Hurricanes (Facts on File Dangerous Weather Series)

Dangerous Weather

Hurricanes Revised Edition

Michael Allaby ILLUSTRATIONS by Richard Garratt

For Ailsa —M.A. To my late wife, Jen, who gave me inspiration and support for almost 30 years —R.G. Hurricanes, Revised Edition Copyright © 2003, 1997 by Michael Allaby All rights reserved. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For information contact: Facts On File, Inc. 132 West 31st Street New York NY 10001 Library of Congress Cataloging-in-Publication Data Allaby, Michael. Hurricanes / Michael Allaby.—Rev. ed. p. cm.—(Dangerous weather) Includes bibliographical references and index. ISBN 0-8160-4795-2 1. Hurricanes. I. Title. QC944 .A44 2003 551.55′2—dc21

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Facts On File books are available at special discounts when purchased in bulk quantities for businesses, associations, institutions, or sales promotions. Please call our Special Sales Department in New York at (212) 967-8800 or (800) 322-8755. You can find Facts On File on the World Wide Web at http://www.factsonfile.com Text design by Erika K. Arroyo Cover design by Nora Wertz Illustrations by Richard Garratt Printed in the United States of America VB Hermitage 10

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Contents Preface: What is a hurricane? Introduction

WHY HURRICANES ARE TROPICAL What happened when Mitch struck? Convection Lapse rates and stability Where hurricanes happen Intertropical convergence and the equatorial trough Jet stream Hurricane and storm tracks Weather fronts Global wind systems

AIR AND SEA Ocean currents and sea-surface temperature General circulation of the atmosphere Trade winds and doldrums George Hadley and Hadley cells Potential temperature Warming, convection, and low pressure Adiabatic cooling and warming Air pressure, highs, and lows Storm clouds How clouds are classified Latent heat and dewpoint Evaporation, condensation, and the formation of clouds

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1 1 4 6 9 11 16 16 19 22

24 24 26 33 35 36 37 41 43 46 47 50 52

INSIDE THE STORM How a hurricane begins Christoph Buys Ballot and his law Vortices The Coriolis effect Conservation of angular momentum What happens inside a hurricane Why the wind blows Wind force and Admiral Beaufort

57 57 59 64 69 71 72 73 78

HURRICANES, TYPHOONS, AND CYCLONES

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Hurricanes in the United States and Caribbean Hurricanes that reach Europe Air masses and the weather they bring Depressions and the jet stream Asian typhoons and cyclones Monsoon Arctic and Antarctic hurricanes

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WHAT A HURRICANE CAN DO Hurricane damage Kinetic energy and wind force Saffir/Simpson Hurricane Scale Daniel Bernoulli and how hurricanes can lift roofs Storm surges Historic hurricanes

LIVING WITH FIERCE STORMS How hurricanes are named and tracked How hurricane damage is predicted Will global climate change bring more hurricanes? El Niño The solar spectrum Protection and safety

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Appendixes Hurricanes in history Cyclone names SI units and conversions

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Bibliography and further reading

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Index

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Preface What is a hurricane? It begins far out at sea, way out to the east, over the central Atlantic. At first it amounts to nothing much, merely a body of air in which the atmospheric pressure is lower than in the surrounding air. This is a depression. There are no fronts associated with it, so it has not formed at the boundary between two large air masses with different temperatures and pressures, but for all that it is unexceptional. Had it formed farther north, say at around 50° N, it would be no different from the many depressions that each year drift eastward across the Atlantic. Some fill, as air flows into them, increasing the pressure until it is the same as that in the surrounding air, and no more is heard of them. Others bring dull weather, with rain or snow, to northwestern Europe. If there is a large difference in pressure between the depression and the surrounding air, they may also bring wind to drive the rain and make sure anyone caught outdoors without rain gear is thoroughly soaked. Depressions are unpleasant, but they cause no harm. They are not dangerous. But this is not 50° N. It is the Tropics, and already the depression is being watched carefully by scientists alert for the first sign of trouble. They study photographs of the clouds forming inside the disturbance, transmitted by a satellite high above. These allow the scientists to trace how the depression develops and track its movements. Ships and aircraft passing through the system radio measurements of the winds, pressure, and air temperature around them to the weather center. Perhaps the depression will fill. In that case it and the clouds it produces will vanish from the satellite pictures. This depression does not fill. The pressure at its center falls further. After a couple of days it has dropped by 20 mb. Again, a drop in pressure of this magnitude is not uncommon in middle latitudes, but in the Tropics it is unusual. At this point the meteorologists start to pay even closer attention.

Starting to turn As air pressure falls, the depression begins to turn counterclockwise, rotating majestically around its own center, and the prevailing easterly winds start to carry it westward. Air is drawn into the region of low pressure, its speed proportional to the difference in pressure inside and outside the depression. Falling pressure causes wind speeds to increase.

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Hurricanes Clouds grow around the center, building into great, towering masses. Seen from space, they form a spiral shape. Beneath them, the rain falls in torrents. When the wind speed exceeds about 25 MPH (40 km/h), the weather system is officially classified as a tropical depression and identified by a number, for example TD14. When the wind speed exceeds about 40 MPH (64 km/h), it becomes a tropical storm and may be given a name. Tropical storms are fairly common during the summer and fall. Forecasters considered 2001 an average year. There were 15 named tropical storms over the Atlantic in that year, of which nine grew into hurricanes. Four of those were classed as major hurricanes—Saffir/Simpson category 3 or higher. In 2000 there had been 15 tropical storms. Forecasters expected 2002 to be another average year for Atlantic storms. They predicted there would be 11 named tropical storms, of which six would develop into hurricanes, two of them major. Pressure continues to fall and wind speed continues to increase. When it exceeds 74 MPH (119 km/h), the tropical storm becomes a hurricane. Of the 15 tropical storms in 2000, eight grew into hurricanes. Still it moves westward and still it is strengthening. Now the spiral of cloud is about 125 miles (202 km) in diameter. As it enters the Caribbean and approaches the first of the inhabited islands, it begins to swing north.

Lashing rain, screaming wind It arrives with lashing rain and a screaming wind. Trees are uprooted and thrown about like sticks. Flimsy buildings are demolished. Roofs are lifted from more substantial houses, cars and trucks are overturned, windows are shattered, and flying debris adds to the damage caused by the wind itself. Then the sea arrives. Driven by the wind, huge waves sweep over lowlying coastal areas. Winds of more than 100 MPH (160 km/h) can raise 15-foot (4.5-meter) waves and these may combine with the tide to produce a massive storm surge, with spray and foam blowing still farther inland. By now the hurricane has reached its maximum ferocity. Its winds are much stronger on the right-hand side than on the left-hand side. As the diagram shows, this is because the hurricane itself is moving in a generally westward direction. To the right of its center, the speed of its own movement adds to the wind speed. To its left, the wind blows in the opposite direction to that in which the hurricane is traveling, so wind speed is reduced. It turns north, heading now toward the coast of the United States and devastating the islands in its path. In September 1999, Hurricane Floyd struck the Bahamas with 155-MPH (250 km/h) winds, then moved northward along the U.S. coast as far as New York and New Jersey. It caused at least 57 deaths and meant 2.3 million people had to be evacuated from their homes in Florida, Georgia, and the Carolinas. Estimates of the cost of the damage it caused to property ranged from $3 billion to more than $6 billion. Better preparedness and highly efficient emergency services mean hurricanes are less deadly than they used to be. In 1900, a hurricane caused 8,000 deaths in Galveston, Texas.

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direction of storm movement (northwest) speed 55 mph 230

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Wind speeds around a hurricane. The movement of the whole storm means the wind speed is higher on one side than on the other. Once it moves inland, however, the hurricane is doomed. It needs water to sustain it, and away from the sea it is starved. Slowly it weakens and eventually dies, although it may retain enough force to cause substantial damage as far north as Pennsylvania. A hurricane releases no radiation, of course, but it may have as much energy as a one-megaton hydrogen bomb, and it can be as destructive. It is the largest, fiercest storm our atmosphere is capable of producing.

Introduction Several years have passed since the first edition of this book was published. Much has happened during those years, and the decision to update the book for a second edition gives me a welcome opportunity to report at least some of them. In doing so, I have substantially altered, expanded, and in some places rewritten the original text. There have been more hurricanes, of course. I wrote the first edition before Hurricane Mitch devastated Central America in 1998. That was one of the most savage storms of the century. Overall, though, hurricanes are not growing fiercer or more frequent. Indeed, in some parts of the world they are now much rarer than they used to be. All the same, no matter how the climates of the world may change in years to come, there is little chance that hurricanes will disappear entirely. Climate research has intensified in recent years. Concern over the possibility that we may be altering the global climate has stimulated funding agencies to increase the resources available for evaluating the likelihood of global warming and its consequences. If we are to understand the extent of this threat—if it is a threat—scientists need to learn much more about the ways the Sun, atmosphere, and oceans interact to produce our day-to-day weather. New discoveries are now being made at an unprecedented rate, and, although there is still a long way to go before the global climate is fully understood, we are learning more about it almost every day. This new edition takes account of the most recent relevant findings. Updating the text has also given me an opportunity to expand it in order to provide more detailed explanations. Mitch deserves, and receives, a chapter to itself. I have retained the use of sidebars to display detailed explanations or interesting items of information without interrupting the main flow of the text. This edition contains more sidebars than were found in the first edition. These explain concepts from atmospheric science, such as potential temperature, the link between the jet stream and midlatitude depressions, why the wind blows at all, and the conservation of angular momentum—the principle that explains the way hurricane winds accelerate as air spirals inward. Measurements are given in familiar units, such as pounds, feet, miles, and degrees Fahrenheit, throughout the book, but in each case I have added the metric or scientific equivalent. All scientists now use standard international units of measurement. These may be unfamiliar, so I have added them, with their conversions, as an appendix. I have also

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Hurricanes added appendices that list some of the most notorious hurricanes of the past and the set of names that are used to identify tropical cyclones in different parts of the world. The first edition contained no suggestions for further reading. These have been added in this edition. The sources include a number of books that you may find useful, but a much larger number of Web addresses. If you have access to a computer, these will allow you to learn more about hurricanes and about climate generally quickly and free of charge. We have decided to omit the photographs from the first edition and instead to increase the number of diagrams and maps. These provide more useful information than photographs of such subjects as flying roofs and wind-bent palm trees. My friend and colleague Richard Garratt has drawn all of the illustrations. As always, I am deeply grateful to him for his skill in translating my crude drawings into such accomplished artwork. I am grateful, too, to Frank K. Darmstadt, my editor at Facts On File, for his hard work, cheerful encouragement, and patience. If this “new, improved” edition of Hurricanes encourages you to pursue your study of the weather further, it will have achieved its aim and fulfilled my highest hopes for it. I hope you enjoy reading the book as much as I have enjoyed writing it for you. — Michael Allaby Tighnabruaich Argyll, Scotland www.michaelallaby.com

What happened when Mitch struck?

WHY HURRICANES ARE TROPICAL What happened when Mitch struck? It helps to know the storm is approaching. You can bolt the doors, seal the windows, and take shelter or simply evacuate. Nowadays the warnings from the National Weather Service give people ample time to prepare. That is why hurricanes cost fewer lives today than did those that people endured a century ago. But there are exceptions. Very rarely there is a storm so ferocious and so big that for many people there can be no escape. A storm like that happened in 1998. That was the year the Caribbean was hit by the deadliest storm to strike the region since the Great Hurricane of October 1780. It disrupted communications to such an extent that a week passed before the outside world received the first news of the scale of the devastation. Mitch was born on October 8, as a disturbance called a tropical wave in the airflow over southern West Africa. The wave crossed the African coast, then traveled over the Atlantic, where the west-southwesterly highlevel winds prevented it from developing further. It passed through the eastern Caribbean Sea on October 18, and by October 20 satellite images showed that an organized cloud pattern was developing. On October 21, what was then a tropical depression developed in the southern Caribbean Sea. By the following day the depression had intensified. When the wind speed around its center exceeded 25 MPH (40 km/h), the depression was reclassified as a tropical storm. It had become Tropical Storm Mitch, its cloud pattern marking it clearly on the photographs being transmitted to the storm watchers on the ground by the U.S. GOES-8 satellite, stationary over the equator. Mitch was hardly moving, but already it was vigorous. Its winds were blowing at about 45 MPH (72 km/h)—a fresh gale on the Beaufort scale of wind force—and strengthening fast. The pressure at the center had dropped to 29.5 inches of mercury (1,000 millibars). This is only slightly lower than the average sea-level pressure, of 29.9 inches of mercury (1,013.25 mb). At that stage Mitch was far out at sea, approximately due south of the western tip of Jamaica and due east of central Nicaragua. During October 23, Mitch drifted a little to the northeast, farther away from the Central American mainland, and it continued to intensify. Its core pressure fell to 29.4 inches of mercury (997 mb). The drop

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Rain and mud Mitch moved very slowly during the last few days before it crossed the coast. Inflowing air gradually filled the depression, and as the core

What happened when Mitch struck?

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Track of Hurricane Mitch, October –November 1998 pressure rose, so the winds slackened. Convection continued, however (see sidebar). Convection produced and sustained the towering storm clouds— the clouds that make all hurricanes so clearly visible in satellite images. The storm clouds produced torrential rain. The wind may have weakened, but the rain was incessant. In total, Mitch is estimated to have dumped about 75 inches (1,900 mm) of rain onto Central America. In places the rain was sometimes falling at the rate of 12–24 inches (305–610 mm) a day.

