treated population. The 95% confidence intervals for these two proportions barely overlap. Additional physical analysis of these experiments [98] lent further strength to these statistical conclusions that a real effect was produced by water seeding a t the heavier rate. The fact that no observable effects on stimulation of growth of drops to sizes yielding a radar echo were found in the Midwest a t even the heavier

287

PHYSICS O F CLOUD MODIFICATION

TABLE VIII. Observed proportion of spray-treated and untreated tradewind cumuli which developed radar echoes and the 95 and 99% confidence intervals for these proportions. (From [58].) Proportion of clouds with echoes Confidence intervals Observed Untreated Treated a t rate of 130 gal/mi Treated at rate of 450 gal/mi

0.23 0.19 0.48

95%

99%

0.144.34 0.07-0.36 0.334.63

0.12-0.37 0.05-0.42 0.31-0.69

seeding rate, and that a significant effect was obtainable in the Caribbean cumuli with the 450-gal/mi rate and not with the 130-gal/mi rate, raises questions concerning the earlier attempts of Coons et al. [96] whose seeding rate was either 1 or 50 gal/mi depending on which scheme was used, and the experiments carried out by Bowen [97] with a seeding rate of only about 10 gallmi. However, the latter is not directly comparable with the Chicago seeding of large drops in cloud tops for Bowen’s method involved seeding with smaller drops in cloud bases. Here, as in so many other places in the field of cloud modification, seemingly contradictory results cannot yet be adequately resolved for sheer lack of sufficient data. From the data a t hand, this method of spraying very large drops into the tops of convective clouds appears a t present to be too costly to warrant large-scale application. However, since positively identified effects have at, least been obtained by this method, since there may still be many unexplored possibilities for improving this particular technique, and since there may be better prospects in Rowen’s less costly approach of spraying much smaller drops (though still large compared with cloud drops) into the bases of convective clouds, the water-seeding methods clearly warrant more careful investigation on a research basis. At this point, it should be almost unnecessary to comment on the indispensable role which fundamental knowledge concerning the entire accretion process must play in efforts to optimize this kind of cloud treatment. 4.3.2. The Salt-Seeding Technique. I n Sections 3.2, 3.3.3, and 4.3, it has been pointed out that there may sometimes occur situations wherein addition of very large salt particles to natural nuclei populations entering cloud bases might accelerate t,he growth processes dependent upon accretion. There has been only a small number of attempts to test this scheme. Schaefer has reported [99] attempts to develop means of generating large numbers of particles of NaCl. A simple coke-burning furnace was built to melt and vaporize NaCl in a way that formed about loll particles per second with radii of the order of 1 p . The physics of the evaporation-

288

JAMES E. MCDONALD

condensation technique of generating such particles limits the upper size readily obtainable, and for the particles with radii of the order of 10 p or larger needed to stimulate accretion, Schaefer recommended some form of mechanical pulverization. No seeding experiments using NaCl or other hygroscopic salt were described by Schaefer. In 1954, Davies [loo] carried out experiments in East Africa in which finely ground salt was carried into clouds by means of balloons which were exploded by gunpowder a t predetermined altitudes, showering the salt particles into the cloud. Of 64 cumuli so seeded, some form of precipitation was observed in 47 cases within a time lapse of from 7 to 35 min (average 24 min) after treatment. Of these 47 cases, 37 gave precipitation that actually reached the ground. These are statistics typical of those so frequently encountered in the field of cloud modification where the results seem tantalizing, but where lack of randomization stands firmly in the way of drawing any meaningful conclusions. The cumuli of the summer monsoon of India have very warm base temperatures and often precipitate before their tops reach the rather high freezing level of that area and season, implying that collision and coalescence processes must be dominant in natural precipitation in such clouds. After making two years of counts of hygroscopic nuclei in Pakistan, and finding that the average surface-air content of giant nuclei with masses greater than gm was about 5 to 10 particles per cubic meter (too few to be the chief agent in starting accretional growth, unless a process akin to Langmuir’s chain-reaction process can occur), Fournier d’Albe et al. [94], set up a program of experimental salt seeding in the Punjab. Salt was ground to desired sizes, stored in sealed tins, and dispersed into the air from a hand-operated centrifugal hot-air blower of economical design. Seeding was restricted to days when the surface monsoonal flow from the east was present, and this occurred on 39 days during the two-month period in 1954 when operations were underway. The results of an analysis of rainfall in the area felt to be chiefly influenced by seeding, as contrasted with rainfall in adjoining control areas, indicated that the seeded area received more rainfall than could be expected on the basis of the concurrent amounth in the control areas. However, it was not possible to conclude with assuranre that this was due to the seeding itself, for the same reasons that have arisen in most seeding projects involving only the target-and-control comparison scheme, namely, that this scheme does not safely overcome the subtle but all-important effects of natural rainfall variability. Although a few other attempts a t salt seeding have been made, these two are the only ones of which I now know wherein even approximate deductions as to seeding effect can be made. They appear to be favorable results but neither is conclusive, because of limitations in statistical design.

PHYSICS OF CLOUD MODIFICATION

289

They scarcely exhaust the possibilities for research on this problem. Until better techniques for counting salt particles have been developed and widely used to gather now-rare data on averages and fluctuations in natural giantnuclei counts in time and space especially in the free atmosphere rather than just at the surface, and until the relative importance of intrinsic coalescence processes as compared with effects of rare giant nuclei is ascertained, salt seeding will be pursued in about as dim a light as have many other seeding efforts. The alternative to hard-won physical insight is a sustained program of properly randomized seeding trials from which reliable deductions may ultimately be drawn solely from observed precipitation amounts.

4.4 Other Types of Cloud-Modification Techniques All of the foregoing remarks have been concerned with efforts to induce more precipitation to fall from clouds by various artifices. There have been other objectives recognized in cloud-modification studies, including dissipation of cloud decks for aviation purposes, suppression of hail to reduce crop damage, suppression of thunderstorm electrical activity to prevent lightning fires over forested areas, and some thought has even been given to attempts at increasing cloud coverage indirectly in order to control solar radiation receipts at the earth’s surface. None of these objectives has received as much research attention as that of stimulating precipitation, so only a few comments will be made on these other techniques. Aufm Kampe, et al. [101, lola] have seeded numerous stratus decks with both dry ice and silver iodide released from aircraft. Conversion of a subcooled cloud of high visual opacity to one composed of a much smaller number of large ice crystals quickly renders the ground visible from points above the deck, even when the latter is as much as 2000 ft thick, according to their reports. Similar effects have been found by earlier workers, and these results constitute particularly clear-cut evidence that artificial treatment can most assuredly induce the Bergeron transition in subcooled clouds. Aufm Kampe el al. have reported that dry ice is generally more effective than AgI in stratus-clearing, and that seeding rates of as much as 10 lb of dry ice per mile will not lead to overseeding in convective clouds (though overseeding in stratus-clearing has, of course, somewhat different meaning than in rain stimulation). Their experience indicates that normally present cloud turbulence diffuses the ice particles laterally at such a rate that the seeded strip grows to a width of two miles in about 30 min. Suppression of hail and suppression of lightning are still only superficially understood [102, 1051. In each instance, the goal is that of so overseeding that the available subcooled water is converted into too many small ice particles to support hail and lightning development. Since hail

290

JAMES E. MCDONALD

damage is often a problem in areas where additional precipitation is tmicficial, these hail-suppression efforts may work at cross-purposes with w:i ter requirements, and the same can he true in lightning suppression. I’reseiit knowledge does not enable a seeder to determine very precisely just what manipulations of the natural clouds will yield hail- and lightning-suppression without net sacrifice of precipitation. Whether or not future developments may make these and other miscellaneous modification techniques significant is impossible to predict. What can be stated with confidence is that here, as in all other applications, we see a pressing need for much more basic information as to physical processes in natural clouds.

5. THEEVALUATION OF MODIFICATION EXPERIMENTS The problem of cloud modification to stimulate precipitation seems to have been a n instructive one for meteorologists as a group in areas other than just physical meteorology. No other important meteorological research problem, other than a few on merely a laboratory scale, now has or ever before had, as an essential part of its rationale, the manipulation of natural processes as distinguished from the passive observation of those processes. In some experimental fields it is quite simple to apply treatment A and then to determine unambiguously whether effect X is or is not produced as a result of A . In other fields no such straightforward approach can be utilized because so many uncontrollable factors other than A operat,e concurrently to cause or to inhibit X that extreme danger of falling in to the post hoc fallacy arises. Cloud modification is very much a field of this latter type. I n the first few years of modification efforts there was a disconcertingly large amount of post hoc reasoning used. The admonitions of a relatively small number of statisticians seem to have been chiefly responsible for correcting, only a few years ago, the most extreme shortcomings of statistical logic applied to modification studies, though these shortcomings were certainly appreciated by many meteorologists a t a much earlier date. Although the subject of evaluation of cloud-seeding programs is largely outside the scope of this article, it is desirable to discuss evaluation difficulties just enough to see the important role they have played in the history of the past decade of modification efforts. Evaluation subtleties arise from the large inherent variability of natural precipitation, and from the meteorologists’ present inability to predict cloud behavior with high enough accuracy to state what would have occurred in the absence of treatment. These circumstances have thrown the problem into the area of statistical inference, and there, broadly speaking, it must lie until substantial gains in physical meteorology have been achieved. There is a bewildering degree of variability in condensation and precipitation phenomena, both in the small- and large-scale processes. At any

29 1

PHYSICS OF CLOUD MODIFICATION

instant on, say, a summer day, cumuli in different parts of the sky over a level plain may be evolving in strikingly different ways. In mountainous country, seemingly similar peaks only a few miles apart may, for subtle dynamic reasons, be causing updrafts of such different nature that one summit will be cloud-capped while the other is clear, and a short time later the situation may be reversed. One day’s cloud history is often wholly dissimilar to the next. One summer’s rain patterns in a given area may be different from those of the preceding summer to an extent not yet explicable in terms of any familiar principles of synoptic climatology. As one deals with increasingly longer time periods, this variability is suppressed more and more for simple probability reasons, but even when one considers precipitation totals for time periods of as long as one year, the historic variability is still uncomfortably large. To display this characteristic, for the case of United States stations, Table IX has been prepared. The annual coefficient of variation (standard deviation divided by mean) of even the least variable of the stations listed there, Iowa City, Iowa, is seen to amount to 0.14. This statistic grows generally larger as one turns to more arid regions. The same figure for Yuma, Arizona, located in the driest section of the Unit’ed States, is 0.62. Since cloud-seeding activities are perhaps most pressingly needed in the arid regions of the world, one recognizes an inherent bias in the direction of having the greatest background of meteorological “noise” in the precipitation record of those very stations most likely to be encountered in evaluation of seeding experiments. TABLEIX. Natural variability of rainfall.* (From [104].) Station Iowa City, Ia. Boston, Mass. Cleveland, Ohio Kansas City, Kans Portland, Ore. Bismarck, N. Dak. Cheyenne, Wyo. Ogden, Utah Wichita, Kans. Sheridan, Wyo. Tucson, Ariz. Phoenix, Ariz. San Diego, Calif. Yuma, Ariz.

Years of record

Mean annual precipitation (inches)

Coefficient of variation

70 60 60 63 60 66 70 60 52 47 87 75 60 83

35.2 41.0 34.0 36.1 42.2 16.3 14.6 16.2 29.2 15.0 11.2 7.6 9.9 3.2

0.14 0.16 0.16 0.18 0.19 0.25 0.25 0.25 0.26 0.27 0.30 0.40 0.44 0.62

* The table gives length of record, annual mean precipitation, and coefficient of variation of annual precipitation for each station.