Convection When a fluid—a gas or liquid—is heated, its molecules move faster and farther between collisions. Consequently, the fluid expands. Expansion means that a given volume of the fluid contains fewer molecules than it did before the fluid expanded. This must be so, because the same amount of fluid is now taking up more space. Because it contains fewer molecules, the expanded fluid is less dense than fluid that has not expanded. At this point gravity takes over. Volume for volume, a dense fluid weighs more than one that is less dense. Place the two side by side and the dense fluid will sink downward, displacing the less dense fluid. This pushes the less dense fluid upward. So the dense fluid sinks and the less dense fluid rises. It is not that the less dense fluid rises of its own accord, simply because it has become less dense. For that to happen some kind of antigravitational force would have to be acting on it. It does not rise by itself, but is pushed upward by the denser fluid sinking beneath it.

Suppose that a fluid is heated from below. This is the way the atmosphere is heated, by contact with the land and sea surface. The air in contact with the surface grows warmer, so it expands and becomes less dense. Denser air sinks to displace it, and so the warm air rises. As it rises, the air moves farther and farther from the source of heat, and so it cools and as it does so its density increases (see sidebar, page 41). Meanwhile, the air that sank to displace it is now in contact with the source of heat and is being warmed. So it now expands, denser air displaces it, and the process continues. This is convection, one of the three processes by which heat is transferred from one place to another (the others are radiation and conduction). Once the pattern is established, fluid circulating vertically by convection forms what is called a convection cell. The diagram shows what happens. Air in contact with the surface of the warm, tropical ocean is heated and rises by convection. Convection is the mechanism that sustains a hurricane.

warm water rises, cools at surface, then sinks

Convection

heat

What happened when Mitch struck? As Mitch approached Honduras, the first place to suffer was the popular vacation island of Guanaja, where tourists enjoy sailing and diving in ordinarily tranquil tropical waters. Guanaja, 11 miles long and 3.5 miles wide (18 × 6 km), about 40 miles (64 km) from the coast, took the full force. The island was devastated, and for a time it lost all contact with the outside world. Then the hurricane reached the mainland of Honduras, close to its border with Nicaragua. Honduras is a mountainous country, and as Mitch moved inland it was forced to rise as it crossed the mountains. This made the air even more unstable (see sidebar), squeezing still more water out of the sky. Mitch crossed Honduras, then attacked Guatemala, Belize, and El Salvador. By November 2, the storm had weakened to nothing more alarming than a tropical depression. That is when, still drifting northwestward, it crossed the Yucatán Peninsula in Mexico and reached the warm waters of the Bay of Campeche. This reinvigorated the storm, and Mitch was once more classified as a tropical storm. It changed track, now heading northeastward, across the western side of the Yucatán Peninsula and back into the Gulf of Mexico. Now it was heading straight for Florida. It reached Key West on November 4, traveling at 26 MPH (42 km/h) and with winds gusting to nearly 80 MPH (130 km/h). During the rest of that day and the next Mitch dropped between 6 and 8 inches (150–200 mm) of rain over the southern part of the state. It also generated several tornadoes. One traveled from Marathon to Key Largo, injuring at least seven people. The tornadoes injured a total of 65 people and damaged 645 homes. Another tornado caused damage at Miramar, north of Miami. Around 100,000 people lost their electrical power. Two people died in Monroe County when a fishing boat capsized. The cost of the damage caused by Mitch in the United States was estimated at $40 million. On November 5, Mitch moved away from Florida toward the Bahamas. By this time it had been downgraded to an extratropical storm. It left the Tropics and finally dissipated.

Honduras Of all the countries Mitch touched, Honduras suffered the most. At its most intense, the hurricane was producing more than 4 inches (102 mm) of rain an hour. The town of Choluteca received 18.37 inches (467 mm) in a single day and 35.89 inches (912 mm) over the entire period of the storm, from October 25 through 31. This was the heaviest rainfall, but La Ceiba was not far behind, with 11.19 inches (284 mm) on a single day and a storm total of 34.52 inches (877 mm). To put this in perspective, in an average year New York City receives 3–4 inches (76–102 mm) of rain a month; about 17.5 inches (445 mm) falls between January 1 and May 31.

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Lapse rates and stability Air temperate decreases (or lapses) with increasing height. The rate at which it does so is called the lapse rate. When dry air cools adiabatically, it does so at about 5.5°F for every 1,000 feet (10°C per km). This is known as the dry adiabatic lapse rate (DALR). When temperature of the rising air has fallen sufficiently, its water vapor will start to condense into droplets. This temperature is known as the dewpoint temperature, and the height at which it is reached is called the lifting condensation level. Condensation releases latent heat, which warms the air. Consequently, thereafter the air cools at a slower rate, known as the saturated adiabatic lapse rate (SALR). The SALR varies, but averages 3°F per 1,000 feet (6°C per km). The actual rate at which the temperature decreases with height is calculated by comparing the surface temperature, the temperature at the tropopause (in middle latitudes about –67°F; –55°C), and the height of the tropopause (about 7 miles;

11 km in middle latitudes). The result is called the environmental lapse rate (ELR). If the ELR is less than both the DALR and SALR, rising air will cool faster than the surrounding air, so it will always be cooler and will tend to subside to a lower height. Such air is said to be absolutely stable. If the ELR is greater than the SALR, air that is rising and cooling at the DALR and later at the SALR will always be warmer than the surrounding air. Consequently it will continue to rise. The air is then absolutely unstable. If the ELR is greater than the DALR but less than the SALR, rising air will cool faster than the surrounding air while it remains dry, but more slowly once it rises above the lifting condensation level. At first it is stable, but above the lifting condensation level it becomes unstable. This air is said to be conditionally unstable. It is stable unless a condition (rising above its lifting condensation level) is met, whereupon it becomes unstable.

SALR

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Lapse rates and stability. If the environmental lapse rate (ELR) is less than both the dry (DALR) and wet (SALR) adiabatic lapse rates, the air is absolutely stable. If the ELR is greater than the SALR, the air is absolutely unstable. If the ELR is less than the SALR but greater than the DALR, the air is conditionally unstable.

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What happened when Mitch struck? The devastation affected the entire country and was so great that President Carlos Flores issued an appeal to the international community: “We will get back on our feet . . . May the Lord illuminate us and give us all strength. We are making an urgent and anguished appeal to the international community, to all countries, to international financial organizations and to aid organizations so that they heed this SOS. Our capacity for suffering and pain was never before put to such a hard test.” The official estimate was that 5,657 Hondurans had been killed, 8,052 were missing and unaccounted for, and 11,762 were injured. More than 400,000 people were living in temporary shelters. Altogether 1.9 million people were affected. More than 25 small villages had been washed away completely. More than a week after the hurricane had passed there were still survivors clinging to rooftops. Supplies had to be carried by helicopter, because most of the country’s secondary roads had been destroyed and more than 90 bridges had been wrecked or damaged. In the capital city, Tegucigalpa, some buildings that were more than 350 years old were washed away, and one-third of all the buildings in the city were damaged by flooding. One entire neighborhood was demolished, and many of the damaged buildings that remained standing were structurally unsound, in danger of falling. In the country as a whole, at least 70,000 houses were damaged or destroyed. Out in the countryside, at least 70 percent of all farm crops were destroyed, including 80 percent of the banana crop. Coffee warehouses and storage facilities were flooded. The value of the lost crops was estimated at $900 million. The total damage throughout Honduras was estimated at more than $5 billion.

Nicaragua On October 29 and 30, the rain bands associated with Mitch became stationary over the western part of the country. During that time the land was lashed by tens of inches of rain. The Malacatoya River rose by more than 50 feet (15 m), overflowing roads and destroying them. On Friday, October 30, a lake in the crater of Mount Casita, a dormant volcano, overflowed and part of the crater wall collapsed. This released a mudslide that moved in a southwesterly direction, eventually covering an area 10 miles (16 km) long and 5 miles (8 km) wide. At least four villages between Casita and the town of Posoltega were completely buried in mud. More than 2,000 people died in the mudslide, and some survivors were stranded for several days, until the mud dried sufficiently for rescuers to be able to walk across it. Entire landscapes were restructured. Rivers widened and in some places adjacent rivers merged. New lakes appeared and hills were washed away. The death toll for the entire country was about 3,800, with up to 7,000 missing. More than half a million people lost their homes. Farm crops were devastated. The total cost of all the damage was estimated at about $1 billion.

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El Salvador The west of the country suffered more severely than any other region. Mitch drew in moisture from the Pacific, and that is where the storm dropped it as rain. Estimates of the numbers of dead and missing varied. About a week after the hurricane passed, the National Emergency Committee of El Salvador estimated that 239 people had died and there were 135 missing. The Red Cross estimated there were 400 deaths and 600 people were missing. It is unlikely that the true number of deaths will ever be known. In all, almost 56,000 people were affected and around 10,000 houses were destroyed. Many more were damaged.

Guatemala Mitch arrived on Sunday, November 1, and moved northwestward across the country. The authorities had time to evacuate nearly 6,000 people from the path of the storm, but on November 4 the Red Cross estimated that 27,000 Guatemalans were still housed in shelters, unable to return home. The floods destroyed or severely damaged approximately 19,000 homes, 32 bridges, and 40 roads. The hurricane also destroyed about 95 percent of the country’s banana crop, 50–60 percent of the other farm crops, and almost onethird of the cattle. Officials estimated that 258 people lost their lives and 120 were missing.

Belize The authorities knew Mitch was coming and evacuated more than 75,000 people from Belize City and the offshore islands. They were accommodated in temporary shelters in the capital, Belmopan. Flood damage was extensive to houses and farm crops, but the timely evacuation saved many lives. Mitch left 11 people dead or missing.

Costa Rica Although Costa Rica lay far to the south of the hurricane’s track, nevertheless heavy rain caused flooding along the whole of its Pacific coast. Many people had to be evacuated, and up to seven people were killed.

Mexico The country was well prepared. There were plans to evacuate threatened areas, and the Mexican Red Cross positioned emergency teams in the Yucatán Peninsula. Nine people were killed, five of them in a car that was washed from the road near Tapachula, in the southwest of the country close to the Guatemala border.

Where hurricanes happen

Recovery Mitch struck with such ferocity and caused such widespread destruction that it will be years before the worst affected countries recover fully. The United Nations, as well as U.S. AID and many nongovernmental organizations responded with help. The United States provided $80 million in assistance. After visiting the region, former Presidents George Bush and Jimmy Carter called for the restructuring and scaling back of the international debt owed by Honduras and Nicaragua. Spain contributed $105 million, and Sweden said it would donate $100–200 million over three years. The humanitarian organizations flew in food and emergency goods and materials. Recovery will take time and the sorrow of bereavement will take time to heal. Mitch demonstrated the power our atmosphere is capable of wielding and the vulnerability of our towns and cities, roads, bridges, and crops. The scale of the disaster was appalling. Yet, in time, there will be a full recovery. Catastrophe draws people together and forges links that are not quickly broken when the worst of the pain has passed.

Where hurricanes happen Hurricanes begin as tropical depressions. These are areas where the atmospheric pressure is just a little lower than it is in the air surrounding it, and they are strictly a tropical phenomenon. It would be impossible for a hurricane to form over Minnesota, for example, or over Europe, although a hurricane traveling from the Tropics might reach such places. It would have greatly weakened during its long journey away from the sources of the energy it needs to sustain it (and meteorologically it would no longer be classed as a hurricane, because some of its essential characteristics would have changed). Hurricanes form in the Tropics because it is only in the Tropics that the necessary conditions ever occur. The map shows where they form and the directions in which they move from their “breeding grounds.” What we in the Western Hemisphere call a hurricane goes by different names in other parts of the world, although nowadays the name hurricane is tending to replace the other names—if you call it a hurricane, everyone will know what you mean. If it forms in the Bay of Bengal, however, its traditional name is cyclone (which, confusingly, is also the meteorological term for a depression in middle latitudes). Over most of the Pacific it is called a typhoon, although near Indonesia and the Philippines it is sometimes called a baguio. Baguio is the name of a town in Luzon, Philippines, unfairly blamed for the catastrophically violent weather that so often assaults the islands. If it occurs near Australia, it is a typhoon, but some people call it a willy-willy (or willy-willi or willynilly). Willy-willy also describes a dust storm or desert whirlwind, and

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Hurricane-prone areas. The map shows the areas where hurricanes develop and their direction of travel.

although you will still find the name in books, meteorologists no longer use it. These are all the same, despite their different names. Southerly winds along the western coast of Mexico generated by a hurricane in the southeastern North Pacific Ocean are known as cordonazo de San Francisco, or the “lash of St. Francis.” The name refers to the fact that these winds are most likely to occur around October 4, which is the Feast of St. Francis. Meteorologists call all of them tropical cyclones, although they, too, are tending increasingly to use the name hurricane. The word hurricane is from the Spanish huracán, derived in turn from Hurakán, the Caribbean god of stormy weather. Typhoon has two derivations. The Greek typh on ¯ means whirlwind and, more probably in this case, the Cantonese tai fung means “big wind.” A cyclone is an area of low atmospheric pressure; its opposite is an anticyclone, which is an area of high pressure. As the name suggests, a tropical cyclone is one that forms in the Tropics. Like cyclones elsewhere in the world, it is a distinct area of low pressure. In the Tropics it becomes much more intense than in higher latitudes, but otherwise it is similar.

How it begins For a tropical cyclone to develop, first there must be a fall in atmospheric pressure over a fairly large area. The difference need not be great. A drop

Where hurricanes happen

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of a mere 0.6 inch of mercury, or 0.3 lb. per square inch (20 mb) over about two days is sufficient. This is enough to produce only a fairly weak depression. A fall of this magnitude is common in temperate latitudes, although it is unusual in the Tropics, where the air pressure is fairly constant over very large areas. Under certain circumstances this small fall in pressure is enough to trigger the development of a more persistent tropical depression from which a hurricane (tropical cyclone) may grow. There are several events that can cause this gentle drop in pressure. Depressions much deeper than this are constantly developing, traveling eastward, and filling in middle latitudes. A pocket of low-pressure air can sometimes become detached from the edge of one of these middle-latitude weather systems. It can then spill over into the Tropics as a tongue of low pressure (called a trough) that extends toward the equator at a high altitude. Alternatively, a low-pressure system on land may drift out over the sea, or a wave may develop along the equatorial trough (see sidebar). This

Intertropical convergence and the equatorial trough The trade winds blow toward the equator in both hemispheres. Consequently, there is a region close to the equator where the winds meet. When two bodies of air flow toward each other they are said to converge. The meeting of the trade winds is therefore the intertropical convergence, and the region where it happens is the intertropical convergence zone (ITCZ). Because the ITCZ forms a boundary between air from the two hemispheres, it is sometimes called the intertropical front (ITF), although it is not a front strictly comparable to those in middle latitudes between polar and tropical air.

The ITCZ is more strongly developed over the oceans than over the continents, and even over the oceans it is evident only as an average. The convergence of the trade winds varies in strength and disturbances form in it and then travel westward. The ITCZ rarely occurs in the areas affected by the doldrums (see the sidebar on trade winds and doldrums in the section on ocean currents and sea-surface temperatures, page 33). The position of the ITCZ changes through the year. The map shows its approximate location in February and August, as this is revealed by bands of clouds that are clearly visible on satellite images. As (continues) A

30°N A

A equator F

F February

A

A

F

F

F F F

F

30°S

ITCZ. The map shows the approximate position of the ITCZ and equatorial trough in February and August.

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(continued) the map shows, the ITCZ is more often to the north of the equator than to the south, and it seldom coincides with the equator itself. Instead, its position coincides with the thermal equator. This is the region where the surface temperature is highest. Any change in the sea-surface temperature is likely to cause the ITCZ to move. The highest sea-surface temperature also produces the most highly developed convection, with the formation of convective clouds and heavy rain.

Both convergence and convection cause air to rise. This reduces air pressure near the surface and produces a region of high pressure in the upper air, where air diverges. The diagram shows what happens. The low surface pressure is known as the equatorial trough. The trough does not coincide precisely with the ITCZ, but is a short distance from it on the side farthest from the equator.

high

direction of air flow

Convergence. Air converges and rises, producing low surface pressure and high pressure in the upper troposphere.

low

will produce a depression that detaches itself and moves away from the equatorial trough, which reforms behind it. No matter what causes it, once it has formed the depression moves westward as an easterly wave (“easterly” because it comes from the east and “wave” because it produces a kink or wave in the otherwise regular flow of air).

Need for warmth and Coriolis Minor depressions can form anywhere, but one will grow into a hurricane only if it crosses a large expanse of very warm sea, where the surface

Where hurricanes happen temperature is at least 76°F (24°C); hurricanes are most likely to form where the sea-surface temperature is about 80°F (27°C). At any lower temperature convection is just not vigorous enough to generate a storm the size of a hurricane. This confines the birthplace of hurricanes to the Tropics. In latitudes higher than about 20° the sea-surface temperature is usually too low. Close to the equator the sea is often warm enough to start a hurricane, but hurricanes never form in latitudes lower than 5°, because this close to the equator moving air follows paths that are very gentle curves—almost straight lines, in fact. There is nothing to set the circulation turning. What is needed is the Coriolis effect, to swing the air moving toward the lowpressure area into a circular path. There is no Coriolis effect at the equator (see sidebar, page 69), and within 5° of the equator it is not strong enough to cause the necessary swing. Vorticity (see the section on vortices, page 64) will cause moving air to swing into a curved path and eventually start it rotating. After that the conservation of its angular momentum (see sidebar, page 71) will accelerate the air as it converges into an ever smaller radius. Without assistance from the Coriolis effect, however, this will not generate winds of hurricane force. For a hurricane to form closer than 5° from the equator by vorticity alone, air would need to converge on the low-pressure region from such a vast area that there is simply not enough air available. Even at 5° latitude, air within a radius of more than 300 miles (480 km) would need to contract to a radius of about 20 miles (32 km) to produce winds of about 100 MPH (160 km/h). In contrast, at latitude 20°, air contracting from a radius of about 90 miles (145 km) to 20 miles would produce 100-MPH winds. These areas are calculated without taking account of friction, which reduces angular momentum and slows the rate at which wind speed increases. Allow for friction, and the areas must be increased. Taken together, the need for a high sea-surface temperature and a sufficiently strong Coriolis effect confine the region in which hurricanes can form to a belt over the oceans between latitudes 5° and 20° in both hemispheres. They also restrict the time of year. The ocean needs time to warm up after the winter, even in the Tropics, and it is usually only in late summer and fall that the water is warm enough, although occasionally hurricanes develop outside these seasons.

Hurricanes occur in the west Some tropical cyclones form in the east of the North Pacific, but the great majority do not develop until a tropical depression has crossed to the western side of an ocean. This is also due, indirectly, to the Coriolis effect. Over the Tropics, air moves vertically in Hadley cells (see sidebar, page 35). As it moves away from the equator at a great height, the Coriolis effect causes the air to swing to the right in the Northern Hemisphere and to the

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Hurricanes left in the Southern. In both hemispheres, this is a swing to the east, and it makes the layer of high-level air moving away from the equator deeper on the eastern side of the Hadley cells than on the western side. This highlevel air sinks while still over the Tropics and warms adiabatically (see sidebar, page 41). This limits the upward movement of warm air rising from the surface by convection, because the rising air meets a layer of air that is less dense than itself and can rise no farther. A layer of warm air overlying cooler air is called a (temperature) inversion, and air rising from below cannot usually penetrate it. This one is known as the trade wind inversion. Sometimes the convection currents are vigorous enough to break through the trade wind inversion, but it is much easier for them to do so on the western side of the Hadley cells than on the eastern side. This concentrates areas where tropical cyclones develop on the western side of oceans. It is only there that condensation in the rising air can reach a high enough altitude to fuel the storm. Depressions can form over land or sea, in dry or moist air. For it to develop into a hurricane, however, a tropical depression must gather enough water vapor to provide it with a layer of very moist air. This layer must be deep enough to supply the depression with the latent heat of condensation it needs to maintain the instability of the air, which is what sustains it (see sidebar, page 6). The only way a tropical depression can find the water it needs is by traveling a long distance over a warm sea. If the depression moves over a continental land mass, it will not gain moisture and therefore it will not develop into a hurricane. Many tropical depressions end in this way. Those that do grow into hurricanes have already crossed an ocean—and since tropical weather systems travel from east to west, driven by the prevailing easterly winds, that is another reason why most hurricanes begin on the western side of oceans.

Equatorial trough and the jet stream There is one more difficulty. The equatorial trough rarely lies exactly at the equator. It moves north and south with the seasons, and hurricanes develop only within about 60 miles (96 km) of it. In the South Atlantic the trough never moves south of 5°. This is too close to the equator for the Coriolis effect to exert a strong enough influence on moving air, and so hurricanes never occur in the South Atlantic. It is not only the equatorial trough that moves with the seasons. So does the jet stream (see the sidebar, page 16). In winter it is quite far to the south, crossing northern Florida, but in summer it moves north. The map shows its approximate positions in January and July. In fact, there are two jet streams, the subtropical and polar, but the subtropical is by far the more constant, and so the term jet stream usually refers to the subtropical jet stream, which is the one shown here. The position of the jet stream is important, because a tropical cyclone cannot develop unless air is able to spiral upward unhindered. This requires that the wind speed and direction are more or less the same at all

Where hurricanes happen

15

heights. (The technical term is to say that there is little or no wind shear.) Often they are not, a fact pilots use by studying the winds at different heights to decide the best altitude at which to fly. The spiraling air will be dissipated if it encounters a strong wind, and for this reason tropical cyclones cannot develop directly beneath the jet stream. It is very unlikely, therefore, that a hurricane could develop over the Caribbean Sea in winter, because the jet stream is then much too close (and it also blows more strongly in winter than in summer). In summer, however, any southward movement by the jet stream can encourage the development of tropical cyclones. It will never come so close as to interfere with their spiraling structure, but the edge of it may carry away air from the top of the cyclone, which will intensify the storm by drawing more air upward. By late summer and fall, the jet stream is starting to move south, so this is a favorable time for tropical cyclones to grow. The area in which hurricanes can be born, therefore, is quite strictly confined. They develop on the western side of oceans. Their requirement for warm water restricts them to latitudes lower than 20°, but the need for the Coriolis effect to set them turning means they are unable to develop within 5° of the equator. They must also be between 5° and 10° of the equatorial trough (to its north in the Northern Hemisphere and to its south in the Southern) when the trough itself is more than 5° away from the equator. Many begin to form in or close to the doldrums (see sidebar, page 33), where the air is moving only slowly and they can grow

Jet stream. The map shows the approximate position of the mean jet stream in January and July, and the regions where wind speed is greatest.

July position January position Areas of maximum wind speed

30°N

equator

30°S

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Hurricanes

Jet stream During World War II, when high-altitude flying was new, aircrews sometimes found their journey times radically different from those they had calculated prior to takeoff. The effect was not reliable enough to predict, but when flying from west to east they could find their airspeed dramatically increased, and when flying in the opposite direction they were just as dramatically slowed. They had discovered a narrow, wavy, ribbon of wind blowing at speeds comparable to those of their aircraft. They called it the jet stream. If they approached the jet stream from above or below, pilots found the wind speed increased by about 3.4–6.8 MPH for every 1,000 feet of altitude (18–36 km/h per 1,000 m). If they approached from the side, it increased by the same amount for every 60 miles (100 km) of distance from the core of the jet stream. At the center of the stream the wind speed averages about 65 MPH (105 km/h) but it sometimes reaches 310 MPH (500 km/h). There are several jet streams. The polar front jet stream is located between about 30° N and 40° N in winter and about 40° N and 50° N in summer. There is an equivalent jet stream in the Southern Hemisphere. The subtropical jet stream is located at

about 30° throughout the year in both hemispheres. These jet streams blow from west to east in both hemispheres. In summer there is also an easterly jet at about 20° N extending across Asia, southern Arabia, and into northeastern Africa. This jet stream blows from east to west. The jet streams are thermal winds. That is to say they are generated by the sharp difference in temperature across the front separating two air masses. This difference is greatest close to the tropopause, which is why the jet streams occur at high altitude—the polar front jet stream at about 30,000 feet (9,000 m) and the subtropical jet stream at about 40,000 feet (12,000 m). The polar front jet stream is associated with the polar front, separating polar air and tropical air. The temperature difference responsible for the subtropical jet stream occurs only in the upper troposphere, on the high-latitude side of the Hadley cells. The polar jet stream is quite variable, and often it is not present at all. The subtropical jet stream is more constant. Consequently, the term jet stream often refers simply to the subtropical jet stream, and this is the one that is usually shown on maps.

undisturbed. They develop in late summer and fall, when the sea has warmed, and when both the equatorial trough and the jet stream have started to move south.

Hurricane and storm tracks On average, two-thirds of all the tropical cyclones that occur each year happen in the Northern Hemisphere, and half of those begin over the western side of the North Pacific Ocean. They are Asian typhoons, and many of them scream into the China Seas to assault the coasts of China and Japan. Hurricanes, affecting the Caribbean Sea and western North Atlantic Ocean, account for only one-sixth of the Northern Hemisphere

Hurricane and storm tracks total and only a little more than one-tenth of the world total. About one in 10 of all the tropical cyclones that occur in the Northern Hemisphere are cyclones—the name given to storms that develop in the northern Indian Ocean (those in the southern Indian Ocean are typhoons). Tropical cyclones of all kinds occur mainly in the late summer and fall. Atlantic hurricanes and Asian typhoons are most likely to strike between July and October, baguios in the region of Indonesia and the Philippines between September and November. In the Southern Hemisphere, most tropical cyclones develop between December and March. Typhoons in the Indian Ocean usually form near Madagascar and South Pacific typhoons near Australia. In summer, the intertropical convergence zone (ITCZ) and equatorial trough move north. The map in the sidebar on page 11 shows their approximate positions in February and August. As the ITCZ crosses the Bay of Bengal in May or June, cyclones develop close to it and in September, as it moves south again, there is a second cyclone season.

Carried by the wind Tropical cyclones develop in air that is moving westward. This reflects the trade-wind air movement in the Tropics (see the sidebar, page 33). Should they leave the Tropics, the storms enter a region where the prevailing winds blow in the opposite direction, from west to east, and they are affected by that movement. Air pressure is generally low in the Tropics, due to the ITCZ and equatorial trough, but around 30° from the equator lies the edge of a subtropical belt of mainly high pressure. Tropical cyclones form and travel not far from this region, and many reach the edge of a local high-pressure system (an anticyclone). In the Northern Hemisphere air circulates in a clockwise direction around areas of high pressure (and counterclockwise in the Southern Hemisphere). This anticyclonic circulation will carry with it any storm that approaches closely enough to be caught. It is the entire storm that moves, of course, and its anticyclonic movement around an area of high pressure has no effect on the speeds of the winds blowing cyclonically (counterclockwise in the Northern Hemisphere, clockwise in the Southern) around its low-pressure center. As a hurricane approaches one of the subtropical anticyclones on the side nearest the equator, it swings around the edge on a track that carries it away from the equator. Most hurricanes are then traveling at 10 to 15 MPH (16 to 24 km/h), but as they enter higher latitudes they sometimes accelerate to twice that speed. Storms that form in middle latitudes are also carried by the prevailing winds, which in this case are westerlies. As the map shows, storms that form in lower latitudes, over the Gulf of Mexico and off Cape Hatteras, are immediately caught in the subtropical anticyclonic circulation. This carries them northward and into the prevailing westerly winds.