292

JAMES E. MCDONALD

The fact that root-mean-square deviations of totals for 12-month periods can, as a consequence of purely natural fluctuations, range as high as 60 % of the corresponding average values leads to evident difficulty in reliably identifying seeding increases of the order of only 10 % in programs carried on for only a few seasons. In modification work, one always confronts the thorny problem of disentangling natural variability from true seeding effects. One central question is always lurking in the background: I s i t probable that the precipitation patterns that appeared immediately following seeding would have appeared even i f no seeding had been carried on? 5.1. Physical Evaluation

If meteorologists only knew so thoroughly all of the intricacies of cloud physics that a given cloud could be examined just prior to cloud-modification treatment to ascertain what its natural evolution was going to be, treatment could then be applied and the actual outcome contrasted with the predicted behavior. During the past dozen years, there has always been a certain segment of meteorological opinion which tacitly refused to admit the present impossibility of this approach. That is, many have insisted that one could “see” the effects of seeding, simply through observing individual cases. These and other workers, of course, may indeed have “seen” true seeding effects, but the difficulty of proving this to the satisfaction of anyone fully aware of present ignorance of cloud dynamics and cloud microphysics is not to be underestimated. Several ways of attempting more nearly physical than purely statistical evaluations have been tried ; and often physical and statistical techniques have been fused, as in the University of Chicago dry-ice and water-seeding experiments where appearance or nonappearance of radar echoes became the datum fed into the statistical machinery. There is so much room for improvement in physical evaluation methods that it seem5 almost irrelevant to describe existing attempts. However, every time radar, photographic, or aircraft observations of clouds can be obtained before, during, and after seeding, a step in the direction of physical evaluation is being taken. The in-cloud observations by research aircraft are potentially most informative, for in this way, drop-sizes, liquid water contents, updraft strengths, presence or absence of the ice phase and of hail or graupel particles, temperatures, and other very pertinent quantities can be obtained. In fact, however, not only is it difficult and expensive to operate suitable planes in this way, but also the whole area of flight instrumentation for cloud physics observat‘ions is sorely in need of far more attention than it has yet received. It does not seem an overestimate of the present state of basic cloud physics theory to suggest that if truly complete physical observational data on a treated cloud could be turned over to the theoretician, he would,

PHYSICS OF CLOUD MODIFICATION

293

hy dint of sufficient calculation based on basic physical principles, be able to determine conclusively whether seeding was or was not significant ill its effects. This emphasizes the need for striving for all possible physical control in future seeding experiments for, even if the above-mentioned ideal of complete data-coverage is still far from attainable, exact physical measurements of even portions of the problem greatly reduce the statistical burden of evaluation. 5.2. Statistical Evaluation

In one way or another, every seeding experiment is subjected to some kind of statistical evaluation, even if only vaguely in the mind of the empiricist who has scattered dry ice into a cloud and notes what he feels to be an evolution not customary in other clouds he has previously observed. From this implicit and qualitative form of “statistical” evaluation (which was not infrequently the only kind of judgment made in some of the seeding trials of a decade ago), cloud-modification evaluation methods have progressed steadily toward more and more sophisticated schemes as one or another program has come under the scrutiny of meteorologists and statisticians who were distressed a t early evaluat,ion approaches. Because the subject is technically involved and basically lies outside the scope of this discussion, I shall not try to summarize more than the major outlines of the evolution of evaluation methodology. But to attempt to do at least this much here seems highly desirable, for one cannot understand the development of the field of cloud modification without appreciating the influence which statistical evaluation difficulties have exerted on the whole subject. Because silver iodide seeding has been carried on more extensively than any other type of seeding, the following is to be assumed to apply to that kind of operation unless otherwise specified. One of the first evaluation refinements to be introduced, beyond that oi noting whether or not rain fell soon after seeding in a given area, was to compare the total rainfall measured at the ground in what was called (often on doubtful grounds) the target area with the normal rainfall for that same area as deduced from climatological records. This so-called ‘lper cent of normal method” took as its criterion of success the appearance of per cents of normal in excess of 100%. But often the seeded time periods were only of the order of a month and the data in Table IX, plus standard probability arguments, show that even with times of the order of a year, one could not incontrovertibly assert that anything but a very large per cent of normal could not have been due to chance meteorological fluctuations. After a time, the per cent of normal method was abandoned by scientifically minded evaluators. An obvious improvement lay in examining the per cent of normal in

294

JAMES E. MCDONALD

the “target area” with the same quantity in one or more “control areas” which were selected so that they lay as near as possible to the target arei without, however, lying in the prevailing direction of flow of seeding agent from the target. At this stage of evaluation evolution, appearance of n larger per cent of normal in the target area than in one or niore control areas was offered as evidence of seeding efficacy. This was becoming more convincing, but operators who used this method had then to answer the following question. How often, in the historic past, might even larger differences in per cent of normal between these two geographic areas (the seeder’s target and control areas) have occurred as a consequence of natural processes? Self-evident as such a question should seem it is still not always asked, or if asked, it is not always satisfactorily answered. The question stated near the end of the preceding paragraph led next to resort to regression methods. The historic (preseeding) rainfall records of all available stations in the target area were correlated against contemporaneous records for stations in the several control areas, and the standard error of estimate (or other equivalent statistic) was then computed for the regression of target on control area amounts. The time period involved might be units of a year, a season, a month, or even individual storm periods. By plotting on a regression diagram the point representing simultaneous values of target and control for the current (seeded) period, and computing the residual deviation of the target ordinate above (if an apparent increase had occurred) the regression-line ordinate corresponding to the given period’s control-area precipitation, a probability statement could be formulated expressing the likelihood that the observed target excess over regression prediction could have arisen solely as a result of random fluctuations measured by the yardstick of the historic standard error of estimate. This regression method was a very real improvement over earlier methods, and, despite weaknesses, it would be capable of discerning real effects if seeding could be conducted for a long enough period of time and if enough historic data were available for use in calculating the basic regression relations. However, statisticians soon pointed out that this regression method may contain pitfalls, too. One difficulty fairly easily suppressed hinges upon the concept of heteroscedasticity, that is the property of a bivariate distribution by virtue of which the variance of the dependent variate is not the same for all intervals of the independent variate. In some geographical areas, the precipitation heteroscedasticity occurs in the sense that the dispersion is greatest for high values of the control-area precipitation. If, then, a seeded year is one in which generally heavy precipitation occurs, use of the ordinary standard error of estimate will lead to bias in the direction of regarding as significant a positive deviation from regression

PHYSICS OF CLOUD MODIFICATION

296

that is not really significant. To correct this error, various coordinate transformations such as the square-root, cube-root, incomplete-gamma [105], and logarithmic transformations may be used, though all must be recognized to be only empirical adjustments a t the present time. But still more fundamental objection is raised on the ground that any kind of historic regression method may, when applied to a seeding project carried on for only a few years, give quite misleading evaluation results because of the varying proportions of differing “storm types” that may influence the experimental area. Imagine that two distinctly different types of rain-bringing storms tend to affect the vicinity of a given cloud-seedingproject area during the season in question. Suppose, further, that some years tend to be characterized predominantly by Type A , other chiefly by Type B, for reasons of oscillations in the general circulation. Finally, let Type A storms have such characteristics that they tend to deposit relatively large amounts of rain on the target area but little on the seeder’s control area, while Type B storms have circulation characteristics such that they tend to distribute rain in just the opposite manner. Then, if the seeder unwittingly prepares his regression relation from a limited amount of available data that came from a period of years when it happened that Type A predominated, and if a secular trend brings, a t about the time seeding begins, a shift to relatively more of Type B storms, the seeder will be misled by his regression analyses into concluding that his efforts are less effective than they may actually be. Conversely, if he should happen to base his historic regression relation on data from a period of years when Type B predominated and the seeding period begins after a swing to relatively more storms of Type A , his regression methods will incorrectly lead him to believe that his seeding is producing significant results when it really may be doing nothing at all, or might even be yielding decreases over what would have occurred in the absence of seeding. In short, the hypothesis of storm-types confronts regression methods with a very basic uncertainty. The first discussion of storm types in this particular context of seeding evaluation was present by Jeeves et al. [lo61 in 1953. The reality of storm types is not in doubt; but the importance of their quantitative effects on evaluation statistics has not been settled [105]. (Note that here is a problem lying basically in the field of synoptic climatology which, like so many in cloud physics proper, could not be decisively answered on the basis of existing information when it was encountered in the course of work in cloud modification. Many others might be cited in this same category.) The statisticians, disturbed with the above shortcomings of even the regression methods, are satisfied onIy with seeding designs wherein one or another acceptable form of randomization is incorporated into the

296

JAMES E. MCDONALD

experiment from the start of operations. Details are not pertinent here, but the basic point is that the seeder must do something equivalent to first selecting the cloud or time interval as “seedable,” and after announcing his decision, he must then effectively flip a coin (randomize) to decide whether he will or will not actually seed that “seedable” situation. After many repetitions of this process, two sets of results will have been obtained, those for the seedable-and-seededand those for the seedable-but-not-seeded cases. These data, processed in any of a number of ways, fulfill the philosophic requirements posed by sampling theory and enable the statistician to define confidence limits or significance limits for the observed effects of treatment (whatever the measure of seeding efficacy might be in a given case). Whereas several aircraft seeding investigations had been started as much as five years ago with randomized designs, it was not until 1957 that any ground-generator silver iodide seeding project was set up in this fashion in the United States. The latter is now underway near Santa Barbara, California, and will require a t least three winter seasons of operation before meaningful results will be forthcoming. The detailed design of this experiment is somewhat too involved to be summarized here. A brief account of some aspects of this undertaking has been published [107]. Randomization can assume many possible forms in seeding trials, but in all cases its functionissimple: It greatly reduces (and in theory eliminates) bias entering into the selection of those cases that go, respectively, into the seeded and the unseeded populations between which an evaluative comparison is ultimately to be made. In the absence of randomization, hidden bias of many kinds may enter, sometimes being forced into the design unwittingly by imposition of other criteria. To give a single example, decision to count only treated clouds which last for, say, 30 min following treatment may be made in what seems the sensible effort to avoid diluting the results with clouds which at time of seeding were so near dissolution that they are not fair tests of seeding efficacy. But the lifetimes of all clouds are so short, and the extent to which wholly natural drop-growth processes may succeed in approaching a state of precipitation is so dependent upon cloud lifetime, that such a criterion may inherently bias the selection of ultimately analyzed data quite strongly in the direction of counting only clouds that were going to precipitate regardless of seeding. This single example suffices to document the fact that the real problem throughout evaluation is ignorance of cloud physical details. Why did randomization not appear in evaluation work a dozen years ago? Partly, I feel sure, because there was nothing in the background of earlier meteorological practice comparable to this problem of detecting effects of treatments of complex natural phenomena. And, of course, even

PHYSICS OF CLOUD MODIFICATION

297

in fields where such problems are highly typical, as for example in agricultural research, it has not been more than about two or three decades since these and related ideas have infused standard practice. But, in addition, an obstacle to randomization in those numerous projects where some water using group was retaining the services of a commercial seeder lay in the reluctance of the client to pay for a program in which roughly half of all seedable storms must be allowed to pass by unseeded. Perhaps, viewed in this light, a decade of postponement of randomized projects in the hope that an answer would be otherwise forthcoming was only about what one could expect from groups desperate for more water. The consequence has been, however, that much effort has gone into contract seeding without past or present knowledge of whether the clients did or did not receive any extra precipitation. It is to be hoped that increasing numbers of contractseeding projects in this country will be set up on a randomized basis, now that one such (Santa Barbara County, California) project has been established. Quick and easy answers cannot be expected; in adopting this resort, we pay with time for information which we cannot now obtain immediately for the reason that fundamental physical research on clouds and precipitation has progressed too short a distance into its complex subject. 6 . CONCLUDING REMARKS

It would, of course, be unreasonable to expect that a mere dozen years of research should have led to final answers in a problem with as many ramifications as have come to be recognized in the physics of cloud modification. In retrospect, one sees that this complexity was not fully appreciated when present seeding techniques were introduced in 1946-1947. From the viewpoint of pure research, one of the great benefits of the recent interest in cloud seeding has been its stimulation of effort to unravel this web of interacting factors that enter into the precipitation problem. I believe that most persons in close contact with current research in cloud physics feel that the surface of this field has barely been scratched. It will have been noted by the reader that in the discussions of each of the four main seeding techniques, those involving dry ice, silver iodide, water spray, and giant salt nuclei, it has been necessary to admit that key aspects of the problem which might hold very real promise have simply not been adequately examined up to the present time. Also, even the most sceptical of those who reject as valid evidence the occasional reports of apparently very marked changes in cloud behavior after seeding has been performed must admit that those statistically insignificant cases may have been instances where, though in ignorance of the detailed reasons, the seeder, quite by chance, carried out his operations in what was exactly the optimal procedure. It is this aspect of cloud modification, the prospects of advances

298

JAMES E. MCDONALD

that may await studies not yet begun, rather than the often indecisive results of work already completed, that I believe deserves principal emphasis in summarizing the present position of the field. In short, the great need is for much more intensive research effort on all fronts, for the importance of water in our economy is mounting so rapidly that no prospects for augmenting our limited supplies can be overlooked. ACKNOWLEDGMENT 1 am indebted to Dr. Louis J. Battan for critically reading this manuscript and for providing many helpful suggestions.