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Storm tracks across North America. In middle latitudes, storms travel from west to east, carried by the prevailing westerlies. Storms that form in the Gulf of Mexico travel north and then east.

Hurricanes

Colorado o

Gul

These North American storms form over land or in coastal waters, and they can be severe, with thunderstorms and winds of gale force. In places they can produce tornadoes. Tornadoes often form beneath the eyewalls of tropical cyclones, too, but there the similarity ends. Apart from covering a much larger area and being a great deal fiercer, tropical cyclones grow from slight disturbances in air that is otherwise at almost exactly the same temperature and pressure over a large area. Midlatitude storms, on the other hand, form in association with weather fronts—the boundaries between masses of air at different temperatures and pressures (see sidebar, page 19). Over the world as a whole, the total force of all the westerly winds is balanced by an equal force of easterly winds (see sidebar, page 22). This must be so. There is friction between moving air and the surface of land and sea. This slows the wind—which is why the wind is usually stronger in air that is well clear of the ground and winds over the sea, where there is less friction, are stronger than those over land, where there are hills, trees, buildings, and other obstructions to cause friction. Friction slows the wind, but the uneven surface that gives rise to it also provides purchase for the wind, so the wind tends to push the Earth in the direction it is blowing. This is the force—due to friction—that drives surface sea waves and ocean currents. The effect on land is very slight, but it is not zero. If the

Hurricane and storm tracks

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winds blowing from west to east were always stronger than those blowing from east to west, over millions of years that drag would accelerate the rotation of the Earth, because it would amount to a force acting in the same direction. Eventually we would notice the days growing shorter. If the east-to-west winds were stronger, the Earth’s rotation would be slowed

Weather fronts During World War I, a team of meteorologists led by the Norwegian Vilhelm Bjerknes (1862–1951) discovered that air forms distinct masses. Because each mass differs in its average temperature, and therefore density, from adjacent masses, air masses do not mix readily. He called the boundary between two air masses a front. Air masses move across the surface of land and sea, and so the fronts between them also move. Fronts are named according to the temperature of the air behind the front compared with that ahead of it. If the air behind the advancing front is warmer than the air ahead of it, it is a warm front. If the air behind the front is cooler, it is a cold front. Fronts extend from the surface all the way to the tropopause, which is the boundary between the lower (troposphere) and upper (stratosphere) layers of the atmosphere. They slope upward, like the sides of a bowl, but the slope is very shallow. Warm fronts have a gradient of 1° or less, cold fronts of about 2°. This means that when you first see, high in the sky, the cirrus clouds marking the approach of a warm front, the point where the front touches the surface is about 350 to 715 miles (570 to 1,150 km) away. When you see the first, high-level sign of an approaching cold front, the front is at the surface about 185 miles (300 km) away. Cold fronts usually move across the surface faster than warm fronts, so cold air tends to undercut warm air, raising it upward along the cold front. If the warm air is already rising, it will be raised even faster along the front separating it from cold air. The front is then called an ana-front, and there is usually thick cloud and heavy rain or snow associated with it. If the warm air is sinking, an advancing

cold front will raise it less. This is a kata-front, usually with only low-level cloud and light rain, drizzle, or fine snow. The diagram shows these frontal systems in cross-section, but with the frontal slopes greatly exaggerated. After a front has formed, waves start to develop along it. These are shown on weather maps, and as they become steeper, areas of low pressure form at their crests. These are frontal depressions, which often bring wet weather. Just below the wave crest, there is cold air to either side of a body of warm air. The cold front moves faster than the warm front, lifting the warm air along both fronts until all the warm air is clear of the surface. The fronts are then said to be occluded, and the pattern they form is called an occlusion. Once the fronts are occluded and the warm air is no longer in contact with the surface, air to both sides of the occlusion is colder than the warm air. Occlusions can still be called cold or warm, however, because what matters is not the actual temperature of the air, but whether air to one side of a front or occlusion is warmer or cooler than the air behind it. In a cold occlusion the air ahead of the front is warmer than the air behind it, and in a warm occlusion the air ahead is cooler, but both of these are cooler than the warm air that has been lifted clear of the surface. The diagram shows this in cross section. As the warm air is lifted, clouds usually form and often bring precipitation. Eventually; the warm and cold air reach the same temperature, mix, and the frontal system dissipates. Often, however, another similar system is following behind, so frontal depressions commonly occur in families. (continues)

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(continued) direction of movement

A t o p o au se

warm air

direction of movement

B t o p o au se

cold air

warm air

Weather fronts. A) At an ana-front air is rising along both fronts. B) At a kata-front air is subsiding along both fronts.

Hurricane and storm tracks

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direction of movement

C tr o p o p a u s e

warm air

cool air

cold air

direction of movement

D tr o p o p a u s e

warm air

cool air

cold air

warm front cold front occluded front

Occluded fronts. C) A cold occlusion, with the air behind the occlusion cooler than the air ahead of it. D) A warm occlusion, with the air behind the occlusion warmer than the air ahead of it. In both cases the air to either side of the occlusion is cooler than the warm air that has been lifted clear of the surface.

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Hurricanes and the days would grow longer. This does happen from time to time, but the change in day length amounts to no more than a few thousandths of a second so we do not notice it, and the change does not accumulate. A temporary increase in winds from one direction is soon balanced by an increase in those from the opposite direction.

Where do they go? Most of the tropical cyclones forming off the west coast of Central America follow tracks that take them westward into the middle of the Pacific, where they die without causing harm. Unfortunately, these are the exception. The great majority of tropical cyclones form on the western side of oceans, not far from continents, and move along tracks that carry them directly toward inhabited regions. Cyclones in the Indian Ocean swing north and head for the subcontinent. Bangladesh is especially vulnerable to them. Asian typhoons also swing north and head for Indonesia, the Philippines, China, and Japan.

Global wind systems Rising air produces low atmospheric pressure at the surface, into which air flows. Subsiding air produces high surface pressure, with air flowing outward. According to the three-cell model of atmospheric circulation, air rises at the equator, producing a region of low pressure into which trade winds blow from north and south. The winds converge at low level, in the intertropical convergence zone (ITCZ), a narrow belt to either side of the equator (see sidebar, page 11). In the Tropics, therefore, the prevailing winds are the trades, blowing from an easterly direction in both hemispheres. Air descending on one side of the low-latitude Hadley cells produces high pressure at around latitude 30° in both hemispheres. Near the surface, the subsiding air diverges, some flowing back toward the equator and some flowing toward the poles. The air moving toward the poles is deflected by the Coriolis effect, to the right in the Northern Hemisphere and left in the Southern (see sidebar, page 69). In both hemispheres this produces prevailing westerly winds—southwesterly in the Northern Hemisphere and northwesterly in the Southern Hemisphere.

There is a second region of low pressure at about 60° in both hemispheres. This is where the westerly winds flowing toward the poles encounter air spilling toward the equator from the high-pressure region over the poles, where air is subsiding and diverging. The two types of air meet at the polar front and rise. Air flowing away from the polar high-pressure regions is also deflected by the Coriolis effect. Deflection to the right in the Northern Hemisphere and left in the Southern produces prevailing easterly winds. Over the world as a whole, the prevailing surface winds form three belts in each hemisphere. Winds are easterly between 0° and 30°, westerly between 30° and 60°, and easterly between 60° and 90°. The diagram shows the pattern. At higher levels, the prevailing winds in the Tropics become westerly. The midlatitude winds are westerly at all altitudes. High-level polar winds are westerly. In midlatitudes, these are the directions from which winds blow most commonly, but on any particular day they may blow from a different direction due to the passage of weather systems.

Hurricane and storm tracks

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polar easterlies 60° N polar front midlatitude westerlies 30° N

trades 0°

L trades

H

30° S

midlatitude westerlies L

polar front

H

polar easterlies

60° S prevailing pressure H high

L low

Caribbean and Atlantic hurricanes head for the islands of the Caribbean, then for the Gulf of Mexico, or Florida and the United States mainland. Although they start weakening as soon as they leave the warm, tropical ocean, tropical cyclones can travel a considerable distance over land while retaining enough energy to cause serious damage. Their tracks continue to swing, however, and this may carry them back out to sea, where they are once more in contact with warm water. Renewed evaporation and convection often reinvigorates them. Atlantic hurricanes occasionally cross the ocean and reach Europe, although they weaken greatly as they pass over the cold North Atlantic water. At the same time, the air inside the eye becomes cool rather than warm. Since a warm eye is one of the defining characteristics of a tropical cyclone, strictly speaking the storms that reach Europe are no longer hurricanes. With the exception of the early summer cyclone season in the Bay of Bengal, tropical cyclones form in late summer and fall over the oceans throughout the Tropics. They travel westward, then curve away from the equator. Those that survive long enough continue to follow curved tracks that eventually carry them in an easterly direction. Because almost all of them start life on the western side of oceans, a short westward journey is enough to take them over land and through inhabited areas. It is where they form and the direction in which they move that makes them so dangerous.

Global wind belts. There are three wind belts in each hemisphere. Overall, the force exerted on the Earth by the westerlies is exactly balanced by the force of the easterlies.

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AIR AND SEA Ocean currents and sea-surface temperature

Plane of the ecliptic. The plane of the ecliptic is the imaginary disk with the Sun at its center and the Earth’s orbital path as its edge.

Our planet is warmed by the Sun, but it is not warmed evenly. The equator faces the Sun directly. To either side of the equator, the Tropics at 23.5° N and S (Cancer in the Northern Hemisphere and Capricorn in the Southern Hemisphere) mark the limits of the region where the Sun is directly overhead at noon on at least one day in the year. These latitudes are equal to the angle by which the rotational axis of the Earth is tilted from the plane of which the Earth’s orbit around the Sun marks the edge (called the plane of the ecliptic). The diagram explains this. The Tropics receive much more energy than the polar regions, where on at least one day each year the Sun never rises above the horizon and on at least one day each year it does not descend below it (see sidebar, page 26). The Arctic and Antarctic Circles are at 66.5° N and S (90 – 23.5 = 66.5). It is not surprising, therefore, that the Tropics enjoy a warm climate and the Arctic and Antarctic are cold.

Ocean currents and sea-surface temperature

What difference do the atmosphere and oceans make? If the Earth had no atmosphere and no liquid water at its surface, these differences would be much greater, and the difference in temperature between day and night would be as extreme as it is on the Moon. There the maximum daytime temperature is about 230°F (110°C) and the minimum nighttime temperature about –275°F (–170°C). The Moon has no atmosphere to speak of (there is a very thin atmosphere, amounting to a few gas molecules in every cubic inch of space close to the surface). On Earth, however, the Sun warms the surface of land and sea, and the air is heated by contact with the warmed surface. Air and water can move, and in doing so they transport warmth away from the equator, warming the cold places and cooling the hot places. When it is warm or cold, when the Sun shines or it is cloudy, and when it rains or snows, naturally enough we assume it is the air which brings us our weather. This is true, of course, but the oceans also play a very important part. Indeed, although it is the air that produces our dayto-day weather, to a large extent it is the oceans that give us our climates. If you doubt this, compare the climate of London, England, at about 51.5° N and St. John’s, Newfoundland, at 47.5° N. In January, the average daytime temperature in London is 43°F (6°C), and the average July daytime temperature is 71°F (22°C). The January and July daytime temperatures in St. John’s are 29°F (–2°C) and 68°F (20°C)—yet St. John’s is four degrees farther south than London. What makes this difference? The North Atlantic Ocean does. The climatic influence of the oceans is due to certain remarkable properties of water. The first of these is its thermal capacity. This is the amount of heat energy that must be applied to a given mass of a substance to raise its temperature by a specified amount. Water has a very high thermal capacity. At 59°F (15°C), which is the average temperature at the surface of the Earth, it requires 51 calories of heat to raise the temperature of one ounce of water by one degree Fahrenheit (4.1855 joules per gram per kelvin; 1 kelvin (K) = 1°C). This means that water absorbs a large amount of heat before its temperature changes at all, and then it warms only slowly. Its thermal capacity varies at different temperatures, but only very slightly. Dry soil, with a thermal capacity of about 10 calories per ounce per degree Fahrenheit (0.84 J g–1 K–1), warms much more quickly than water. Air, passing from land to sea in summer, is warmed by contact with the land, then cooled as it passes over the sea. Their different thermal capacities also mean that water is much slower than dry land to give up its heat. In winter, air may be warmed as it passes over the ocean and cooled when it crosses land. Water moderates climates because of its high thermal capacity. It slows the rate of summer warming by absorbing heat with little change in

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Hurricanes

General circulation of the atmosphere The Tropics, of Cancer in the north and Capricorn in the south, mark the boundaries of the belt around the Earth where the Sun is directly overhead at noon on at least one day in the year. The Arctic and Antarctic Circles mark the boundaries of regions in which the Sun does not rise above the horizon on at least one day of the year and does not sink below the horizon on at least one day in the year. Imagine a beam of sunlight just a few degrees wide. As the drawing shows, this beam illuminates a much smaller area if the Sun is directly overhead than it does if the Sun is at a low angle in the sky. The amount of energy in each beam is the same, because they are of the same width, so energy is spread over a smaller area directly beneath the Sun than it is when the Sun is lower. This is why the Tropics are heated more strongly than any other part of the Earth and the amount of heat we receive from the Sun decreases the farther we are from the equator. Solar energy warms the surface of land and water. The air is warmed by contact with the surface. As it is warmed, the air expands. This makes it

less dense than the air immediately above it, so it is lifted by denser air flowing beneath from a colder region. This air is heated in its turn. Where the surface is heated strongly and air in contact with it is expanding, there will be a region of low atmospheric pressure at the surface. The equatorial belt is a region of generally low pressure. At high altitude, the air moving away from the equator subsides at about latitude 30°. Its subsidence is not due to its temperature or density (see sidebar, page 36), but to general atmospheric movements, including the flow of low-level air toward the equator. The high-level air is very dry, having lost its moisture as it rose and cooled adiabatically. Where the sinking air reaches the surface, the atmospheric pressure will be high. The edges of the Tropics and the subtropics, where equatorial air is sinking, are regions of generally high pressure, one in each hemisphere. Although the air is very cold while it remains at a great height, it warms adiabatically as it sinks and is compressed, so air in the tropical–subtropical regions is warm. Over the poles, very cold air sinks to the surface. This produces generally high pressure.

its own temperature and it slows the rate of winter cooling by gradually releasing the heat it spent the summer absorbing. By late fall, the sea-surface temperature in the northern North Atlantic is usually a few degrees higher than the air temperature over adjacent land. Oceans cover about 70 percent of the Earth to an average depth of rather more than 12,000 feet (3,660 m). Their total volume is more than 325 million cubic miles (1,354 km3). That is a very large amount of water spread over a very large area. Consequently, its moderating effect on climate is huge.

Transporting heat This is only one way the oceans influence the global climate. They also move heat from low to high latitudes by themselves, independently of the air.

Ocean currents and sea-surface temperature

Between the low-latitude high pressure and the high-latitude high pressure there is, in each hemisphere, a belt of generally low pressure. Air movements carry warm air away from the Tropics and cool air away from polar regions. This distributes the warmth we receive from the Sun more evenly than would be possible if the Earth had no atmosphere. Although the Earth is heated most strongly in the Tropics, all parts of the planet receive some

27

warmth from the Sun, and land and water respond differently. Land warms and cools much faster than water. As air moves, it is warmed or cooled by the surface over which it travels. Together, the transport of heat from low to high latitudes and the difference in the effect of heating on land and water generate the general circulation of the atmosphere. It is this circulation that produces regional climates and our day-today weather.

high p

Sun low pressure high pressure low pressure high pressure

Sunshine and pressure. The Sun’s radiance is much stronger in the Tropics than in higher latitudes. The general circulation of the atmosphere produces latitudinal belts of high and low pressure.

Water near the equator is strongly warmed by the Sun and flows north and south away from the equator as warm currents, its place being taken by cold currents flowing away from the poles. Air passing over a warm current is warmed by it. The difference in average temperatures in Britain and Newfoundland, mentioned above, occurs in part because the British coast is bathed by the extension of the warm Gulf Stream known as the North Atlantic Drift, while Newfoundland is washed by the cold Labrador Current, flowing south from the Arctic. The Gulf Stream and North Atlantic Drift form part of a gyre, a largescale system of currents flowing in a clockwise direction around the North Atlantic. The map shows the principal components of this system. It begins just north of the equator as the westward-flowing North Equatorial Current. This turns north off the North American coast, where it is joined by the Equatorial Counter Current (bringing water across the equator) and becomes the Antilles Current (not named on the map), and

28

Hurricanes then the Florida Current. As it passes the Gulf of Mexico it becomes known as the Gulf Stream, with the still waters of the Sargasso Sea bordering it to the east. At about 40° N, in the latitude of Spain and Portugal, it turns east across the ocean and then south, returning to the Tropics as the Canary Current to become the North Equatorial Current again. At about 40° N the Gulf Stream divides and the North Atlantic Drift (also called the North Atlantic Current) heads away in a northeasterly direction, washing the coast of northwestern Europe and passing around the north coast of Norway, where it becomes the Norwegian Current flowing into the Arctic. The Arctic is warmer than the Antarctic because it is covered mainly by sea fed by warm currents. The huge continent of Antarctica dominates the Antarctic climate.

The Great Conveyor This gyre strongly affects the climates of lands bordering the North Atlantic, and it forms part of what used to be called the Atlantic Conveyor when it was thought to affect only the North and South Atlantic. It is now known to be an even bigger system that influences climates over much of the world. Consequently it has been renamed the Great Conveyor. The Great Conveyor begins near the Arctic Circle, where ice forms on the sea surface. What happens next is due to two more remarkable properties of water. As the temperature of water falls, its molecules lose energy and move more slowly. This causes them to crowd together more closely, so that a given volume of water contains more of them. The water then weighs more as its density increases, reaching a maximum at 40°F (4°C). Below this temperature, ice starts to form as water molecules arrange themselves into crystals. Ice crystals are open at the center, and because they are open, freezing makes water expand and its density decreases. Ice is less dense than water that is just above freezing temperature, which is why ice floats. If this were not so, ice would accumulate on the bottom of lakes and the sea rather than on top, with disastrous consequences for the animals living on and in the bottom sediments. At the edge of the sea ice and just beneath it, water is just a little above freezing temperature, and so it is denser than the warmer water around it. At the same time, as ice crystals form and link together in salt water, the water molecules bond to one another directly. This breaks the bonds they had with the sodium and chlorine of salt. Common salt is sodium chloride (chemical symbol NaCl); a salt molecule comprises one atom of sodium (Na) joined to one atom of chlorine (Cl). The sodium atom carries a positive charge (Na+) and the chlorine atom a negative charge (Cl–). The water molecule (H2O) is also charged, because the two hydrogen atoms (H+) are both on the same side of the molecule as the oxygen atom (O–). When salt dissolves in water, its Na+ and Cl– separate. The Na+ joins its opposite charge on the O–, and the Cl– joins the H+. NaCl is the commonest of the salts found in seawater, but the

Ocean currents and sea-surface temperature

60° W

30° W

0°

29

30° E

Labr

Norweg ia

80° N

ad

60° N

or

Cu r rent

ic D r Atlant North

. nC

ift

ream Gulf St

aC

.

N o rt

h E q u ato

na

20° N

ry

C.

F r id

40° N

rial Current

E q u at

o rial C o u n

Ben gu

rren

Cu

Per

B

z il ra

rent Cur

u urr nt

t

ela

20° S

Guinea C

uatorial Cur th Eq ren

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Sou

ter Current

.

0° Equator

Ca

alklan

40° S nt urre dC

ind Drift West W

F

60° S

Atlantic currents. The major currents form gyres rotating clockwise in the North Atlantic and counterclockwise in the South Atlantic.

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Hurricanes other salts dissolve in the same way. When water molecules join together to form ice crystals, the H+ of one water molecule links to the O– of another (by hydrogen bonding). This detaches—“squeezes out”—the Na+ and Cl–. Isolated, these join together once more as NaCl. The NaCl then dissociates once more as it dissolves in the water into which freezing has “pushed” it. The diagram shows the sequence of events when salt dissolves and is detached when the water freezes. The ice is made from freshwater, and the salt removed from it and then redissolved makes the adjacent water saltier. The difference is not great, but it is enough. The average salinity of North Atlantic water is

1

2

Cl-

Na+

Na+

salt O-

H+

O-

H+

water

H+

H+

Cl-

3

Na+

sodium and chlorine separate

-

O

Cl-

H+

4

Na+

Cl-

sodium and chlorine rejoin to form salt

H+ ice forming

O-

Separating of salt when water freezes. 1. Salt (NaCl) enters water (H2O). 2. Na attaches to O, Cl to H. 3. When water freezes, Na and C1 are detached. 4. Na and Cl rejoin to form NaCl.

H+

1 Salt (NaCl) enters water (H2O). 2 Na attaches to O, Cl to H.

H+

O-

H+

3 When water freezes, Na and Cl are detatched.

4 Na and Cl rejoin to form NaCl. H+

Ocean currents and sea-surface temperature 34.9‰ (parts per thousand, pronounced “per mil”) and that of water near to the edge of the ice about 35.5‰. Salt water is denser than freshwater, because a given volume contains as many water molecules as a similar volume of freshwater at the same temperature, but also the sodium and chlorine atoms of the salt dissolved in it. Colder and saltier than the surrounding water, the water at the edge of the sea ice sinks beneath it. It fills a basin on the northern side of a ridge that extends along the ocean floor between Greenland and Scotland, then spills over the ridge between Iceland and Scotland. It is then known as Iceland Scotland Overflow Water (ISOW). It is denser than the water of the deep ocean, and it sinks all the way to the floor at a depth of about 10,000 feet (3,000 m). As more water sinks behind it, the deep water forms a current that flows southward, closely following the edge of the North American continent (which is several hundred miles from the coast). It takes more than 20 years for the water to reach the equator. It has now become the North Atlantic Deep Water (NADW). As it continues south, more very salt water joins it, spilling out of the Mediterranean Sea at the Strait of Gibraltar. The NADW flows all the way to Antarctica, where it joins the West Wind Drift (also called the Antarctic Circumpolar Current). It then branches several times, its branches flowing northward into the Indian, Pacific, and finally Atlantic Oceans. It forms the cold Benguela Current, which passes the western coast of Africa, crosses the equator, then the North Atlantic to North America, and finally returns to its starting place. By that time it has risen to the surface, ready to sink once more. The map shows the route the Conveyor follows. It is a slow process. Deep-water currents flow very slowly, perhaps at no more than about 150 feet (45 m) a day, and once it has descended to the ocean floor, the water does not mix with surface water. In the Atlantic, it takes 500 to 800 years for deep water to return to the surface (and in the Pacific it takes twice as long). The Gulf Stream, in contrast, flows at about 6 MPH (10 kmh) as a set of narrow streams. Throughout the whole of human history, the Conveyor has remained stable. It has always flowed more or less as it flows now, but this was not so in the more remote past. Then it changed at intervals, and quickly. Sometimes it flowed more strongly, sometimes it ceased to flow altogether, and its variations wrought major climatic changes. About 10,000 years ago, which is the last time it departed from the behavior we now think of as normal, the North Atlantic Drift ceased to flow and within a few decades the whole of the Northern Hemisphere was plunged into near ice-age conditions. That change, and probably others that preceded it, was caused by fresh meltwater flowing into the North Atlantic, in that case from the retreat of the ice sheet covering much of North America as the most recent ice age came to an end. At other times the Conveyor was suppressed by the sudden release of large numbers of huge icebergs for reasons scientists are at present unable to explain. Being less dense, the freshwater floated above the salt water like a huge raft and prevented dense water from sinking to form the NADW. Some scientists fear that global warming

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Hurricanes

60 N

30° N

equator

30° S

60° S

The Great Conveyor. This system of currents carries warm water away from the equator and cold water toward it.

might increase the amount of rain and snow falling over high latitudes and that this might suppress the formation of the NADW, disrupting the Conveyor and causing a sharp cooling in the climate of northwestern Europe. The risk of this is quite small, however.

Driven by the wind The Great Conveyor drives the gyres that circulate in all the oceans, but it has help from the wind. Surface currents are driven by the wind. To either side of the equator, the winds blow predominantly from the east. These are the trade winds, from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere (see sidebar). It is the trade winds that drive the Equatorial Currents in both hemispheres. There are also gyres, driven by the trade winds, in the South Atlantic and the North and South Pacific and a smaller and rather more complex one in the southern part of the Indian Ocean. In the North Pacific, the Kuroshio Current is very similar to the Gulf Stream and flows at a similar speed, washing the shores of Japan. In the Northern Hemisphere all the gyres flow in a clockwise direction, and in

Ocean currents and sea-surface temperature

33

Trade winds and doldrums At the tropical margins of the Hadley cells, air is sinking. It warms adiabatically as it sinks, but gathers no moisture, so by the time it reaches the surface it is hot and dry. When it reaches the surface, the air diverges, some flowing toward the equator, some away from it. The air flowing toward the equator is swung to the right in the Northern Hemisphere and to the left in the Southern, producing north-easterly winds to the north of the equator and south-easterly winds to its south. These are the trade winds. Trade is an old German word for “track,” and the trade winds earned their name because they always blow in the same direction. They occur over nearly half the world and they are very reliable, in speed as well as direction, although they are stronger in winter than in summer.

There is not one Hadley cell, however, but several. This means that the trade winds blow from the eastern margin of each cell (in both hemispheres). They converge where they meet, near the equator, but if you picture the winds as arrows, there are gaps between their “shafts.” The winds in these gaps are light and variable, and often the air is quite still. Sailing ships could be becalmed in these regions. This was not merely inconvenient. So far as the sailors were concerned it was dangerous. They could sit, day after day, under the scorching heat of the Sun, while their supply of freshwater dwindled. They called these areas the doldrums. They were also known as the horse latitudes, because horses that were being carried as cargo would sometimes die from thirst when the water supply ran short, and the carcasses would be thrown overboard.

Tropic of Cancer doldrums doldrums equator doldrums Tropic of Capricorn

Trade winds and doldrums. The areas of light winds called the doldrums occur in particular places.

34

Hurricanes the Southern Hemisphere they flow counterclockwise. These major currents are very distinctive. There is a sharp difference in temperature between a warm or cold current and the water on either side of it. The temperature of the Gulf Stream, for example, is a fairly constant 64–68°F (18–20°C), regardless of the temperature of the adjacent ocean water. Some currents are even visible. “Kuroshio” means “black water.”