REFERENCES 1. Hutchinson, G. E. (1954). The biochemistry of the terrestrial atmosphere. I n “The Earth as a Planet” (G. P. Kuiper, ed.), 751 pp. Univ. Chicago Press, Chicago, Illinois. 2. Fox, C. S. (1952). “Water,” 148 pp. Philosophical Library, New York. 3. Sutcliffe, R. C. (1956). Water balance and the general circulation of the a t mosphere. Quart. J . Roy. Meteorol. SOC.82, 385-395. 4. Braham, R. R . , Jr. (1952). The water and energy budgets of the thunderstorm and their relation to thunderstorm development. J . Meteorol. 9, 227-242. 5. Huff, F. A. and Stout, G. E. (1951). A preliminary study of atmospheric moisture-precipitation relationships over Illinois. Bull. Am. Meteorol. SOC.S2, 295297. 6. Reitan, C. H. (1957). The role of precipitable water vapor in Arizona’s summer rains. Tech. Repts. on Meteorol. and Climatol. Arid Regions, N o . 2, 18 pp. Inst. Atm. Physics, Univ. Arizona, Tucson, Arizona. 7. Los Alamos Scientific Laboratory (1950). “The Effects of Atomic Weapons,” 456 pp. U. S. Govt. Printing Office, Washington, I>. C. 7a. Mason, B. J. (1957). ”Cloud Physics,” 481 pp. Oxford Univ. Press, London and New York. 8. Houghton, H. G. (1950). A preliminary quantitative analysis of precipitation mechanisms. J . Meteorol. 7, 363-369. 9. Junge, C. E. (1958). Atmospheric chemistry. Adoances in Geophys. 4, 1-108. 10. Howell, W. D. (1949). The growth of cloud drops in uniformly cooled air. J. Meteorol. 6, 134-149. 11. Houghton, H. G. (1938). Problems connected with the condensation and precipitation processes in the atmosphere. Bull. Am. Meteorol. SOC.19, 152-159. 12. Kraus, E . B., and Smith, B. (1949). Theoretical aspects of cloud drop distributions. Australian J . Sci. Res. A2, 376-388. 13. Squires, P. (1952). The growth of cloud drops by condensation, I, 11. Australian J . Sci. Res. A6, 59-86, 473-499. 14. Junge, C. E. (1956). Recent investigations in air chemistry. Tellzcs 8, 127-139. 15. Woodcock, A. H. (1952). Atmospheric salt particles and raindrops. J . Meteorol. 9, 200-212. 16. Woodcock, A. H., and Blanchard, D. C. (1955). Tests of the salt-nuclei hypothesis of rain formation. Tellus 7, 437-448. 17. Krastanow, L. (1941). Beitrag eur Theorie der Tropfen- und Kristallbildung i n der Atmosphaere. Meteorol. 2. 68, 3745.

PHYSICS OF CLOUD MODIFICATION

299

18. Turnbull, D., and Fisher, J. C. (1949). Rate of nucleation in condensed systcms. J . Chem. Phys. 17, 71-73. 19. Mason, B. J. (1952). The spontaneous nucleation of supercooled water. Quart. J . Roy. Meteorol. SOC.78, 22-27. 20. McDonald, J. E. (1953). Homogeneous nucleation of supercooled water drops. J . Meteorol. 10, 416, 433. 21. Schaefer, V. J. (1946). The production of ice crystals in a cloud of supercooled water droplets. Science 104, 457459. 22. Bigg, E. K. (1953). The formation of atmospheric ice crystals by the freezing of droplets. Quart. J . Roy. Meteorol. SOC.79, 510-519. 23. Jacobi, W. (1955). Homogeneous nucleation in supercooled water. J . Meteorol. 12, 408-409. 24. Pound, G. M., Madonna, L. A., and Peake, S. L. (1953). Critical supercooling of pure water droplets by a new microscopic technique. J . Colloid Sei. 8, 187-193. 25. Fournier d’Albe, E. M. (1949). Some experiments on the condensation of water a t temperatures below 0°C. Quart. J . Roy. Meteorol. SOC.76, 1-14. 26. Schaefer, V. J. (1951). Snow and its relationship to experimental meteorology. I n “Compendium of Meteorology,” (T. F. Malone, ed.) 1334 pp. Am. Meteorol. SOC.,Boston, Massachusetts. 27. Isono, K. (1955). On ice-crystal nuclei and the other substances found in snow crystals. J . Meteorol. 12, 456462. 28. Kassander, A. R . , Jr., Sims, L. L., and McDonald, J. E. (1956). Observations of freezing nuclei over the southwestern United States. Sci. Rept. No. 3, 18 pp. Inst. Atm. Physics, Univ. Arizona, Tucson, Arizona. 29. Smith, E. J., and Heffernan, K. J. (1954). Airborne measurements of the concentration of natural and artificial freezing nuclei. Quart. J . Roy. Meteorol. Sac. 80, 182-197. 30. Murgatroyd, R. J., and Garrod, M. P. (1957). Some recent airborne measurements of freezing nuclei over southern England. Quart. J . R o y . Meteorol. Sac. 83, 528-533. 31. Findeisen, W. (1938). Die kolloidmeteorologischen Vorgange bei der Niederschlagsbildung. Meteorol. 2. 66, 121-133. 32. Simpson, G. C. (1941). On the formation of cloud and rain. Quart. J . Roy. Meteorol. SOC. 67, 99-133. 33. Schmauss, A., and Wiegand, A. (1929). “Die Atmosphare als Kolloid,” 74 pp. Vieweg, Braunschweig, Germany. 34. Humphreys, W. J. (1922). How fast can a droplet grow by falling in a cloud? Bull. Am. Meteorol. SOC.3, 59-60. 35. Milham, W. I. (1912). “Meteorology,” 549 pp. Macmillan, New York. 36. Langmuir, I. (1948). The production of rain by a chain reaction in cumulus clouds a t temperatures above freezing. J . Meteorol. 6, 175192. 37. Swinbank, W.C. (1947). Collision of cloud droplets. Nature 169, 849-850. 38. Blanchard, D. C . (1950). The behavior of water drops a t terminal velocity in air. Trans. Am. Geophys. U n . 31, 836-842. 39. Gunn, K . L. S., and Hitschfeld, W. (1951). A laboratory investigation of the coalescence between large and small water drops. J . Meteorol. 8, 7-16. 40. Bowen, E. G. (1950). The formation of rain by coalescence. Australian J . Sci. Res. A3, 193-213. 41. Ludlam, F. H. (1951). The production of showers by coalescence of cloud droplets. Quart. J . Roy. Meteorol. SOC.77, 402-417.

300

JAMES E. MCDONALD

42. Weickmann, H. K., and aufm Kampe, H. J. (1953). Physical properties of rumnlus clouds. J. Meteorol. 10, 204-211. 43. Marshall, J. S., Hitschfeld, W., and G u m , K. L. S. (1955). Advances in radar weather. Advances i n Geopkys. 1-56. 44. Reynolds, S. E., and Braham, R. R., Jr. (1952). Significance of the initial radar echo. Bull. Am. Meteorol. SOC.33, 123. 45. Battan, L. J. (1953). Observations on the formation and spread of precipitation in convective clouds. J . Meteorol. 10, 311-324. 46. Adderley, E. E. (1953). The growth of raindrops in cloud. Quart. J. Roy. Meteorol. SOC.79, 380-388. 47. Telford, J. W., Thorndike, N. S., and Bowen, E. G. (1953). The coalescence between small water drops. Quart. J. Roy. Meteorol. SOC.81, 241-250. 48. Telford, J. W. (1955). A new aspect of coalescence theory. J. Meteorol. 12, 436444. 49. Pearcey, T., and Hill, G. W. (1957). A theoretical estimate of the collection efficiencies of small droplets. Quart. J. Meteorol. SOC.83, 77-92. 50. East, T. W. R., and Marshall, J. S. (1954). Turbulence in clouds as a factor in precipitation. Quart. J . Roy. Meteorol. SOC.80,2647. 51. Arenberg, D. (1939). Turbulence as a major factor in the growth of cloud drops. Bull. Am. Meteorol. SOC.20, 444448. 52. East, T. W. R. (1957). An inherent precipitation mechanism in cumulus clouds. Quart. J . Roy. Meteorol. SOC.83, 61-76. 53. Houghton, H. G. (1951). On the physics of clouds and precipitation. In “Compendium of Meteorology” (T. F. Malone, ed.), 1334 pp. Am. Meteorol. SOC., Boston, Massachusetts. 54. Ludlam, F. H., (1948). The forms of ice-clouds. Quart. J. Roy. Meteorol. SOC.74, 39-56. 55. Smith, E. J. (1949). Experiments in seeding cumuliform cloud layers with dry ice. Australian J. Sci. Res. A2, 78-91. 56. Squires, P., and Smith, E. J. (1949). The artificial stimulation of precipitation by means of dry ice. Australian J . Sci. Res. A2, 232-245. 57. Bowen, E. G. (1952). Australian experiments on artificial stimulation of rainfall. Weather 7 , 204-209. 58. Braham, R. R., Jr., Battan, L. J . , and Byers, H. R. (1957). Artificial nucleation of cumulus clouds. In “Cloud and Weather Modification: A Group of Experiments,” Meteorol. Monographs, 11, 47-85. group of field experiments. Meteorol. Monogr. 2 (11). 59. Braham, R. R., Jr., and Sievers, J. R. (1957). Overseeding of cumulus clouds. I n “Artificial Stimulation of Rain.” Proc. Conf. Phys. of Cloud and Precipitation Particles, Woods Hole Oceanog. Inst., Pergamon, London, in press. 60. Vonnegut, B. (1947). The nucleation of ice formation by silver iodide. J. A p p l . Phys. 18, 593-595. 61. Vonnegut, B. (1949). Nucleation of supercooled water clouds by silver iodide smokes. Chem. Revs. 44, 277-289. 62. Smith, R. B., in ref. [62] and Boucher, R. J. (1956). Operation overseed. Final Rept. 116 pp. Advisory Comm. on Weather Control, December 1955-January 1956. Mt. Washington Obs., Milton, Massachusetts. 63. Warner, J., and Twomey, S. (1956). The use of silver iodide for seeding individual clouds. Tellus 8, 453459. 64. Reynolds, S. E., Hume, W., Vonnegut, B., and Schaefer, V. J. (1951). The elyect

PHYSICS OF CLOUD MODIFICATION

301

of sunlight on the action of silver iodide particles as sublimation nuclei. Bull. A m . Meteorol. SOC.32, 47. 65. Inn, E. C. Y. (1951). Photolytic inactivation of ice-forming silver iodide nuclei. Bull. Am. Meteorol. SOC. 32, 132-135. 66. Vonnegut, B., and Neubauer, R . (1951). Recent experiments on the effect of ultraviolet light on silver iodide nuclei. Bull. Am. Meteorol. SOC.32, 356. 67. Reynold¶, S. E., Hume, W., and McWhirter, M. (1952). Effects of sunlight and ammonia on the action of silver iodide particles as sublimation nuclei. Bull. A m . Meteorol. SOC.33, 26-31. 68. Birstein, S. J. (1952). The effect of relative humidity on the nucleatingproperties of photolysed silver iodide. Bull. Am. Meteorol. Soc. 33, 431-437. 69. Smith, E. J., Heffernan, K. J . , and Seely, B. K. (1955). The decay of ice-nucleating properties of silver iodide in the atmosphere. J . Meteorol. 12, 379-385. 70. Smith, E. J., and Heffernan, K. J. (1956). The decay of ice-nucleating properties of silver iodide released from a mountain top. Quart. J . Roy. Meteorol. SOC.82, 301-309. 71. Wyckoff, R. W. G. (1948). “Crystal Structures,” Vol. 1. Interscience, New York. (Looseleaf .) 72. Manson, J. E., (1955). X-ray diffraction study of silver iodide aerosols. J . Appl. Phys. 26, 423425. 73. Mason, B. J., and Hallett, J. (1956). Artificial ice-forming nuclei. Nature 177, 681-683. 74. Manson, J . E. (1957). Ice nucleation and the structure of nuclei crystal faces. In “Artificial Stimulation of Rain,” (H. K. Weickmann and W. E. Smith, eds.). 427 pp. Pergamon Press, London. 75. Pruppacher, H. R., and Sanger, R. (1955). Mechanismus der Vereisung unterkuhlter Wassertropfen durch disperse Keimsubstanzen. I, 11. 2.angew. Math. u . Phys. 6 , 407-416, 486-493. 76. Schaefer, V. J. (1954). Silver iodide and lead iodide as ice-crystal nuclei. J . Meteorol. 11, 417419. 77. Mason, B. J . , and Hallett, J. (1957). Ice-forming nuclei. Nature 179, 357-359. 78. Birstein, S. J. (1955). The role of adsorption in heterogeneous nucleation. I. Adsorption of water vapor on silver iodide and lead iodide. J . Meteorol. 12, 324331. 79. Birstein, S. J. (1956). The role of adsorption in heterogeneous nucleation. 11. The adsorption of water vapor on photolysed silver iodide. J . Meteorol. 13,395398. 80. Braham, R . R., Jr., Seely, B. K., and Crozier, W. D. (1952). A technique for tagging and tracing air parcels. Trans. Am. Geophys. Un. 33, 825-833. 81. Crozier, W. D., and Seely, B. K. (1954). Concentration distribution in aerosol plumes 3 t o 22 miles from a point source. Trans. A m . Geophys. Un. 36, 46-52. 82. Kassander, A. R., Jr. (1957). A study of the trajectories and diffusion patterns of ground-generated airborne particles under orographic wind flow conditions. Sci. Rept. N o . 6 , 16 pp. Inst. Atm. Physics, Univ. Arizona, Tucson, Arizona. 83. “Proceedings of the Conference on the Scientific Basis of Weather Modification,” April 10-12, 1956. Inst. Atm. Physics, Univ. Arizona, Tucson, Arizona. (Round mimeo abstracts.) 84. McDonald, J . E. (1956). Scientific, basis of weather modification. Science 124, 86-87. 85. Thom, H. (1957). An evaluation of a series of orographic cloud seeding opera-

302

JAMES E. MCDONALD

tions. Tech. Rept. N o . 2,78 pp. Advisory Comm. on Weather Control, Washington, D. C. 86. Bergeron, T . (1949). The problem of artificial control of rainfall on the globe. I, 11. Tellus 6 ( I ) , 32-33; 6 (3), 14-32. 87. Ludlam, F. (1955). Artificial snowfall from mountain clouds. Tellus 7, 277-290. 88. Kramer, H . P., and Rigby, M. (1950). Selective annotated bibliography on cloud physics and “rain making”. Met. Abstr. Bibl. 1, 174-205. 89. Thuronyi, G . (1955). Annotated bibliography on weather modification. Met. Abstr. Bibl. 6, 1433-1513. 90. Woodcock, A. H. (1950). Condensation nuclei and precipitation. J . Meteorol. 7, 161-162. 91. Lodge, J. P. (1955). A study of sea-salt particles over Puerto Rico. J . MeteoroE. 12, 493-499. 92. Twomey, S. (1955). The distribution of sea-salt nuclei in air over land. J . Meteorol. 12, 81-86. 93. Reitan, C. H., and Braham, R. R., J r . (1954). Observations of salt nuclei over the midwestern United States. J. Msteorol. 11, 503-506. 94. Fournier d’Albe, E. M., Lateef, A. M. A., Rasool, S. I., and Zaidi, I. H. (1955). The cloud-seeding trials in the central Punjab, July-September 1954. Quart. J . Roy. Meteorol. Soc. 81, 574-581. 95. Langmuir, I. (1950). Studies of tropical clouds. Occasional Rept. N o . 26, 17 pp. Project Cirrus, Contract No. W-36-039-SC-38141. General Electric Res. Lab., Schenectady, New York. 96. Coons, R . D., Jones, E . L., and Gunn, R. (1949). Second partial report on the artificial production of precipitation, cumuliform clouds, Ohio, 1948. U . S. Weather Bur. Res. P a p . N o . 31, 58 pp. Washington, D. C. 97. Bowen, E. G. (1952). A new method of stimulating convective clouds t o produce rain and hail. Quart. J . Roy. Meteorol. Soc. 78, 3745. 98. Braham, R . R . J r . , Battan, L. J . , and Byers, H. R . (1955). Artificial nucleation of cumulus clouds. Rept. N o . 24, 106 pp. Cloud Physics Project, Contract No. AF19(604)618. Dept. Meteorol., Univ. Chicago, Chicago, Illinois. 99. Schaefer, V. J. (1953). Laboratory, field, and flight experiments. Final Rept. Part I. Project Cirrus, Contract No. DA-36-039-SC-15345. General Electric Res. Lab., Schenectady, New York. 100. Davies, D. A. (1954). Experiments on artificial stimulation of rain in East Africa. Nature 174, 256-258. 101. aufm Kampe, H. J., Kelly, J . J., and Weickmann, H. K . (1956). Seeding experiments in supercooled stratus-type clouds. Paper presented at the Conference on the Scientific Basis of Weather Modification, Univ. Arizona, Tucson, Arizona, April 1Ck12, 1956. 101a. aufm Kampe, H. J., Kelly, J . J . , and Weickmann, H. K . (1957). Seeding experiments in subcoded stratus clouds. I n “Cloud and Weather Modification: A Group of Experiments,” Meteorol. Monographs, 11, 86-111. 102. Weickmann, H. (1957). Current understanding of the physical processes associated with cloud nucleation. Beit. Pkys. Atrn. 30, 97-118. 103. Thorn, H. (1957). A method for the evaluation of hail suppression. Tech. Rept. No. 4, Adv. Comm. for Weather Control, Washington, D. C., 42 pp. 104. McDonald, J. E. (1956). Variability of precipitation in an arid region: A survey of characteristics for Arizona. Tech. Repts. on Meteorol. Clinmtol. of Arid Regions, N o . 1, 88 pp. Inst. Atm. Physics, Univ. Arizona, Tucson, Arizona.

PHYSICS O F CLOUD MODIFICATION

303

105. Thom, H. (1957). A statistical method of evaluating augmentation of precipitation by cloud seeding. Tech. Rept. N o . 1, 62 pp. Advisory Comm. on Weather Control, Washington D. C. 106. Jeeves, T. A . , LeCam, L., Neyman, J . , and Scott, E. L. (1955). On the methodology of evaluating cloud seeding operations. Weather Modification Operations in California, Bull. 16, 271 pp. California State Water Resources Board, Sacramento, California. 107. Neyman, J., McDonald, J. E., Elliott, R. D., Reynolds, R. R., and Scott, E. L. (1957). Unusual opportunity for studies of atmospheric physics. Bitll. A m . Meteorol. SOC.38, 175-177.

This Page Intentionally Left Blank

AUTHOR INDEX Number in brackets are reference numbers and are included t o assist in locating references in which the authors' names are not mentioned in the text. Numbers in italics indicate the page on which the reference is listed.

A Abbes, H., 12, 14, 49 Aboud, A. A., 176(132), 182(132), 183,216 Adderley, E. E., 259, 300 Ainsworth, J., 161(35), 211 Airy, G. B., 124, 160 Alcaraz, A., 61(45), 77(45), 87 Allen, C. W., 173(112, 1131, 216 Anderson, J. Pamelia, 99(15), 114 Andrbe, S. A,, 14, 49 Andrews, F., 55, 86 Arenberg, D., 261, SO0 Argence, E., 208(253), 921 Arnold, K., 112, 116 Aschenbrand, L., 192, 200, 207(248, %9), 218, 219, 221 Astoin, N., 185(170), 190, 191(186), 194, 197(202, 203), 199, 300, 217, 218, 219 aufm Kampe, H. J., 258, 289, 300,302

B Baer, P., 191, 194, 218 Ballard, S.S., 176(141), 177, 216 Banerji, S. K., 63(51), 73, 80, 87, 90, 92 Bartman, F. L., 161, 218 Bastamov, S. L., 12, 44, 49 Batchelor, C. D., 99(16), 114 Bates, D. R., 166, 173(118), 196, 200, 201, 203, 204(216), 205, 207, 208(232, 234, 250, 256), 209(262), 213, 216, 218, 219, 2.90, 221

Blth, M., 63(61), 64(61), 66(83), 67, 69(61), 70(61), 71(83), 75, 80,(133,156, NO), 81, 82, 87, 88, 89, 90. 91. 92 Battan, L. J., 259, 266, 267 (58), 285, 286(98), 287(58), 300 30.2 Bauer, E., 208(254), 221 Baum, W. A., 169, 213 Belensiefer, E., 60(37), 86

Benioff, H., 56, 58, 68(78), 76(78), 77, 86, 86, 91 Bergeron, T., 280,902 Bergstrand, E., 98(9), 113 Berkner, L. V., 38, 49 Bernard, P., 62(50), 63(64), 66, 69, 70(64), 73, 74, 77, 80, 87, 88, 91 Bernasconi, C., 59, 86 Berroth, A., 98(6), 118 Bertelli, T., 55, 86 Best, N. R., 161, 166(33), 211 Biese, E., 14, 60 Bigg, E. K., 246, 299 Birge, R. T., 185(160), 187(160), 217 Birkeland, K., 45, 49 Birstein, S. J., 271, 276, 277, 801 Blanchard, D. C., 241(16), 256, 261, 298, 299 Bomford, G., 101(19), 114 Bonchkovsky, V. F., 66, 72, 88 Bondi, H., 173(111), 216 Bondsdorff, J., 97(4), 113 Borgen, C. N. J., 12, 49 Bossolasco, M., 59(26), 86 Boucher, R. J., 269, 270, 300 Bowen, E. G., 256,257(40), 258,259(47), 261(47), 266,284,286,287,299, 300, 80% Boyd, L. G., 39, 62 Bradford, D. C., 71, 89 Bradley, J. E. S., 177(153), 181(153), 185 (153), 188(153), 199(153), 217 Braham, R. R., Jr., 228,233,259,266(58), 267(58), 279(80), 283, 285, 286(98), 287(58), 298, 900, 301, 902 Brand, F., 59(32), 86 Brein, R., 99(14), 113 Breitfus, L. L., 12, 49 Brix, P., 156(26), 176(144), 177, 211, 216 Buettner, K., 60(37), 86 Buissoo, H., 177(145), 216 305

306

AUTHOR INDEX

Bunge, A. A., 12, 49 Burnight, T.R., 171, 172(103, 104), 215 Byerly, P., 81, 92 Byers, H. R., 266, 267(58), 285, 286(98), 287(58), 300, 302 Byram, E. T., 161, 162, 165(59), 166, 167, 168(59), 170(100), 171(40, 75, 105), 172(40, 59, 100, 105), 178, 181, 204, 206(59), 207, 208(40), 209, 212, 213, 214, 215

C Caloi, P., 60(41), 61, 87 Cannon, C. G., 177(153), 181(153), 185, (153), 188(153), 199(153), 217 Carder, D. S., 59(28), 64(68), 66(68), 86, 88

Carpenter, P. H., 14, 49 Cederholm, B. J., 197(200), 198, 219 Chakrabarty, S.K., 60, 86 Chapman, S.,38, 49, 154, 155(2), 165, 204, 205, 210 Choong, S.P., 195, 218 Choudhury, D. C., 208(252), 221 Chubb, T. A., 161(40), 162(40), 165(59), 166(40), 167(75), 168(59), 170(40), 171(40, 75, 105, 106, 107), 172(40, 59, 105, 106), 173(107), 178(59), 181(75), 204(59), 206(59), 207(75, 107), 208(40, 105), 209(105), 212, 213, 215 Clark, K. C., 177(149), 182(149), 185, 189, 190, 217 Clearman, H. E., 169, 170, 214 Coffin, E. M., 197(200), 198, 202(224), 219, 220

Cohen, E. R., 156(24), 211 Colborne, D. C., 134, 135, 151 Collins, G., 176(139), 178, 182(139), 216 Coons, R. D., 284, 302 Corkan, R. H., 144, 162 Coulomb, J., 78, 81, 91 Craig, R. A., 165(55), 205(55), 212 Crozier, W. D., 279(80, 81),SO1 Curry, J., 176(140), 177(140), 216 Curtis, J. P., 176(131, 132), 182(132), 183(132), 185(131), 190, 216

D Darbyshire, J., 65(74), 74, 88, 90 Darbyshire, M., 65(74), 88

Davies, D . A., 288, 302 Dawson, H. P., 14, 49 Deacon, G. E. R., 74, 76(142), 90, 91 Deb, S.,209(263), 221 Debenham, F., 46, 49 Defant, A., 141, 161 de Graaff-Hunter, J., 93(1), f l 3 de Jager, C., 171, 173, 216 Deming, L. S.,208, 221 Depperman, C. E., 72(119), 90 Dietrich, G., 125, 150 Dinger, J. E., 65(73), 66(84), 71(112), 74, 88, 90 Ditchburn, R. W., 177(151, 152, 153), 178(151), 181, 185(153), 188, 199, 202, 204, 217, 219, 220 Donn, W. D., 72, 75, 90 Donn, W. L., 56, 61, 66, 75, 85, 87 Donovan, R. A., 200(207), 219 Doodson, A. T., 118, 131, 133, 135, 136, 138, 139, 140(27), 141, 143, 144, 147, 160, 151, 152

Dow, W. G., 161(41), 166(71), 212, 213 DuMond, J. W. M., 156(24), 211 Duncan, A. B. F., 200(210), 202, 219,220 Durand, E., 161(33), 166(33), 170,Zl1,214 Diitsch, H. U., 165(53), 205(53), 212

E East, T. W. R., 261, 300 Ehler, A. W., 201, 219 Eichhorn, H., 103(25), 114 Ekholm, N. G., 14, 49 Elliott, R. D., 296(107), 303 Ellis, F. W., 14, 61 Elterman, L., 161, 211 Elwert, G., 173(115, 116, 117), 215 Eschenhagen, M., 14, 49 Essers, E., 59(31), 86 Ewing, M., 56(17), 66, 75, 76, 77, SO(157, 158), 81, 82, 84(180, 181, 182), 86, 91, 92

F Fabry, C., 165(54), 212 Fairbairn, L. A., 144, 151 Ference, M., 161, 212 Findeisen, W., 253, 265, 299 Fisher, G. H., 74, 90 Fisher, J. C., 245, 299 FIeming, J. A . , 49,60

307

AUTHOR INDEX

Flory, P. J., 177(156), 217 Foner, S. N., 155(12), 211 Fortsch, O., 83(177), 92 Fournier d’Albe, E. M., 247, 283(94), 288, 299, 302 Fox, C. S., 225(2), 298 Freidman, H., 161(40), 162(40), 165(59), 166(40, 73), 167(73, 75), 168(59), 170 (40, loo), 171(40, 75, 105, 106, 107), 172(40, 59, 100, 105, 106, 110), 173(107), 178(59), 181(75), 204(59), 206(59), 207 (75), 208(40, 105), 209 (73, 105, 107), 212, 213,214,215