Doldrums and horse latitudes There are regions in the Tropics, however, where winds are usually light and often do not blow at all. They are places where the Sun beats down mercilessly, often in very still air. In the days of sailing ships, sailors dreaded these areas and tried to avoid them, because ships could be becalmed there for weeks on end. Sailors called these places the doldrums. In those days ships often carried cargoes of horses, which might die if the ship were becalmed long enough for freshwater to become scarce. When this happened, the dead horses were thrown overboard. The winds were most likely to fail at around latitude 30° N and S. Sailors called these the horse latitudes. The doldrums, horse latitudes, and the trade winds are all produced by the convective circulation of tropical and subtropical air in Hadley cells (see sidebar). Thousands of feet below the surface of the sea, cold, dense water flows south, and its replacement causes the general circulation of ocean water. At the surface, water is also driven by the trade winds. The resulting currents carry away water that has been heated by the tropical Sun, so although tropical waters are always warm, they are cooler than they would be were they not constantly moving and being replaced by cooler water flowing in from higher latitudes. Similarly, if warm water were not fed into them constantly, the oceans in the far north and south would be even colder than they are. In the doldrums, however, the lack of wind means the surface water is not moving so fast (see sidebar, page 33). It remains exposed to the hot Sun for longer, and from time to time patches of it become very warm indeed. If the sea-surface temperature rises above about 80°F (27°C) over a large area, the scene may be set for a hurricane to develop in the air warmed by contact with the sea. An area of warm sea in the right place is not the only condition that is necessary for a hurricane to form (see “Where hurricanes happen,” page 9), but it is probably the most important one.

Hurricane seasons There are no seasons at the equator, but there are in the Tropics to its north and south, and it is during the tropical summer that the sea is heated most strongly. Because of the high thermal capacity of water, the sea warms slowly through the summer and temperatures high enough to trigger the development of hurricanes are not reached until mid to late

Ocean currents and sea-surface temperature

35

summer, so hurricanes are most likely later in the summer and in the fall. In the western North Atlantic and Caribbean, and also in the western Pacific, July to October is the peak season for them, although they can occur as early as May and as late as December. They are most frequent from September to November in southeastern Asia and between April and June and again in October and November in the Indian Ocean. December to March is the hurricane season throughout the Southern Hemisphere, but hurricanes never develop in the Atlantic south of the equator.

George Hadley and Hadley cells When European ships began venturing far from their home ports, into the Tropics and across the equator, sailors learned that the trade winds are very dependable in both strength and direction. They made use of them, and by the end of the 16th century their existence was well known. Many years passed, however, before anyone knew why the trade winds blow so reliably. Like many scientific explanations, this one developed in stages. Edmund Halley (1656–1742), the English astronomer, was the first person to offer an explanation. In 1686 he suggested that air at the equator is heated more strongly than air anywhere else. The warm equatorial air rises, cold air flows in near the surface from either side to replace it, and this inflowing air forms the trade winds. If this were so, however, the trades either side of the equator would flow from due north and south. In fact, they flow from the northeast and southeast. There the matter rested until 1735. In that year George Hadley (1685–1768), an English meteorologist, proposed a modification of the Halley theory. Hadley agreed that warm equatorial air rises and is replaced at the surface, but said that the rotation of the Earth from west to east swings the moving air, making the winds blow from the northeast and southeast. Hadley was right about what happened, but not about the reason for it. This was discovered in 1856 by the American meteorologist William Ferrel (1817–91), who said the swing is due to the tendency

of moving air to rotate about its own axis, like coffee stirred in a cup. In accounting for the trade winds, Hadley had proposed a general explanation for the way heat is transported away from the equator. He suggested that the warm equatorial air moves at a great height all the way to the poles, where it descends. This vertical movement in a fluid, driven by heating from below, is called a convection cell and the cell Hadley described is known as a Hadley cell. The rotation of the Earth prevents a single, huge Hadley cell from forming. What really happens is more complicated. In various equatorial regions, warm air rises to a height of about 10 miles (16 km), moves away from the equator, cools, and descends between latitudes 25° and 30° N and S. These are the Hadley cells. When it reaches the surface in the Tropics, some of the air flows back toward the equator and some flows away from the equator. Over the poles, cold air descends and flows away from the poles at low level. At about latitude 50° it meets air flowing away from the equatorial Hadley cells. Where the two types of air meet is called the polar front. Air rises again at the polar front. Some flows toward the pole, completing a high-latitude cell, and some flows toward the equator until it meets the descending air of the Hadley cell, which it joins. There are three sets of cells in each hemisphere. This is called the three-cell model of atmospheric circulation by which warm air moves away from the equator and cool air moves toward the equator.

36

Hurricanes

Potential temperature Cold air is denser than warm air, because its molecules are closer together. Consequently, a given volume of cold air has a greater mass than a similar volume of warm air and so it weighs more. Warm air rises because it is less dense than the cold air above it. The cold, dense air sinks beneath the warm, less dense air, and pushes it upward. Air temperature decreases with height. If you climb to the top of a mountain, you expect the air to be colder there. High mountaintops are covered in snow, even in summer, and climbers take warm clothes with them. Why is it, then, that the cold, dense air at the top of a mountain, or at the top of the troposphere, does not simply sink to the surface? How does it manage to stay up? To answer that you must imagine what would happen to the air if it did descend. Suppose, for example, that the air is fairly dry, with no clouds in the sky, and the temperature near to ground level is 80°F (27°C). Near the tropopause, 33,000 feet (10 km) above the surface, suppose the air temperature is –65°F (–54°C). The air near the tropopause is dense, because of its temperature, but this really means it is denser than the air immediately above it. Because air is very compressible, its density also decreases with height.

If the high-level air were to subside all the way to ground level, as it descended it would be compressed and it would heat adiabatically (see sidebar, page 41). Because it is dry, the air would warm at the dry adiabatic lapse rate (DALR; see sidebar, page 00). The DALR is 5.4°F per 1,000 feet (9.8°C per 1,000 m). As the air descends 33,000 ft (10 km), its temperature will rise by 5.4 × 33 = 178.2°F (98°C). Add this increase to its initial temperature, and its temperature when it reaches the ground will be 178.2 – 65 = 113.2°F (44°C). This is much warmer than the actual ground-level temperature of 80°F (27°C). The air could not reach the ground, because it would be less dense, and therefore lighter, than the air below. The temperature air at any height above the surface would have if it were subjected to sea-level pressure of 1,000 mb (100 kPa, 29 in. of mercury) and warmed adiabatically as it was compressed is known as its potential temperature (usually symbolized by Φ, which is the Greek letter phi). Potential temperature depends only on the actual pressure and temperature of the air. Meteorologists calculate the potential temperature of air to determine its stability.

In the world as a whole, there are about 80 hurricanes in most years, 54.6 in the Northern Hemisphere and 24.5 in the Southern Hemisphere (the average from 1958 to 1977). In the Atlantic, 1995 was an exceptional year, with 11 hurricanes, compared with the 1885–1994 average of 4.9 per year. That is more hurricanes than there had been for more than 60 years and, for the first time in more than 70 years, four named tropical storms were in the Atlantic at the same time. In 2000 there were a more typical six in the Atlantic, and a further 21 in other parts of the Northern Hemisphere, making a total of 27. In 2001 there were nine Atlantic hurricanes and 20 typhoons in the North Pacific, making a total of 29. There were eight typhoons in the Southern Hemisphere in the 2000–01 season. The world total was therefore 35, making 2000 a quiet year. Forecasters predicted six Atlantic hurricanes and 21 North Pacific typhoons in 2002.

Warming, convection, and low pressure Outside the Tropics, hurricanes cannot develop, because the sea is never warm enough. They are exclusively tropical, but once formed, hurricanes can and sometimes do travel long distances. Living outside the Tropics does not guarantee protection from these fiercest of all tropical storms. Occasionally they reach northern Europe, much weakened after so long a journey, but still with enough energy to wreak considerable havoc, and made worse because Europeans do not expect them, prepare inadequately if at all, and are invariably taken by surprise.

Warming, convection, and low pressure Before airplanes were invented, there were many arguments among scientists and engineers about the feasibility of building a flying machine that is heavier than air. Some scientists said the whole idea was ridiculous, and in December 1903 the New York Times published an editorial strongly opposing the foolish waste of public funds on experiments by Samuel Pierpoint Langley (1834–1906). Langley, an American astronomer, had designed an airplane powered by a steam engine. Several pilotless models flew successfully, but it proved impossible to scale the machine up to a size large enough to carry a human. The federal government funded three attempts. It was after the third failure that the Times denounced the project, saying it would be 1,000 years before humans could fly. Nine days later, on December 17, the Wright brothers proved them wrong. “Lighter-than-air” machines were the alternative to “heavier-thanair” machines, and the two varieties competed vigorously for many years. Airships—which were lighter-than-air—entered passenger service, and in 1936 the German Hindenburg began carrying passengers between Germany and the United States. It was not until several airships crashed in the 1930s, culminating in the crash of the Hindenburg at Lakehurst, New Jersey, on May 6, 1937, that people began to think “heavier-than-air” airplanes were safer. Airships were big. The Hindenburg was 804 feet (245 m) long, powered by four 1,100-horsepower diesel engines, and carried up to 50 passengers in considerable comfort. It carried fuel for its engines and, because the North Atlantic crossing took about 65 hours, food and sleeping accommodations for its crew and passengers. Clearly, it was heavier than air, but its engines and cabins were housed in a structure below the main hull, and it was the hull that accounted for almost all of its great size. The hull was filled with hydrogen. Modern airships (for the craft are making something of a comeback) use helium. This is more expensive than hydrogen but has the great advantage of being nonflammable.

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Hurricanes Both hydrogen and helium are lighter than the nitrogen and oxygen of which air is composed, and the difference in weight between the gas in the hull and an equivalent volume of air is equal to the weight of the hull, engines, and accommodations. An airship is made from materials weighing more than air, but the “lifting gas” compensates for their weight. In effect, an airship is a balloon equipped with engines and that can be steered. Some balloons also use helium as a lifting gas, but most do not. Ballooning has become a popular sport, and the balloons that drift peacefully across the countryside in summer are kept aloft by air itself. They are “hot-air” balloons.

Buoyancy and Archimedes Below the envelope of the balloon itself is the basket in which the passengers ride, and immediately above the basket there is a powerful burner, fuelled by gas. When the burner fires, hot air rises into the envelope and the balloon rises. When the burner is not lit, the air in the envelope slowly cools and the balloon starts to descend. Like an airship, a balloon has weight, but unlike an airship, a hot-air balloon uses no lifting gas that is lighter than air. It is lifted by air itself. This is possible because when air is heated its density decreases. The principle underlying this was discovered in 1787 by the French physicist Jacques A. C. Charles (1746–1823) and confirmed more accurately in 1802 by another French physicist, Joseph L. Gay-Lussac (1778–1850), so some

expanded volume

Buoyancy. Molecules are farther apart in the hot gas than they are in the cold gas; therefore a given volume of hot gas contains fewer molecules than the same volume of cold gas. Because it contains fewer molecules, the mass of the hot gas is less than that of an equivalent volume of cold gas.

original volume

Cold gas

Hot gas

Warming, convection, and low pressure

inserted body

volume of fluid displaced

people call it Charles’s law and others Gay-Lussac’s law. It states that if a given mass of gas is held at constant pressure, its volume is proportional to its temperature. This is expressed mathematically as V = kT, where V is the volume, T is the temperature measured in kelvins (1K = 1°C = 1.8°F), and k is a constant. If the gas is heated, therefore, it will expand to occupy a larger volume of space (increase T and V must increase in proportion). It follows that the original volume will contain less of the heated gas, and, because it contains less gas, it will have less mass and so it will weigh less. The diagram on page 38 shows what happens. Archimedes (c. 287–212 B.C.E.), the Greek mathematician and physicist, discovered that when a body is immersed in a fluid, such as water or air, it displaces an amount of that fluid equal to its own volume. This reduces the weight of the body by an amount equal to the weight of the fluid it displaced. The rule describing this is called Archimedes’ principle. The diagram on page 00 illustrates what happens. The reduction in weight is like a force pushing the body upward. The force is called buoyancy. It is positive if it acts in an upward direction, but air can also experience negative buoyancy, forcing it downward. The amount of buoyancy acting on a body—or on a “parcel” of air—can be calculated if you know the mass and density of the body (or air), the density of the surrounding fluid, and the gravitational acceleration (g = 32.18 feet per second per second; 9.807 m s–2). Meteorologists determine whether air is stable or unstable (see sidebar, page 4) by calculating the buoyancy force acting on it.

What happens when air is heated Air is a mixture of gases. These exist as molecules moving freely in all directions. They collide with one another frequently and ricochet in new

39

Archimedes’ principle. When a body is immersed in a fluid it displaces an amount of fluid equal to its own volume. This reduces the weight of the body by an amount equal to the weight of the displaced fluid.