Fritz, H., 43, 50

H Hallett, J., 273(73), 276, 301 Hansen, W., 144, 145, 146, 161, 152 Hardtwig, E., 71(113), 72, 83, 90, 92 Harradon, C . , 50 Harris, R. A., 124, 150 Harrison, A. J., 197(200), 198, 202(224), 219, 220

Haskell, N. A., 81, 92 Havens, R. J., 161(33, 34), 162, 166(33, 34), 167, 209, 211, 213 Haycock, 0. C., 166(69), 213 Hecker, O., 55, 56, 77, 85, 86 Heddle, D. W. O., 177(151, 152), 217 Heffernan, K. J., 249, 272(69), 273, 279, 299, 301

Gailar, N., 167(75), 171(75), 181(75), 207(75), 213 Gale, D. I., 161(33), 166(33), 211 Galitzin, B., 63(59), 64(59), 77, 87, 91 Garrod, M. P., 249, 299 Gaydon, A. G., 156(25), 211 Geddes, A. E. M., 74, 90 Genatt, S. H., 99(16), 114 Geussenhainer, O., 68, 81, 89 Gherzi, E., 63, 71, 72, 78, 87,89, 91 Gilmore, M. H., 57,58(28), 65,66,71(108), 72, 76(21, 138, 139), 86, 88, 89, 91 Giorgi, M., 62, 87 Gluckauf, E., 164(48), 212 Goldberg, L., 164(52), 200(207), 212, 219 Goldsbrough, G. R., 133, 134, 135, 151 Goto, K., 190(180), 218 Granier, J., 185(170), 190, 191(186), 194, 200, 227, 218, 219 Greely, A. W., 14, 50 Grinevetsky, L. F., 14, 50 Griswold, K., 176(134), 216 Grunmach, L., 59, 86 Guenther, S., 55, 85 Gunn, K. L. S., 256, 259(43), 260(39), 284(96), 299, 300, SO2 Gutenberg, B., 55, 58(23), 60(36, 43), 63(58), 64(58, 67, 69, 75, 76, 78), 66(67, 69,80), 67 (36, 58, 75, 89, go), 68 (43, 78), 69(58), 70(58), 71(36, 89), 73(89), 74 (36, 89), 75(89), 76(78), 77(151), 78(36, 153), 80(159), 81, 82(176), 83(176), 85, 86, 87, 88, 89, 91, 92

Heiskanen, W., 97(4), 100(18), 108, 109, 110, 111(18), 113, 124 Henning, H. J., 196, 201(197), 218 Henriksen, S. W., 99(16), 114 Henry, A. J., 12, 44, 50 Heppner, J. P., 165, 213 Herron, J. T., 155(18), 195, 211 Herzberg, G., 155, 156(26), 176(140, 144), 177(140), 190(179), 210, 211, 216, 218 Hill, G. W., 261, 800 Hinteregger, H. E., 170(101), 176(133), 214,216

Hirono, T., 74(130), 90 Hirvonen, R. A., 99(12, 13), 108, 109(32), 110(32), 113, 11.4 Hitschfield, W., 256, 259(43), 260(39), 299, 300 Hoffmeyer, N. H., 12, 50 Hok, G., 166, 213 Honkasalo, T., 95(2, 3), 113 Hopfield, J. J., 169, 176(137, 138), 182 (137, 138), 185(138, 159, 160), 187(160), 189, 190(138), 196, 214, 216, 217, 218 Horowitz, R., 161(36), 162(36), 163, 164, 166, 168, 211 Hough, F. W., 97(4), 123 Hough, S. S., 132, 151 Houghton, H. G., 235, 238, 250, 251, 252, 254, 256, 257, 258, 265, 298, 300 Howell, W. D., 238, 239, 298 Hoyle, F., 173(111, 118), 208, 215 Hubert, W. E., 65, 711108), 88 Hudson, R. L., 155(12), 211

308

AUTHOR INDEX

Huff, F. A., 229, 298 Hulburt, E. O., 166(73), 167(73), 208, 209(73), 213, 221 Hume, W., 271(64, 67), 800, 301 Humphreys, W. J., 254, 299 Hurzeler, H., 194, 218 Hutchinson, G. E., 225, 298 Hynek, J. A., 39, 68

Jung, F. Rudolf, l02(21), l l 4 Jung, K., 75, 91 Junge, C. E., 237, 240, 298 Jurgens, N . P., 14, 60 Jursa, A. S., 169, 172(88), 197(204), 200 (211), 207(248, 249), 214, 219, 221 Jutsum, P. J., 202(231), 220

I Ignazio, D., 59(26), 86 Inghram, M. G., 1941189), 218 Inn, E. C. Y., 167(77), 170, 175(94), 177(77, 121, 122), 178(121, 1231, 179, 181(123), 182(123), 185(122), 187(122), lsS(77, 122), 191(77, 128), 192(128), 195(126), 196, 199(77), 200(124), 201, 202(122), 207(77, 123), 218, 21.6, 216, 216, 218, 271, SO1 Inoue, Y., 208(257), 209, 221 Inouye, W., 74(130), 90 Isono, K., 248, 299 Iyer, H. M., 76(142a), 91

Kaiiriiiinen, E., 102(22), 114 Kalaja, P., 99(12), 113 Kallmann, H. K., 166, 167, 213 Kammer, E. W., 65(73), 66(84), 88 Kanai, K., 81, 92 Kandel, R. J., 155(13),211 Kane, J. A., 166(68),213 Kaplan, J., 38, 60 Kappes, T., 59(31), 86 Kassander, A. R., Jr., 249(28), 250(28) 279(82), 299, SO1 Kato, S., 201, 209, 219 Kelly, J. J., 289(101, lola), 302 Kerbert, C., 14, 60 Kintanar, R., 61(45), 77(45), 87 Kishinouye, F., 62(49), 66(79), 69, 74, 75(131), 87, 88, 89, 90 Koch, K. R., 14, 60 Knauss, H. P., 176(141), 177, 216 Koll, R. T., 161(34),162(34), 166(34),211 Koomen, M. J., 202, 820 Krakau, E. V., 12, 44, 61 Kramer, H. P., 281(88), SO2 Krastanow, L., 245, 898 Kraus, E. B., 239, 298 Kreplin, R. W., 171(107), 173(107), 207(107), 209(107), 216 Kreusler, H., 174, 177(119), 216 Krug, E. D., 56, 57, 86 Kukkamiiki, T. J., 95(3), 99(12, 13), 102(20), 113, 114 Kupperian, J. E., 171(107), 173(107), 207(107), 209(107), 215 Kuznetsov, N. I., 14, 60

J Jackson, J. E., 166, 208(67), 218 Jackson, J. M., 169(83), 214 Jacobi, W., 246, 299 James, B. R., 14, 60 Jardetzky, W. S., 81(169), 82(169), 92 Jausseran, C., 177(145), 216 Jeeves, T. A., 295, 303 Jeffreys, H., 79(155), 81, 91, 92, 112(36), 114

Jelstrup, G., 103(23), 114 Jelstrup, H. S., 99(14), 113 Jenkins, F. A,, 185(162), 189(162), 217 Jensen, H., 63, 87 Johannin-Gilles, A., 197(201, 202), 199 (202), 219 Johansson, 0. V., 14, 60 Johnson, F. S., 163, 164, 165, 166, 167, 169(80), 170(91, 96, 97), 172(86), 173 (86), 175(96), 195(57), 205, 208, 209(86), 212,213,214

Johnston, H. L., 197(199), 198, 201, 202, 219

Jones, E. L., 284(96), SO2 Jones, L. M., 164(49),812 Jones, 0. A., 76(140), 91 Jones, Sir H. S., 60

K

L Lacaee, J. R., 67, 89 Lacoste, J., 69(99, loo), 70(99), 89 La Cour, D., 46, 51 Ladenburg, R., 174, 177(120), 178(120), 179, 216 Lagerqvist, A., 190(179),218

AUTHOR INDEX

La Cow, H. E., 161(34, 35, 36), 162(34, 36), 163, 164, 166(34), 168, 211 Lamar, W. H., 14, 61 Lambert, W. D., 97(4), 118 Lambrey, M., 190(172), 217 Landsberg, H., 77, 78(148), 91 Langmuir, I., 255, 258, 284(95), 299, SO2 Lanman, C., 14, 61 Laplace, P. S., 130, 160 Lateef, A. .M. A., 283(94), 288(94), SO2 Laursen, V., 24, 46, 61 LeBlanc,F. J.,169(88),172(88),200(211),

309

McMath, R. R., 200,219 McMorrow, D. R., 166(69, 70), 219 McWhirter, M., 271(67), SO1 Madonna, L. A., 246(24), 299 Malitson, H. H., 169(86), 172(86), 173 (86), 208 (86),209 (86), 214 Malone, T. F., 69(104), 89 Mange, P., 206, 209, 220,221 Manson, J. E., 273(72), 274, SO1 Marchant, M. Q., 99(16), 114 Marcou, R. J., 166(69, 70), 219 Markovib, B., 64(65), 88 Markowitz, W., 99(17), 114 214, 219 Marmo, F. F., 155(9), 158(9), 167(77), Le Cam, L., 295(106), 303 Ledersteger, K., 112, 116 175, 177(9, 77), 178, 179(9), 181(9), Lee, A. W., 63(60, 63), 64(60, 63), 66, 184, 185, 188(77), 189, 190(9), 191(72, 69(60, 81), 70(81), 80, 87, 88, 92 128, 129, 184), 192(128), 194(129), Lee, P., 177(150, 155), 179, 181, l82(150, 199(77), 207(77, 245), 208(9, 245), 220, 213, 216, 217, 218, 220, 221 155), 183, 185(155, 169), 188(169), Marr, G. V., 202(231), 220 189(169), 190(155, 169), 202, 217, 220 Leet, L. D., 71(109), 72, 89, 90 Marshall, J. S., 259, 261, SO0 Leifson, S. W., 176(136), 190(136), 196 Martin, C. R., 164(51), 212 (136), 200, 201(136), 216 Mason, B. J., 234, 245, 273(73), 276, 298, 299, SO1 Lemstrom, S., 14, 61 Lenz, R. E., 14, 61 Massey, H. S. W., 203, 208(232), 280 Levallois, J. J., 112, 116 Masuda, K., 67(91), 70(91), 89 Lichtman, S. W., 170(100), 171, 172(100), Mayence, J., 191(183), 192, 200, 218, 219 Meadows, E. B., 165, 213 214 Meinel, A. B., 196, 218 Lien, J. R., 166, 219 Linke, I?., 75, 76, 90 Meissner, O., 69(98), 89 Linsford, L. B., 166(70), 219 Mendel, H., 68(96), 70(96), 73, 75, 89 Little, J. W., 175(130), 176(130), 216 Mercure, R., 170(92), 171(92, 108), 173 Liu, V. C . , 161, 212 (92), 176(132), 182(132), 183(132), 214, Liznar, J., 14, 61 216, ,916 Lock, C., 170, 214 Miche, M., 74, 80, 90 Lodge, J. P., 283(91), SO2 Miescher, E., 190(178, 179), 191, 194, 218 Longuet-Higgins, M. S., 74, 80, 82, Milham, W. I., 254, 299 90, 92 Miller, S. C., 170, 171(92, 108), 173(92), Lo Surdo, A., 60(41), 87 214, $16 Ludlam, F. H., 256, 257, 258, 265, 280, Milne, J., 55(2), 63(63), 64(63), 76, 78, 282, 299, 900,SO2 86, 87 Lutken, C. F., 14, 61 Mintrop, L., 59(29), 86 Lynch, J., 60, 87 Mitra, M., 83(178), 92 Lyttleton, R. A., 173(111), 216 Mitra, S. K., 155, 161, 165(3), 203, 207, 209(246), 210, 220 M Moe, G., 202, 219 McDonald, J. E., 245, 249(28), 250(28), Mohler, 0. C., 200(207), 219 280,291(104),296(107),2~9,~0l,S02,SOS Mohr, E. I., 185(169), 188(169), 189(169), 190(169), 217 Macelwane, J. B., 55, 56, 71(110), 72 Molodenskij, M. S., 112, 116 (110), 76(141), 86, 89, 91

310

AUTHOR INDEX

Moore, C. E., 155(22), 170(91), 211, 214 Mori, K., 190(175), 218 Morrison, J. D., 155(20), 194(189), 211, 218

Moses, H. E., 206, 220 Mottl, J. R., 155(16), 175(16), 179(158), 181(158), 194(158), 202(16), 211, 217 Mukherji, S. M., 63(52), 64(66), 87, 88 Mulliken, R . S., 158(29), 177(29), 178(29), 22 1 Munday, G., 177(153), 181(153), 185(153), 188(153), 199(153), 217 Munk, W. H., 64, 74, 79(122), 88, 90 Murai, G.,74(130), 90 Murdoch, J., 14, 50 Murgatroyd, R. J., 249, 299 Murphy, L., 66(85), 88

N Nagata, T., 60(40), 87 Neubauer, R., 271, 301 Neumayer, G., 14, 60 Newell, H. E., 155, 161, 162(45), 166(74), 167(74), 210, 212, 213 Neyman, J., 295(106), 296(107), 903 Nicholson, A. J. C., 155(20), 211 Nicolet, M., 155, 173(114), 196, 203, 205, 206,207,208(251), 209,220,216,218,220 Nidey, R . A., 169(85), 21.4 Nisimura, E., 60(39), 86 Nordberg, W., 161(39), 163(39), 164(39), 212