40

Hurricanes directions. The speed at which they travel depends on their temperature. Heat is a form of energy, and when gas molecules absorb it, that energy changes from heat into energy, of motion (called kinetic energy). When molecules are traveling faster, collisions between them are more violent and they ricochet farther. This causes them to move, or be “bounced,” farther apart. The distance between them increases. That is why gas expands when it is heated and why a given volume of hot gas contains fewer molecules than a similar volume of cold gas. When the burner fires, hot air expands upward into the envelope of the hot-air balloon. Any given volume of the expanding air contains fewer molecules than the cooler air around it, so the hot air rises to the top of the envelope and denser, cool air is forced out at the bottom to make room for it. After a short time, the air inside the envelope is less dense than the air outside the envelope, so it weighs less than a similar volume of outside air. Being lighter, the warm air rises like an air bubble rising through a bottle of water—just the way Archimedes said it would. It is trapped inside the envelope, however, so it lifts the entire balloon, envelope, burner, basket, passengers, and all. As with the airship, the entire structure is lighter than air because the difference between the weight of the air inside and outside the envelope is equal to the weight of the other components. Now think about what happens when the Sun warms the Earth. Radiation from the Sun has very little effect on the gases of the atmosphere. It passes through them as though they were not there and is not absorbed until it reaches the surface of land or water. There it is absorbed and the surface is warmed. The warmed surface then warms the air in contact with it. Solar radiation always warms air from below, not from above, and it is this warming from below that produces our weather. There is now a layer of air warmed by the surface with which it is in contact. Because it is warmed, it expands and, because it expands, it becomes less dense and weighs less, volume-for-volume, than the surrounding air. It has buoyancy, so it rises. Its place near the surface is taken by cooler air, and this is warmed and made buoyant in its turn. The process continues for as long as the Sun continues to warm the surface, producing a stream of rising air fed from below by inflowing cool air and distributing heat vertically through the atmosphere by convection. Once air expands and starts to rise it begins cooling again. The temperature in the atmosphere decreases with increasing height above the surface. The rate at which temperature falls is known as the lapse rate and averages 5.5°F for every 1,000 feet (10°C per km) in dry air and about 3°F per 1,000 feet (6°C per km) in moist air. Expanding, rising air cools quite independently of the temperature of the air around it. This is called adiabatic cooling (see sidebar, page 41). Eventually, the temperature of the rising air is similar to that of the surrounding air, so its density and volume-for-volume weight are also similar. When it reaches this height, it will rise no farther, because it is

Warming, convection, and low pressure

41

Adiabatic cooling and warming Air is compressed by the weight of air above it. Imagine a balloon partly inflated with air and made from some substance that totally insulates the air inside. No matter what the temperature outside the balloon, the temperature of the air inside remains the same. Imagine the balloon is released into the atmosphere. The air inside is squeezed between the weight of air above it, all the way to the top of the atmosphere, and the denser air below it. Suppose the air inside the balloon is less dense than the air above it. The balloon will rise. As it rises, the distance to the top of the atmosphere becomes smaller, so there is less air above to weigh down on the air in the balloon. At the same time, as it moves through air that is less dense, it experiences less pressure from below. This causes the air in the balloon to expand. When air (or any gas) expands, its molecules move farther apart. The amount of air remains the same, but it occupies a bigger volume. As they move apart, the molecules must “push” other molecules out of their way. This uses energy, so as the air expands its molecules lose energy. Because they have less energy they move more slowly.

When a moving molecule strikes something, some of its energy of motion (kinetic energy) is transferred to whatever it strikes and part of that energy is converted into heat. This raises the temperature of the struck object by an amount related to the number of molecules striking it and their speed. In expanding air, the molecules are moving farther apart, so a smaller number of them strike an object each second. They are also traveling more slowly, so they strike with less force. This means the temperature of the air decreases. As it expands, air cools. If the air in the balloon is denser than the air below, it will descend. The pressure on it will increase, its volume will decrease, and its molecules will acquire more energy. Its temperature will increase. This warming and cooling has nothing to do with the temperature of the air surrounding the balloon. It is called adiabatic warming and cooling, from the Greek word adiabatos, meaning “impassable.”

Adiabatic cooling and warming. Effect of air pressure on rising and sinking air. Air is compressed by the weight of air above it. A “parcel” or “bubble” of air is squeezed between the weight of air above and the denser air below. As it rises into a region of less dense air, it expands. As it sinks into denser air, it contracts.

now denser and heavier than the air immediately above it. It has lost its positive buoyancy and now possesses neutral buoyancy. If the air is heated very strongly by its contact with the surface and if the rising air is constantly replenished by more air being warmed beneath it, it may rise to a very great height. In such a case, adiabatic cooling can reduce its temperature to around –74°F (–59°C) at an altitude of about 7

42

Hurricanes miles (11 km). Solar heating is strongest in the Tropics, and there warmed air rises to about 11 miles (18 km), where the temperature is around –112°F (–80°C). Above this height, varying from about 11 miles (18 km) over the equator and 7 miles (11 km) over the poles, the air temperature no longer falls with increasing height, and at a higher level still it starts increasing as height increases. The region of the atmosphere in which this occurs is called the stratosphere, where the air forms layers (strata) with little vertical movement. The lower boundary of the stratosphere, called the tropopause, traps rising air so it can rise no farther. In equatorial regions, then, air is being heated strongly at the surface and is rising all the way to the tropopause. In fact, it is not quite so simple. Surface heating accounts only partly for the buoyancy of the rising air. Equatorial air is moist and is also heated during its rise by latent heat that is released as water vapor condenses. Also, although adiabatic cooling is the most important cause of its fall in temperature with height, there is some mixing of warm, rising air and the cooler, surrounding air, which cools the rising air.

Air pressure Wherever you stand on the surface of the Earth there is a mass of air above you in what you can imagine as a column extending all the way to the top of the atmosphere. Air has weight. This was discovered in 1644 by the Italian physicist Evangelista Torricelli (1608–47), who worked as an assistant to Galileo (1564–1642) and later succeeded him as mathematician to the court of Tuscany. Torricelli invented an instrument to help him decide whether air has weight. The instrument became known as a barometer, and it is still used today. The weight of air exerts a pressure, just as the weight of any physical object will exert pressure on whatever lies beneath it. Just how much pressure the weight of the air exerts depends on the amount of air present in the column. This can vary. If you climb to the top of a mountain, for example, the column of air above you will be a little shorter, so it will contain less air and the pressure it exerts will be smaller. Pressure can also vary at sea level and for the same reason: if the quantity of air above a particular place (that is to say the number of air molecules) decreases, then the air will weigh less and so the surface pressure will decrease. If the amount of air increases, the surface pressure will also increase. Where the surface pressure is lower in one place than in another place nearby, air will flow from the area of high pressure to the area of low pressure—rather like water flowing downhill—although it does not flow in a straight line owing to the Coriolis effect (see sidebar, page 69). This flow of air, from high to low pressure, is what we feel as wind, and the strength of the wind varies according to the difference in pressure between the two areas (see sidebar, page 43). A hurricane produces very fierce winds

Air pressure, highs, and lows When air is warmed, it expands and becomes less dense. When air is chilled, it contracts and becomes more dense. Air expands by pushing away the air around it. It rises, because it is less dense than the air immediately above it. Air flows in to replace it, lifting it upward, and in turn is warmed by contact with the surface, so it also expands and rises. If you imagine a column of air extending all the way from the surface to the top of the atmosphere, warming from below causes air to be pushed out of the column, so it contains less air (fewer molecules of air) than it did when it was cooler. Because there is less air in the column, the pressure its weight exerts at the surface is reduced. The result is an area of low surface pressure, often called simply a low. In chilled air the opposite happens. The air molecules move closer together, so the air contracts, becomes more dense, and sinks. The amount of air in the column increases, its weight increases, and the surface atmospheric pressure also increases. This produces an area of high pressure, or simply a high. At sea level, the atmosphere exerts sufficient pressure to raise a column of mercury about 30 inches (760 mm) in a tube from which the air has been removed. Meteorologists call this pressure one bar and used to measure atmospheric pressure in millibars (1,000 millibar (mb) = 1 bar = 106 dynes cm–2 = 14.5 lb in–2). Millibars are still the units quoted

in newspaper and TV weather forecasts, but the international scientific unit has changed. Scientists now measure atmospheric pressure in pascals (Pa): 1 bar = 0.1 MPa (megapascals or millions of pascals); 1 mb = 100 Pa. Air pressure decreases with height, because there is less weight of air above to exert pressure. Pressure measured at different places on the surface is corrected to sea-level pressure, to remove differences due only to altitude. Lines are then drawn, linking places where the pressure is the same. These lines, called isobars, allow meteorologists to study the distribution of pressure. Like water flowing downhill, air flows from high to low pressure. Its speed, which we feel as wind strength, depends on the difference in pressure between the two regions. This is called the pressure gradient. On a weather map it is calculated from the distance between isobars, just as the distance between contours on an ordinary map allows the steepness of hills to be measured. As the diagram shows, the steeper the gradient, the closer together the isobars are, and the stronger the wind. Moving air is subject to the Coriolis effect, which swings it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, with the result that clear of the surface, winds flow parallel to the isobars rather than across them (see sidebar, page 59).

steep gradient;

LOW 990 994

998

1002 wind direction

Pressure gradient and wind speed. A wind that blows at right angles to the isobars, like water flowing down a hillside, is called a gradient wind. Above the ground winds flow parallel to the isobars and are called geostrophic winds.

44

Hurricanes because there is a great difference in pressure between the area covered by the hurricane system and the air surrounding it.

Tropical air circulation Over equatorial regions, where the surface is being heated strongly throughout the year and air warmed by contact with it is expanding and rising, the air all the way up to the tropopause is less dense than air to the north and south. This means there is less air (fewer air molecules) over equatorial regions than there are elsewhere and so the surface air pressure is permanently low compared with the pressure elsewhere. When it reaches the tropopause and can rise no higher, the equatorial air spreads away from the equator, moving northward in the Northern Hemisphere and southward in the Southern Hemisphere. It is now extremely cold, because it has cooled adiabatically, and its density increases because it is being fed constantly by more rising air. When the cold air moving away from the equator meets air flowing toward the equator, both sink to the surface. This air is also very dry. At low levels it is moist, because water readily evaporates into it and most of the equatorial regions are covered by ocean. The amount of water vapor air can carry depends on its temperature, however, and as the rising air cools, so more and more of its water vapor condenses (see sidebar, page 52). By the time it reaches the tropopause the rising air has lost almost all of its water vapor. Cold, dry, dense air that originated over the equator sinks all the way to the surface in the Tropics. As it sinks, it warms adiabatically, so by the time it reaches the surface it is hot, but still very dry. It produces deserts throughout the Tropics in both hemispheres. It also produces a region of permanently high surface pressure. Just below the tropopause, air is constantly being fed into the flow from the direction of the equator, increasing the amount (number of molecules) of air between the surface and tropopause and, therefore, its weight and the pressure it exerts at the surface. At the surface, some of the air moves toward the equator and some moves away from the equator. The air moving away from the equator extends the deserts into higher latitudes. The air moving toward the equator replaces the warm rising air and completes the cycle. Air rises over the equator, moves away from the equator and sinks, then flows back to the equator at a low level. This pattern of movement is called a convection cell, and the huge atmospheric convection cell moving air between the equator and Tropics is called a Hadley cell. The diagram shows how it works. It is called a Hadley cell because it was discovered in the 17th century by George Hadley (see “Ocean currents and sea-surface temperature,” page 24). Wind is simply air on the move, and Hadley wanted to explain why tropical winds are so reliable. As the sidebar on page 35 explains, how-

Warming, convection, and low pressure

45

Tropopause

cold air moves away from the equator

latent heat causes more warming

dense air descends

water vapor condenses

warm air rises

equator

ever, we now know that there is not just one convection cell, as Hadley thought, but three distinct systems of cells together making up what meteorologists call the three-cell model of atmospheric circulation. The three-cell model explains how heat is carried from equatorial regions into high latitudes. It also explains the general distribution of pressure systems. Wherever air is rising, surface pressure will generally be low, and where air is sinking, surface pressure will be high. Pressure is low in equatorial regions, high in the Tropics and subtropics, and high at the poles. In midlatitudes pressure is very variable. That is where polar air and tropical air meet, at a boundary that moves north and south according to conditions close to the tropopause. This makes midlatitude weather very changeable and difficult to forecast. Nevertheless, over the region as a whole pressure is low more often than it is high. Warm, rising air is what meteorologists call unstable, because it continues to rise until it reaches a level where the surrounding air has the same density as itself. If the air is moist as well as unstable, towering clouds will form in it. These clouds can cause storms anywhere, but near the equator, where the air is moist and more unstable than it is anywhere else on Earth, they can cause hurricanes—and sometimes do.

air warms adiabatically

latitude 30°

Hadley cell. Air rises at the equator, moves away at high level, subsides in the Tropics, and returns to the equator as the low-level trade winds.