Norlund, N. E., 103(24), l l 4 Nottarp, K., 99(14), 123 Ny, T. Z., 195, 218

0 Oberly, J. J., 169(80), 170(80), 223, 214 O’Keefe, John A., 99(15), 114 Olander, V. R., 97(5), 119 Oliver, J., 76, 80(157), 84(180, 181, 182), 9i, 92 Olsen, J., 12, 44, 51 Omori, F., 55 60(38), 61(48), 86, 87 ,

P Packer, D. M., 170, 214 Paneth, F. A., 164(47), 212 Paulsen, A. F. W. 14, 50

Peake, S. L., 246(24), 299 Pearcey, T., 261, 300 Penndorf, R., 206, 220 Pesonen, U., 99(12), 113 Pfeffer, G. J., 14, 60 Pfleiderer, H., 60(37), 86 Pickar, A. D., 166(67), 208(67), 213 Pierce, A. K., 200(207), 219 Pietenpol, W. B., 169, d l 4 Pissetti, P., 104(27), 124 Poincar6, H., 149, 162 Ponte, G., 60(41), 87 Pound, G. M., 246, 299 Press, F., 56(17), 65(72), 71(72), 75(72), 76(72), 77(151), SO(157, 158), 81(72, 169), 82(169), 85, 88, 91,92 Pressly, E. C., 165, 213 Pressman, J., 207(245, 248), 208(245), 220, 221 Preston, W. M., 167, 175, 177(76), 179, 181, 182(76), 185(76), 188, 199, 207(76), 213

Price, W. C., 155(14, 15, 17, 19, 21), 176(139), 178, 182(139), 195, 196(15), 197, 201(14), 221, 216 Prokof’eva, I. A., 165(56), 212 Proudman, J., 133, 135, 136, 141, 142, 143, 147, 149, 152, 162 Priifer, G., 125, 150 Pruppacher, H. R., 275, 278, 301 Purcell, J. D., 165(57, 58), 169(86, 87), 170(91, 99, 102), 171(99), 172(86, 99), 173(86), 195(57), 204(99), 205(57), 208 (86), 209(86), 212, 214

R Rae, R . W., 51 Ramirez, J. E., 57, 65, 66(20), 71(20), 76(20), 86 Rasool, S. I., 283(94), 288(94), 302 Rathenau, G., 196, 197, 201, 218 Rawer, K., 208(253), 221 Ray, P. H., 14, 50 Reasbeck, P., 164(50), 212 Rebeur-Paschwitz, E. von, 55, 78, 86 Regele, O., 12, 60 Regener, E., 169, 213 Regener, V. H., 169, 223 Reitan, C. H., 229, 283, 298, 302

311

AUTHOR INDEX

Rense, W. A., 169, 170(84, 92), 171(84, 92, 108), 173(92), 176(132), 182(132), 183(132), 214, 216, 216 Reynolds, R . R., 296(107), 303 Reynolds, S. E., 259, 271(64), 300, SO1 Rice, D. A,, 110(34), 114 Richter, C. F., 58(23), 77(151), 86, 91 Rigby, M., 281(88), 302 Rockwood, C. C., 169(80), 213 Rolf, B., 12, 44, 61 Roll, H. U., 59, 74, 86 Romand, J., 200, 219 Romney, C. F., 63(57), 87 Rossiter, J. R., 144, 162 RothB, J. P., 66, 88 Rouard, P., 177(145), 216 Ruijs, J. M., 14, 60 Rune, G. A., 97(4), 113 Rust, F., 14, 60

S Sagrebrin, D. W., 111, 114 Sakai, H., 175(130), 176(130, 134), 179 (158), 181(158), 194(158), 216, 217 Sakuma, S., 60(40), 87 Sandig, H.-U., 99(14), 213 Sanger, R., 275, 278, SO1 Sarkar, D., 60, 86 Sato, T., 208(255), 209(264), 221 Schaefer, V. J., 246, 248, 249, 265, 271 (64), 276(76), 277,287,299,300, 301,302 Scharizer, R., 14, 61 Schiff, H. I., 155(18), 195, 211 Schilling. G. F., 39, 62 Schley, W. S., 14, 63 Schmauss, A., 254, 299 Schneider, E. G., 177(146), 178(146), 216 Schneider, R., 63(54), 67, 87, 89 Scholte, J. G., 80, 81, 82, 92 Schfinemann, H., 77, 91 Schuyler, G. L., 57, 86 Scolnik, R., 202(230), 220 Scott, E. L., 295(106), 296(107), SO9 Seaton, M. J., 201, 203, 204(216), 207, 208(234), 219, 220 Seddon, J. C., 166, 208, 213 Seely, B. K., 272(69), 279(80, 81), 301 Sen-Gupta, P. K., 200, 219 Seya, M., 190(175), 218 Sezawa, K., 81, 92

Shaw, J. J., 57, 86 Shidi, I., 66(79), 88 Sicinski, H. S., 161, 212 Sieberg, A., 55, 78, 86 Sievers, J. R., 267, SO0 Sigl, R., 99(14), 113 Simpson, D. M., 155(14, 17), 195,201(14), 211 Simpson, G. C., 254, 299 Sims, L. L., 249(28), 250(28), 299 Smith, B., 239, 298 Smith, E. J., 249, 266, 272, 273, 279, 299, 300, 301 Smith, R. B., 269, 270, 300 Spencer, N. W., 161(41), 166(71), 212,113 Sponer, H., 185(161), 217 Sprengnether, W., 56(13), 86 Squires, P., 240, 266, 298, 300 Stacey, D. S., 169(83, 85), 21.4 Staff, J. M., 60 Steen, A. S., 14, 62 Sterneck, R., 125, 160 Stokes, G. G., 107(28), 114 Stout, G. E., 229, 298 Strain, C. V., 169(80), 213 Strobach, K., 58, 79(24), 86 Stroud, W. G., 161, 163, 164, 212 Stuart, F., 171(108), 216 Suga, T., 185(163), 189(163), 190(163), 217 Sun, H., 191(185), 194, 201, 202, 218, 219 Sutcliffe, L. H., 190(176), 191(176), 218 Sutcliffe, R., 226, 229, 298 Swinbank, W. C., 256, 299

T Takamine, T., 176(142), 182(142), 185 (163, 164), 189(163, 164), 190, 209, 216 217 Tams, E., 75, 77, 90, 91 Tanaka, Y., 169(88), 172(88), 175(126), 176(142, 143), 178, 182(142), 185(163, 164, 167), 187(167), 188, 189(163, 164), 190(163, 164, 173, 174, 175), 191(182), 195, 196, 200, 202, 209, 21.4, 216, 216, 2 i 7 , 218, 219, 220 Tannenbaum, E., 202, 220 Tanni, Id., 108, 11.4 Tardi, P., 97(4), 113 Telford, J. W., 259, 260, 261, 300

312

AUTHOR INDEX

Thom, H., 280, 289(105), 295(105), 301, 303 Thomson, W., 131, 160 Thorndike, N. S., 259(47), 261(47), 300 Thuronyi, G., 281(89), 306 Tillo, A. A., 14, 62 Tollner, H., 12, 44, 62 Tousey, R., 165(57, 58), l69(86, 87), 170(89, 91, 96, 99, 102), 171, 172(86, 99), 173(86), 175(96), 195(57), 202(230), 204, 205(57), 208(86), 209(86), 212, 214, 220

Townsend, J. W., 165, 213 Tromholt, S., 14, 62 Trommsdorff, F., 56, 57, 86 Turnbill, D., 245, 699 Tutte, W. T., 155(19), 211 Twomey, S., 269, 283(92), 300, SOP

U Ueda, M., 190(177), 218 Ulwick, J. C., 166(69, 70), 213 Ursell, F., 80, 92

V Vaisalii, Y., 98(7), 113 Van Straten, F. W., 71(111), 75(111), 89 van Voorhis, C. C., 174, 177(120), 178 (120), 179(120), 616 Vassy, A., 177(148), 195, 216, 218 Vassy, E., 62 Vegard, L., 208, 221 Vening Meinesz, F. A., 110, 114 Vigroux, E., 196, 218 Villain, C., 125, 160 Vincent, E., 12, 62 Vodar, B., 197(202), 199(202), 219 Volman, D. H., 177(157), 217 Vonnegut, B., 267, 268(60), 271(64), 300, 301

W Wadati, K., 67(91), 70(91), 89 Wadley, T. L., 98(10), 113 Wainfan, N., 170, 177(95, 154), 184, 185(154), 190, 200, 202(154), 214, 217 Walker, W. C., 170(95), 177(95, 154),

184(154), 185(154), 190(154), 191(187), 194, 200(154), 202(154), 214, 217, 218, 220

Walsh, A. D., 155(21), 190(176), 191, 211, 218

Walsh, D. H., 60, 86 Walsh, J. R., 161, 163(39), 164(39), 212 Walz, F. C., 169(83), 21.4 Warfield, C. N., 160(30), 166(30), 211 Warner, J., 269, 300 Watanabe, K., 155(9, 10, 11, 16), 156 (9, 27), 158(9, lo), 165(57), 167, 168, 170 (10,11,96,98,99,101), 171(99), 172(99), 175(94, 96, 130), 176(130, 134, 135), 177 (9, 77, 121, 122, 123), 178, 179, 181(158), 182(9, 123), 183, 184,185(122), 187(158), 188, 189, 190(9), 191(77, 128), 192, 194 (158), 195(57, 126), 197, 198, 199, 200 (124), 201(125), 202(16), 204(99), 205 (57), 207(10, 123), 208(245), 210, 211, 212, 213, 214, 216, 216, 217, 219, 220

Watts, C. B. A., 98(11), 113 Weickmann, H. K., 258, 289(101, lola, 102), 300, 302 Weisner, A. G., 161, 212 Weissler, G. L., 170(95), 177(95, 150, 154), 179, 181, 182(150), 183, 184(154), 185(154, 169), 188, 189, 190(154), 191 (185, 187), 194, 200(154), 201, 202(154), 214, 217, 218, 219, 220

Westerhausen, H., 67(86), 76, 88 Wetzel, W., 60(37), 86 Whewell, W., 124, 160 Whipple, F. J. W., 63(60), 64(60), 67, 68(92), 69(60), 77, 80, 87, 89, 91, 92 Whipple, F. L., 39,62, 161, 211 White, W. B., 166(74), 167(74), 213 Whitten, C. A., 97(4), 113 Wiborg, B. W., 164(50), 212 Wichmann, H., 14, 62 Wiechert, E., 72, 75, 90 Wiegand, A., 254, 299 Wilczek, N. J., 12, 62 Wild, H., 12, 62 Wilkinson, P. G., 158, 177(29), 178(29), 185(168), 187, 188, 197(199), 198, 201, 202, 211, 217, 219 Williams, S. E., 177(147), 179, 216 Wilson, C. D. V., 59(30), 86'

313

AUTHOR INDEX

Wilson, J. T., 65(72), 66(72), 71 (72), 75(72), 76(72), 81(72), 88 Wilson, N . R., 170(102), 214 Witherspoon, A. E.,200, 219 Wohlgemuth, E. E., 14, 62 wolf, H., ioi(i9), io4(26), 114 Woodcock, A. H., 241, 261, 282, 283(15, go), 298, SO2 Woolley, R.v.d.R., 173(112, 113), 215 Worley, R. E., 185(162, 165, l66), 189 (162, 165), 190, 217 Wii, T. Y., 206, 208(254), 220, 221 Wulf, 0.R., 208, 221 Wyckoff, R. W. G., 273(71), 301

Y Yatoukin, It., 39, 52

Z Zaidi, I. H., 283(94), 288(94), SO2 Z&topek, A., 69, 89 Zelikoff, M., 175, 177(121, 122, 123), 178(121, 123), 179, 181(123), 182(123), 185(122), 187(122), 188(122), 192, 197, 198, 200, 201(125), 202(122), 207(123, 248, 249), 215, 216, 219, 821 Zoeppritz, K., 68, 75, 77, 89

SUBJECT INDEX A

electron densities in, 165 layer(s) in, D, 206 E, 207 F, 209 oxygen dissociation, 205 ozone, 204 photochemical processes in, 154, 157, 160 solar ultraviolet in, 169 intensity measurement of, 170, 171, 172 theoretical estimate of, 173 temperatures of, 160, 161, 164 Aurora(e), 17, 33 connection of, with clouds, 3 intensity of, 4, 8 maximum frequency of, 3, 8 observation of, 4, 23 Auroral, forms, Stormers photographic atlas of, 17 physics, 15 ilzimuth, astronomical, 100

Absorption spectrum(a), ultraviolet, ammonia, 202 carbon dioxide, 200 carbon monoxide, 202 hydrogen, 202 methane, 202 nitric oxide, 190 nitrogen, 185 nitrous oxide, 200 oxygen, 176 ozone, 195 rare gases, 202 sodium, 202 water vapor, 196 Adiabatic cooling, cloud droplet growth in, 238 role of, i n condensation, 233, 235 Air-earth’s crust system, waves in, 81 Airglow, 33 Amphidromic systems, 123, 138 degenerate, 128 Anomalies, gravity, 106 Antarctic, ice cap, seismic sounding through, 39 Arctic exploration, 3 German commission on, 4 ocean basin, drifting station, 39 Astronomical coordinates, 99 Atmosphere, upper, absorption processes in, 153, 155 dissociation, 158 excitation, 156 photoionization, 158 predissociation, 158 preionization, 158 composition of, 160, 163 constituents of, carbon dioxide, 164 nitrous oxide, 164 oxygen, 166 ozone, 165 densities of, 160, 161

B Bergeron, process, 244, 253 transition, 267, 289 Bouguer reduction, 111 Bernoulli’s equation, 120

C Chart, tidal see Tidal charts Cloud(s) cirrus, 237, 238 cumulus, 288 tropical, 242 depth of, 226, 257 life times of, 232 effective, 231 observed, 232 314

315

SUBJECT INDEX

stratus, clearing of, 289 subcooling of, 246, 250 terrestrial, 225 Cloud drop(s), 242, 243 aggregation process of, 243 accretion, 244 ice crystal, 244 role of microturbulence in, 26 concentration of, 25 Cloudiness, zonal mean, 266 Cloud modification, evaluation of experiments for, 290 physical, 292 statistical, 293 goal of, 225 influencing precipitation by, 241 physics of, 223 recent developments in, 263 scientific status of, 280 Cloud physics, 224 history of, 234 present status of, 234 Cloud seeding cupric sulphide, 278 dry ice, 264, 265, 266 efficiency of, 265, 267 geophysical effects of, 228 influence of, on hydrologic cycle, 229 randomised pairs, 267,285,286,295,296 recovery from effects of, 230 salt, 287, 288 silver iodide, Weyl’s hypothesis, 276 water spray technique of, 284 Cogeoid, 111 Compensation, isostatic, 101 Condensation, latent heat of, 239 Coriolis force, 122, 123 Cosmic rays, 15,33,42 Currents, tidal, 148

D Deflection of the vertical, 94, lo(! astro-geodetic, 100, 111 gravimetrical, 117 D layer, see Atmosphere, upper, layerk) in Drops, very large, see Raindrops Dust, concentration of, a t cloud latitudes, 248, 249

E Earth, albedo of, 226, 234 figure of, 7, see also Earth, size and shape of rotation of, forces due to, 122 satellites, see Satellites, earth size and shape of, 93 E layer, see Atmospheric, upper, layer(s) in Electronic trilateration, 98 Ellipsoid, homogenous, 103 International, 96, 103 reference, 101 Ellipsoidal Coordinates, 96 Equilibrium tide, 119 Explorer satellites, see Satellites, earth

F Fixations, astronomical, see observations, geodetic F layers, see Atmosphere, upper, Iayer(s) in FPY, see International Polar Year, First Free-air anomalies, 106 reduction, 106

Gal, 105 Gases, atmospheric, absorption cross section of, 173 exchange rate of, 230 Geodesy, 93 Geodetic, azimuth, 97, 113 latitude, 96, 113 line, 96 longitude, 96, 113 Geodimeter, 98 Geoid, 101, 103, 108 determination of, 106 undulations of, 108, 109 Geomagnetism, 34 Geop, 101, 107, 108 Geopotentials, 102 Geostrophic force, 127 Glacial observations, 12, 34

316

SUBJECT INDEX

Gravimeter, sea surface, 41 Gravity, 35 anomalies, 106

H Hail, suppression of, 289 Heat balance, global, 226 Herzberg bands, 157 Hydrography of the oceans, 7 Hydrologic cycle, 225 atmospheric steps in, 233 effect of cloud seeding on, 229

I IGY, see International Geophysical Year Internation ellipsoid, see Ellipsoid, International International Geophysical Year, 2, 24, 155 objectives of,26 participating nations, 28 programs for, 26, 27, 28 antarctic, 28, 30, 31, 32 arctic, 29 rocket, 29 scientific, 33 results of, preliminary, 39-42 International Polar Year, 2 First, 10 observations of, 10 results of, 12 stations of, 10, 11 Second, 14 background of, 14 programs for, 16, 17 national, 20, 21 scientific, 22 results of, 23 scope of, 19 International Years, 42 deficiencies of, 45 future of, 47 Ionic layers, see Atmosphere, upper, layer(s) in Ionospheric physics, 15, 35 Isostatic compensation, see Compensation, isostatic

K Kelvin and Poincard wave#, 142

1 Laplace stations, 94, 97 Laplace’s equation, 100 Latitude, astronomical, 100, 113 determination of, 12, 36 geodetic, 96, 113 reduced, 103 Leveling, high precision, Lightning, suppression of, 103, 289 Longitude, astronomical, 100, 113 determination of, 12, 36 geodetic, 96, 113

M McPherson monochromator, 175 Magnetic, field, secular variation of, 17 variation, charts of, 16 Mareographs, 102 Markowits camera, 36, 99 Meteorological methods, radar, 258 Meteorology, 6, 15, 17, 23, 36 Microseisms, 53 amplitude of, change with depth of, 67 beats in, 63 classification of, 55 correlation of, with meteorological phenomena, 71 cold fronts, 71 cyclones over continent, 71 cyclones over ocean, 71 pressure changes, 72 winds blowing against mountains, 72 with ocean waves, 72, 73 with sunspots, 69 cyclical variations in, 69 eleven year, 69 semi annual, 69 definition of, 54 direction of approach of, 64 energy problem of, 81 history of, 55 instruments for investigation of, 56, see also Microseisms

317

SUBJECT INDEX

tripartite instruments irregular, 63 nomenclature, 54 period of, changes in, during propagation, 83 correlation of, with ocean waves, 74 effect of geological structure on, 83 long, several minutes, 78 ten seconds t o several minutes, 76 connected with local wind, 76 from surf origin of, 83 short, see Microseism, short period period problem of, 82 produced by artificial causes, 59 “noise” in seismic explosions, 59 traffic and industry, 59 water flowing over a dam, 59 propagation of, 65 along ocean bottoms, 65 across continents, 65 barriers to, 67 regular, 60, 62, 63 beate in, 63 from surf, 75 period-amplitude relation of, 63 period-source distance relation of, 63 period, four seconds, 62 period, one to three seconds, 60 short period, 59, 60 connected with meteorological phenomena, 59 produced by local surf, 60 theory of 78 tripartite instruments for recording of, 56 equations for, 57 units and symbols, 55 use of, in meteorology, 74 vectorial recorders for, 58 velocity of, 65 wave types in, 68 channel, 69, 79 Rayleigh, 68, 79 shear, 79

N Normal gravity, 103, 104, 113 Normal section, see Triangulation

Nucleation homogenous, 245, 247 critical threshold of, 262 Nucleus(ei) Aitken, 240 condensation, 236, 247 growth curves for, 239 population of, 241 deposition 247, 248 freezing, 246 grant, 240, 241, 282 sea salt, 282, 283 counts of, 283 role of, in initiating accretion, 283 ice, 251 chemical characteristics of, 248 counts of, 251 physical characteristics of, 248 large, 240 sea salt, 283 silver iodide, 268 generating technique for, 268, 269 nucleating efficiency of, 268 sublimation, 247 Nuclei counts, surface, 283 N values, 106, 107, 112

0 Observations, geodetic, 93 astronomical fixation, 93 gravity measurement, 93 spirit levelling, 93, 101 Ocean-earth’s crust system, waves in, 81 Oceanic tides, see Tides, oceanic Oceanic water, fraction of, in hydrologic cycle, 225 Oceanographic observations, 12 Oceanography, 36 Orthometric height, 102, 108 Ozone, determination of, 23

P Precipitation, ice-crystal hypothesis of, history of, 235 physics, 234 process, 241 accretion, 253 artificial stimulation of, 282 aggregation, 255 Bergeron, 253, 244

318

SUBJECT INDEX

coagulation, 255 coalescence, 255, 283 collision, 254, 283 Telford’s model, 260 ice crystal, artificial nucleation of 9 264 physical theories of, 224 release rates, 228 vertically downward flux of, 230 Proudman’s Tidal theorem, 143 Pulsations, 28

R Radiosonde measurements, 23 Raindrops, 242, 243 accretion efficiency of, 256 coalescence efficiency of, 256 collision efficiency of, 255 Langmuir’s theory, 255 Rain making, see Cloud modification Rocket probing of atmosphere, 42 Reference ellipsoid, see Ellipsoid, International Rydberg series, 182

S Satellites, earth, 29, 33, 40, 41 Sehumann Runge bands, 157, 158, 177 Sea waves, correlation with microseisms of, 59 Seismographs, see Microseisms, instruments for investigation of Seismology, 36 Silver iodide crystal properties of, 273, 274 effect of generator temperature on crystal form of, 274 photolytic decay of, 270, 271, 272 seeding efficacy of, 280 seeding technique, see Cloud seeding Silver iodide particles, counting technique, 262 Snow flakes, growth of, 257 Solar activity, 37 Spherop, 107, 108 Spirit levelling, see Observation, geodetic Sputnik, see Satellites, earth SPY, see International Polar Year, second

Stokes’ formula, 107, 108, 111 Vening Meinesa derivation of, 110 Stokes’ function, 107, 113

T Tellurometer, 98 Terrestrial magnetism, 6, 15 Theodolites, 95 Thunderstorm, latent heat release in, 223 Tidal charts, 124 Dietrich and Villain’s, 125 for Pacific ocean, 127 Harris’, 124 Prufer’s, 125 Sterneck’s, 125 Whewell and Airy’s, 124 Tides, earth, 148 internal, 148 oceanic, 117 effects of, capes, bays, and islands on, 147 coriolis force on, 122, 123 earth tides on, 148 internal tides on. 148 natural period of channels on, 126 equations of motion for, 129 Doodson’s solution for narrow oceans, 139 finite differences method, 144 Goldborough’s solution for polar and zonal oceans, 133 Hough’s solution, 132 Laplace’s solution, 130 narrow sea methods, 141 Proudman and Doodson’s solution, 135 for oceans bounded by meridians, 133 for oceans encircling the earth, 129 harmonic terms for, 118 use of tidal currents to determine, 108 Triangulation, 93, 94 adjustment in, 97 base lines for, 94 measurement of, 94 computation of, 95 electronic trilateration, 98 flare, 98 intercontinental connections, 97

319

SUBJECT INDEX

international connection, methods for, 98 lay out, 94 lunar methods, 98 measurement of angles for, 95 normal section, 96 Tropopause,

equatorial, 16 polar, 16

spectra, see Absorption spectrum(a), ultraviolet

V Vertical, deflection of, see Deflection of the vertical Volcanic tremors, 60

W U

Ultraviolet radiation, penetration of, 203

Water vapour, atmospheric, horizontal flux of, 230 turnover rate of, 228, 229, 230, 231 World days, 37, 38

CUMULATIVE TITLE INDEX TO VOLUMES I-IV

VOLUME I JOHN C. BELLAMY, Automatic Processing of Geophysical Data. . . . . . ARNOLD COURT,Some New Statistical Techniques in Geophysics. . . . BERTBOLIN,Studies of the General Circulation of the Atmosphere. . FRED L. WHIPPLE,Exploration of the Upper Atmosphere by Meteoritic Techniques . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N. C. GERSON, Unsolved Problems in Physics of the High Atmosphere. D. W. PRITCHARD, Estuarine Hydrography. . . . . . . . . . . . . . . . . . . . . . . GEORGEPRIOR WOOLARD, The Earth’s Gravitational Field and Its ........................ Exploitation . . . . . . . . , . . . . . . . . . . JAMES R. BALSLEY, Aeromagnetic Sur ng. . . . . . . . . . . . . . . . . . . . . . ,

2 45 87 119 156 243 281 314

VOLUME I1

J. S. MAHSHALL, WALTER HITSCHFELD AND K. L. S. GUNN,Advances 1 in Radar Weather. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , IRVING I. GRINGORTEN, Methods of Objective Weather Forecasting. . 57 WILLARD J. PIERSON, JR., Wind Generated Gravity Waves.. . . . . . . . 93 J. LAURENCE KULP,Geological Chronometry by Radioactive Methods 179 HUGOBENIOFF,Earthquake Seismographs and Associated Instruments . . . . . . . . . . . . . . . . . . . . . , ,

VOLUMEI11 A. P. CRARY, Arctic Ice Island Research. . . . ...... .. 1 ZDENEKSEKERA, Recent Developments in t y of th ation of Sky Light, . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . 43 PERRY BYERLY, Subcontinental Structure in the Light of Seismological Evidence . . . . . . . . . . . . . . E. C. BULLARD, A. E. MAXWELL, AND R. REVELLE, Heat Flow through . . . 153 the Deep Sea Floor. . . . . *J. A. JACOBS, The Interior of the E a r t h . . . . . . . . . . . . . . . . . . . . . . . . 183 1’. H. JONES AND H. E. SKIBITZKE, Subsurface Geophysical Methods in Ground-Water Hydrology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 S. F. SINGER,Geophysical Research with Artificial Earth Satellites. . 302 ,

,

,

,

VOLUMEI V CHRISTIAN E. JUNGE, Atmospheric Chemistry. . . . . . . . . . . . . . . . . . . . JOSEPH w. CHAMBERLAIN, Theories of the Aurora. . . . . . . . . . . . . . . . . LINCOLN LAPAZ,The Effects of Meteorites upon the Earth (Including Its Inhabitants, Atmosphere, and Satellites) . . . . . . . . . . . . . . . . . . *J. LEITIIHOLLOWAY, JR., Smoothing and FiItering of Time Series and Space Fields. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PAUL J. MELCHIOR, Earth Tides.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,

320

I 110 218

351 392

CUMULATIVE SUBJECT INDEX TO VOLUMES I-IV A

Bennett and Hulburt’s self-focused stream, IV, 148 Chapman and Ferraro’s, IV, 158 Hoyle’s, IV, 177 Lebedinski’s, IV, 178 Lemstrom’s, IV, 111 Maris and Hulburt’s ultraviolet light, IV, 190 Martyn’s, IV, 170 Parker’s, IV, 189 Singer’s shock wave, IV, 186 Stormer’s, IV, 112, 113, 127-146

Aeromagnetic survey, see Surveying, aeromagnetic Aerosols, IV, 3 continental, IV, 29 nature and origin, IV, 17 physical constitution, IV, 14 sea-salt, IV, 20 size distribution, IV, 5 Air pollution, IV, 94 Arctic, Ice Island research, 111, 1 subsurface character, 111, 23 B summer conditions, 111, 18 Borehole, analysis of, 111, 242, also see surficial deposits, 111, 21 Ground-water hydrology T-3 studies, 111, 34 Island rotation, 111, 36 C seismic, 111, 34 Chemistry, atmospheric, IV, 1 Ocean area, 111, 6 precipitation, IV, 71 gravitational observations, 111, 10 Chronometry, geological, 11,179 ice drift, 111, 13 radioactive methods, 11, 179 submarine bathymetry, 111, 8 carbon-14, 11, 198 water masses, 111, 12 helium, 11, 208 weather observations, 111, 6 ionium, 11, 209 Atmosphere, lead, 11, 182 general circulation of, I, 87 potassium, 11, 206 baroclinic model, I, 113 strontium, 11, 204 barotropic model, I , 109 tritium, 11, 213 energy balance, I, 102 fluctuations, I , 103 D physical principles, I, 91 Data, geophysical, automatic processing theory for, I, 107 of.. I,. 1 high, I, 155 present techniques, I , 3 collisional phenomena, I, 199 description, I, 3 composition, I , 188 evaluation, I, 18 physics of, I, 155 unitary records, I , 23 static properties and processes, I, automatic processing, I, 33 179 Deep sea floor, heat flow through, 111, temperature, I , 182 I53 momentum balance, I, 95 local disturbances, 111, 164 terrestrial, I, 158 measured values, 111, 156 Aurora(e), 111, 322; IV, 109-215 Atlantic ocean, 111, 168 theories of, IV, 109 interpretations, 111, 169 AlfvBn’s, IV, 174 321

322

CUMULATIVE SUBJECT INDEX TO VOLUMES I-IV

Mediterranean, 111, 168 Pacific ocean, 111, 166 methods of measurements, 111, 155 sources of heat, 111, 169 Deflection of the vertical, IV, 403; V, 94, 100 Discontinuity, Conrad, 111, 119 MohoroviEib, 111, 117, 146, 147

E Earth, elastic deformations of, IV, 435 Herglotz theory, IV, 435 electrical conductivity of, 111, 227 gravitational field, I, 281; 111, 308 determination of, I, 282 interior of, 111, 183, 304 composition and constitution, 111, 185 core, 111, 192 inner core, 111, 194 mantle, 111, 188 recent experimental data, 111, 200 seismic data, 111, 185 magnetic field of, 111,219, 304 motion in earth’s core, 111, 220 reversals of, 111, 230 secular variation in, 111, 227, 304 polar wandering, 111, 231 satellites, see Satellites, earth thermal history of, 111, 202, 212 core-mantle boundary, 111, 210 inner core, 111, 204 melting point and adiabatic gradients, 111, 206 tides, see Tides, earth Estuaries, I, 243 classification, I, 245 coastal plain, I, 247, 262 deep fiord, I, 256 fiord, I, 253 tidal, I, 268 Estuarine flushing, I, 270

Gravity, anomalies, I , 288; 111, 308 geologic uses of, I, 301 earth crustal studies, I, 301 exploration, I, 303 strength of the earth, I, 304 national base value I, 297 observations, I, 290 exploitation of, in geodetic studies, I, 293 variations in intensity of, IV, 426 waves, 11, 93 wind generated, 11,93 energy spectrum, 11, 140 ergodic theorem, 11, 134 practical wave statistics, 11, 137 propagation of, 11, 163 theory, 11, 95 simple harmonic progressive wave, 11, 103 wave train in deep water, 11,107 wave record analysis, 11, 158 Ground-water hydrology, 111, 241 subsurface geophysical methods, 111, 24 1 borehole-diameter logging, 111, 284 electric logging, 111, 243 application, 111, 256 resistance and resistivity, 111, 243 spontaneous potential, 111,248 flow-meter logging, 111, 289 fluid conductivity logging, 111, 294 radiation logging, 111, 263 Geiger-Mueller counter, 111, 265 ionization chamber, 111,265 neutron detectors, 111, 266 scintillation crystal, 111,266 theory and instrumentation, 111, 263 temperature logging, 111, 278 instrumentation, 111, 279 interpretation, 111, 281

F Forecasting, see Weather, forecasting cj

Geophysics, statistical techniques in, I, 45 circular distributions, I, 75 extremes, I, 53

H Hydrography, estuarine, I, 243

I International ellipsoid, I, 285; V, 96, 103

CUMULATIVE SUBJECT INDEX TO VOLUMES I-IV

International Geophysical Year, V, 2, 24, 155 program of, IV, 440; V, 26 International Polar Year, V, 2 First, V, 10 Second, V, 14 International Years, V, 42 Ionosphere, I, 212; 111, 323 tides, see Tides, ionospheric wind observation, I, 215

1 Love’s numbers, IV, 400

M Magneto-hydrodynamics, basic equations in, 111, 222 Meteorites, IV, 217 achondritic, IV, 237, 338 aerolites, IV, 237, 245 ballistic potential of, IV, 273 chondritic, IV, 337 deeply burried, detectors for, IV, 258 recovered, IV, 235 siderites, IV, 237 siderolites, IV, 237 Meteoritic craters, IV, 307 Meteoritic falls, effects of, IV, 218 Microseisms, V, 53 classification of, V, 55 correlation of, with meteorological phenomena, V, 71 cyclical variations in, V, 69 definition of, V, 54 direction of approach, V, 64 energy problem of, V, 81 history of, V, 55 instruments for investigation of, V, 56 also see Microseisms, tripartite instruments and Seismographs irregular, V, 63 nomenclature, V, 54 period of, V, 83 produced by artificial causes, V, 59 propagation of, V, 65 regular, V, 60 short period, V, 59 theory of, V, 78 tripartite instruments, V, 56 units and symbols, V, 55 vectorial recorders, V, 58

323

velocity of, V, 65 wave types in, V, 68

N N values, V, 106, 107, 112 Normal gravity, V, 103, 104, 113 Nucleation, homogenous, V, 245, 247 Nucleus (ei), Atiken, IV, 4, 11, 17; V, 240 condensation, IV, 73; V, 236,247 giant, IV, 4, 6, 12; V, 240, 241, 282 large, IV, 4; V, 240

0 Observations, geodetic, V, 93 Oceanic tides, see Tides, oceanic Orthometric height, V, 102, 108

P Precipitation, physics, V, 234 process, V, 241 accretion, V, 253 aggregation, V, 255 Bergeron, V, 244, 253 coagulation, V, 255 coalescence, V, 255, 283 collision, V, 254, 283 Proudman’s Tidal Theorem, V, 143 Pulsations, V, 28

R Radiosonde measurements, V, 23 Raindrops, V, 242

S Sassa extensometer, IV, 418 Satellites, earth, 111, 301; V, 29, 40 design characteristics, 111,354 geophysical research with, 111, 301 chemosphere, 111, 320 airglow and aurora, 111, 122 ozone content and distribution, 111,321 cosmic radiation, 111, 337 earth, figure of, 111, 308 earth’s magnetic field, 111, 304 earth’s surface and lower atmosphere, 111, 313 albedo, 111, 313 distribution of thunderstorms, 111, 320

324

CUMULATIVE SUBJECT INDEX T O VOLUMES I-IV

exosphere, 111, 327 gravitational anomalies, 111, 308 interior of the earth, 111,304 ionosphere, 111, 323 electrone density, 111, 323 meteoric particles, 111, 341 solar radiation, 111, 333 Seismographs, 11, 220 carrier-current transducer, 11, 237 components, 11, 254 electromagnetic, 11, 225, 228 moving conductor, 11, 225 Benioff, 11, 226 Ewing and Press’ long period, 11, 227 Galitzin, 11, 225 Sprengnether, 11, 226 Wenner, 11, 225 variable reluctance, 11, 228 Benioff, 11, 234 electrostatic transducer, 11, 235 Benioff, 11, 235 Gane, 11, 236 linear strain, 11, 244 pendulum, 11,224 McComb, 11, 224 Romberg, 11, 224 remote recording, 11, 252 response characteristics, 11, 269 torsion, 11, 220 Anderson and Wood, 11,220 Benioff, 11, 222 Lehner, 11, 223 McComb, 11, 224 Smith, 11, 223 Wenner, 11, 223 Seismology, V, 36, also see Seismographs Sky light, polarization of 111, 43 measurements of, 111, 76 new developments in, 111, 76 results of recent, 111, 82 molecular atmosphere, 111, 59 theory of, 111, 46 Rayleigh scattering, 111, 49 Stokes polarization parameters, 111,46 turbid atmosphere, 111, 72 Space field, smoothing and filtering of, IV, 351 Stokes’ formula, V, 107, 108, 111

Structure, subcontinental, 111,106 investigation techniques, 111, 107 dispersion of surface waves, 111, 115, 138 explosions, 111, 125, 126, 133 quarry blasts, 111, 125, 126, 129, 131, 134 reflection method, 111, 114 refraction method, 111, 107 low velocity layers, 111, 141 Surveying, aeromagnetic, I, 313 advantages, I, 342 applicability, I, 344 associated equipment, I, 322 aerial cameras, I, 322 altimeters, I, 322 electronic navigation aides, I, 322 basic instrument, I, 314 detector element, I, 315 detector output, I, 320 compilation of field data, I, 326 field technique, I, 323 interpretations of results, I, 329

T Tellurometer, V, 98 Tidal charts, V, 124 Dietrich and Villain’s V, 125 Harris’, V, 124 Priifer’s, V, 125 Sterneck’s, V, 125 Whewell and Airy’s, V, 124 Tides, earth, IV, 391; V, 148 effect on speed of rotation of earth, IV, 439 indirect effect of ocean tides on, IV, 411 static theory of, IV, 294 internal, V, 148 ionospheric, I, 221 oceanic, V, 117 amplitude of, IV, 401 Time series, smoothing and filtering of, IV, 351 Trace gases, atmospheric, IV, 44 carbon-dioxide, IV, 45 halogens, IV, 66 methane, IV, 67 nitrous oxide, IV, 57

CUMULATIVE SUBJECT INDEX TO VOLUMES I-IV

ozone, IV, 49 sulfur dioxide, IV, 64 Triangulation, V, 93, 94 electronic trilateration, V, 98 intercontinental connections, V, 97 Tropopause, V, 16

U Ultraviolet radiation, penetration of, V, 203

V Vertical, deflection of, see 1)eflection of the vertical

W Waves, ocean, forecasting of, 11, 168 Weather, forecasting, 11, 57 devices for, 11, 63 algebraic formulas, 11, 63 analogue, 11, 70 regression formulas, I I , M scatter diagrams, 11, 64 stratification of eases, 11, 71 tables of historical data, 11, 65 methods of objective, 11, 57

325

classification, 11, 62 limitations, 11, 59 purpose, 11, 58 suceess and future of, 11, 79 predictors, 11, 72 direct surface observations, 11, 73 surface weather charts, 11, 73 upper-air charts, 11, 74 upper-air observations, 11, 73 radar, advances in, 11, 1 precipitation patterns, 11, 4 “Angels”, 11, 29 clouds, 11, 27 continuous precipitation, 11, 7 hurricanes, 11, 23 lightning, 11, 25 showery precipitation, 11, 17 tornadoes, 11, 24 theory, 11, 32 attenuation, 11, 41 back scattering from spherical particles, 11, 33 fluctuations, 11, 42 quantitative observations, 11, 46 radar equation, 11, 32

Z Zenith telescope, IV, 424

This Page Intentionally Left Blank