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Hurricanes

Storm clouds Rising air cools. As it does so, water vapor may condense into tiny droplets or ice crystals to form clouds. But this is only part of the story. If this were all there is to it, clouds would all be alike and, obviously, they are not. Some are small, white, and puffy, the kind of clouds seen on a fine day in summer and called fair-weather cumulus. Clouds may be in flat layers or billowing heaps. There are white clouds, black clouds, and clouds of every imaginable shade of gray. Some clouds bring fine drizzle, some showers, and these may fall as rain or snow. Other clouds bring torrential rain, snow, or hail, often with thunder and lightning. It is these storm clouds that, in some places and under certain conditions, can cause hurricanes. Clouds are not all the same, and throughout history people tried to find ways to describe the different types clearly. It is not so easy as you might think. In ancient Greece, the philosopher Theophrastus (c. 372–c. 287 B.C.E.), a student of Plato and friend of Aristotle, wrote of “clouds like fleeces of wool” and “streaks of cloud.” Theophrastus was good at classifying things, especially plants. He is often called “the father of botany.” Yet classifying clouds defeated him—the names he suggested are not very helpful. Frenchman Jean Lamarck (1744–1829) was a great naturalist and classifier of plants and animals. He also tried classifying clouds, but with little success. In his system, clouds were described using such words as “sweepings,” “bars,” “grouped,” “piled,” “veiled,” and “dappled.” It was not until 1803 that Luke Howard (1772–1864), a young apothecary (druggist) and amateur meteorologist living in London, published an article setting out his scheme for classifying clouds. This one worked and could be made to include every kind of cloud. The classification he devised formed the basis of the one used to this day. All meteorologists use it in a way that is standardized throughout the world by the World Meteorological Organization (WMO), which is an agency of the United Nations (see sidebar, page 47), so if you describe a cloud by its proper, scientific name, any meteorologist in the world will know what you mean. The system is used just like the one used to name plants and animals. Every language has its own word for “dog,” but call the animal Canis familiaris and zoologists of all nationalities will understand you. The “official” classification is contained in the International Cloud Atlas, published by the WMO. First published in 1896, the Atlas is regularly updated. It comprises two volumes. The first is a loose-leaf manual describing the way clouds and other atmospheric phenomena should be observed. The second contains 196 pages of photographs (161 in color) with captions defining cloud types. There is also an abridged, single-volume version with 72 photographs, some in color and some in black and white, of all the cloud types and a variety of other phenomena, such as fog and waterspouts, with a text describing cloud observation.

Storm clouds

47

How clouds are classified There is an internationally agreed scheme for classifying clouds on the basis of their appearance and structure. Under the scheme, clouds are grouped into 10 distinctive types, called genera (singular genus). The genera are subdivided into species (singular and plural) and species into varieties. There are also accessory clouds (small clouds seen in association with a bigger cloud of a different type) and supplementary cloud features (extensions or protrusions from a cloud). Genera and species names, which are in Latin, have standard abbreviations. Variety names are usually written in full, as are the names of accessory clouds and supplementary cloud features. Stratocumulus (Sc), for example, may form in an almond or lens shape, producing the species lenticularis (len), abbreviated as Sclen. If a cloud appears to consist of bands, it may be given the variety name radiatus. Cloud genera are described as low-level, medium-level, or high-level according to the height at which they most commonly form, although clouds can form at higher or lower levels. Large storm clouds, which have a low-level base but extend to a great height, are counted as low-level clouds, mainly for convenience. Most medium-level clouds have names beginning with the prefix alto- and the names of high-level clouds have the prefix cirr-. The illustration, shows the 10 cloud genera, as well as the “anvil” that often forms where the ice crystals at the top of a cumulonimbus cloud are swept out into this distinctive shape by the wind. The scientific name for the anvil is incus, and it is a supplementary cloud feature.

Cloud genera Low-level c louds Cloud base from sea level to 1.2 miles (2,000 m). Stratus (St). An extensive sheet of featureless cloud that will produce drizzle or fine snow if it is thick enough. Stratocumulus (Sc). Similar to St, but broken into separate, fluffy-looking masses. If thick enough, it also produces drizzle or fine snow.

Cumulus (Cu). Separate, white, fluffy clouds, usually with flat bases. There may be many of them, all with bases at about the same height, and they may merge into a single cloud. Cumulonimbus (Cb). Very large cumulus, often towering to a great height. Because they are so thick, Cb clouds are often dark at the base. If the tops are high enough, they will consist of ice crystals and may be swept into an anvil shape. Medium-level clouds Cloud base from 1.2–2.5 miles (2,000–4,000 m) in polar regions, 4–5 miles (6,000–8,000 m) in temperate and tropical regions. Altocumulus (Ac). Patches or rolls of cloud joined to make a sheet. Ac is sometimes called “wool-pack cloud.” Altostratus (As). Pale, watery, featureless cloud that forms a sheet through which the Sun may be visible as a white smudge. Nimbostratus (Ns). A large sheet of featureless cloud, often with rain or snow, that is thick enough to completely obscure the Sun, Moon, and stars. It makes days dull and nights very dark. High-level cloud Cloud base from 2–5 miles (3,000–8,000 m) in polar regions, 3–11 miles (5,000–18,000 m) in temperate and tropical regions. All high-level clouds are made entirely from ice crystals. Cirrus (Ci). Patches of white, fibrous cloud, sometimes swept into strands with curling tails (“mares’ tails”). Cirrocumulus (Cc). Patches of thin cloud, sometimes forming ripples, fibrous in places, and with no shading that would define their shape. Cirrostratus (Cs). Thin, almost transparent cloud forming an extensive sheet and just thick enough to produce a halo around the Sun or Moon. (continues)

(continued)

35,000

30,000

25,000

20,00

15,000

10,000

5,000

Cloud types

Storm clouds

How clouds form Although the cloud classification is detailed and quite complicated, many of the names it uses describe clouds that are seen only rarely, and then only in some places. There are only 10 main cloud types and their names are not hard to remember. All clouds form by the condensation of water vapor, but the type of cloud that results depends on the height at which it forms and what is happening in the air itself. If the air is being heated strongly from below, so it is rising and heat is moving vertically by convection, the air is said to be unstable. Heaped clouds of the cumulus type (the adjective is cumuliform) will develop in it, and the stronger the vertical movement of air the taller they will be. If there is little vertical movement, and especially if it is slowly sinking, the air is said to be stable. Layered clouds of the stratus type (the adjective is stratiform) will form in it. Both cumuliform and stratiform clouds can produce rain or snow, but stratiform clouds usually deliver steady, persistent precipitation and cumuliform clouds produce showers, which can be heavy. (Precipitation is the general name given to any kind of water falling from a cloud and includes drizzle, rain, hail, sleet, and every type of snow.)

Humidity: how moist is the air? Air may be relatively moist or dry. Clouds cannot form in very dry air, but the amount of moisture in the air varies widely from place to place and time to time. Water vapor is a gas, of course, and you cannot see it, but the amount present can be measured, as the humidity of the air. In a polar desert, where it is so cold that water vapor changes directly into ice, there may be almost no water vapor in the air, but in warm, moist air it may account for as much as 7 percent of the air by weight. Expressed in this way, as the mass of water vapor in a given mass of air not counting the water vapor, the result is known as the mixing ratio or mass mixing ratio. It is usually measured as grams of the gas in one kilogram of air without the gas. Because it is measured in units of mass, with no reference to volume, the mixing ratio is not affected by changes of temperature or pressure. An alternative measure is the absolute humidity. This is the mass of the water vapor that is present in a given volume of air, usually expressed in grams per cubic meter (1 g m–3 = 0.046 ounces per cubic yard). If atmospheric moisture is measured as the mass of water vapor in a given mass of air including the water vapor, it is called the specific humidity. Water vapor accounts for such a small proportion of the total mass of the air, however, that it makes little difference whether the total mass is measured with or without the water vapor. Consequently, for all practical purposes, the mixing ratio and specific humidity are the same, although they are calculated differently.

49

50

Hurricanes The term most commonly used, though, is relative humidity (RH). When weather forecasters state the humidity, relative humidity is what they mean. This is the amount of water vapor in the air as a percentage of the largest amount the air could hold at that temperature. When the relative humidity reaches 100 percent, the air is saturated and water will start to condense. A hygrometer is an instrument that measures the humidity of the air, either with a needle and dial to display the value or a digital display if the instrument is electronic. If you have a hygrometer at home, the value it shows will be the RH.

Stable and unstable air For water vapor to condense into droplets the air must be cooled until its relative humidity reaches 100 percent, a temperature known as its dewpoint. Tiny particles, called cloud condensation nuclei (CCN), must also be present onto which the vapor can condense (see sidebar below). As the water condenses, however, something else happens. Condensation releases heat, known as latent heat. This warms the surrounding air,

Latent heat and dewpoint Water can exist in three different states, or phases: as gas (water vapor), liquid (water), or solid (ice). In the gaseous phase, molecules are free to move in all directions. In the liquid phase, molecules join together in short “strings.” In the solid phase, molecules form a closed structure with a space at the center. As water cools, its molecules move closer together and the liquid becomes denser. Pure water at sea-level pressure reaches its densest at 39°F (4°C). If the temperature falls lower than this, the molecules start forming ice crystals. Because these have a space at the center, ice is less dense than water and, weight for weight, has a greater volume. That is why water expands when it freezes and why ice floats on the surface of water. Molecules bond to one another by the attraction of opposite charges, and energy must be supplied to break those bonds. This energy is absorbed by the molecules without changing their tempera-

ture, and the same amount of energy is released when the bonds form again. This is called latent heat. For pure water, 600 calories of energy are absorbed to change one gram (1 g = 0.035 oz; 600 cal g–1 = 2,501 joules per gram; joules are the units scientists use) from liquid to gas (evaporation) at 32°F (0°C). This is the latent heat of vaporization, and the same amount of latent heat is released when water vapor condenses. When water freezes or ice melts, the latent heat of fusion is 80 cal g–1 (334 J g–1). Sublimation, the direct change from ice to vapor without passing through the liquid phase, absorbs 680 cal g–1 (2,835 J g–1), equal to the sum of the latent heats of vaporization and fusion. Deposition, the direct change from vapor to ice, releases the same amount of latent heat. The amount of latent heat varies very slightly with temperature, so this should be specified when the value is given. The standard values given here are

Storm clouds

correct at 32°F (0°C). The diagram illustrates what happens. Energy to supply the latent heat is taken from the surrounding air or water. When ice melts or water evaporates, the air and water in contact with them are cooled, because energy has been taken from them. That is why it often feels cold during a thaw and why our bodies can cool themselves by sweating and allowing the sweat to evaporate. When latent heat is released by freezing and condensation, the surroundings are warmed. This is very important in the formation of the storm clouds from which hurricanes and tornadoes develop. Warm air rises, its water vapor condenses, and this warms the air still more, making it rise higher.

freezing melting

680 cal/g

80 cal/g

water oxygen

Warm air is able to hold more water vapor than cool air can, and the amount of water vapor air can hold depends on its temperature. If moist air is cooled, its water vapor will condense into liquid droplets. The temperature at which this occurs is called the dewpoint temperature. It is the temperature at which dew forms on surfaces and evaporates from them. At the dewpoint temperature, the air is saturated with water vapor. The amount of moisture in the air is usually expressed as its relative humidity (RH). This is the amount of water present in the air expressed as a percentage of the amount needed to saturate the air at that temperature.

sublimation (direct change between water vapor and ice)

ice

51

evaporation condensation

600 cal/g water vapor

hydrogen

Latent heat. As water changes between the gaseous, liquid, and solid phases, the breakage and formation of hydrogen bonds linking molecules releases or absorbs energy as latent heat.

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Hurricanes

Evaporation, condensation, and the formation of clouds When air rises, it cools adiabatically. If it is dry, at first it will cool at the dry adiabatic lapse rate of 5.5°F every 1,000 feet (10°C per km). Moving air may be forced to rise if it crosses high ground, such as a mountain or mountain range, or meets a mass of cooler, denser air at a front. Locally, air may also rise by convection where the ground is warmed unevenly. There will be a height, called the condensation level, at which its temperature falls to its dewpoint. As the air rises above this level, the water vapor it contains will start to condense. Condensation releases latent heat, warming the air, and once the relative humidity of the air reaches 100 percent and the air continues to rise, it will cool at the saturated adiabatic lapse rate, of about 3°F per 1,000 feet (6°C per km). Water vapor condenses onto minute particles, called cloud condensation nuclei (CCN). If the air contains CCN consisting of minute particles of a substance that readily dissolves in water, water vapor will condense at a relative humidity as low as 78 percent. Salt crystals and sulfate particles are common

examples. If the air contains insoluble particles, such as dust, the vapor will condense at about 100 percent relative humidity. If there are no CCN at all, the relative humidity may exceed 100 percent and the air will be supersaturated, although the relative humidity in clouds rarely exceeds 101 percent. Cloud condensation nuclei range in size from 0.001 µm to more than 10 µm diameter, but water will condense onto the smallest particles only if the air is strongly supersaturated, and the largest particles are so heavy they do not remain airborne very long. Condensation is most efficient on CCN averaging 0.2 µm diameter (1 µm = one-millionth of a meter = 0.00004 inches). At first, water droplets vary in size according to the size of the nuclei onto which they condensed. After that, the droplets grow, but they also lose water by evaporation, because they are warmed by the latent heat of condensation. Some freeze, grow into snowflakes, then melt as they fall into a lower, warmer region of the cloud. Others grow as large droplets collide and merge with smaller ones.

sometimes increasing its buoyancy enough to allow the denser air above it to subside and push it upward. The subsiding air then warms adiabatically, sometimes enough to evaporate the water droplets in it. Evaporation absorbs latent heat from the surrounding air, cooling it, and this may cause the air to sink even farther. If you have ever flown through cloud in an airplane, you will know that it looks just like fog and gives the impression of being nothing more than a great mass of very tiny water droplets hanging motionless in the air. In fact, though, very complicated things are going on inside any cloud (see sidebar above). In highly unstable air, conditions inside a cloud can be extremely violent. If you have ever flown through or even close to cumuliform (heaped) cloud, you may have been advised to fasten your seat belt, because the air is likely to be “bumpy” due to the strong vertical air currents that alternately lift the airplane upward and then drop it again. It is not very likely

Storm clouds

1

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condensation level

2

3

warm air

cold air

that you will have flown through a really big storm cloud. Pilots avoid them, and for good reason. The electrical fields inside them can make compasses and other instruments useless, and the vertical air currents can make the airplane uncontrollable or even tear it apart.

Why air rises. 1. Air is forced to rise over high ground (orographic lifting). 2. Air rises by convection, due to uneven heating of the ground. 3. Air is forced to rise along a weather front.

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Hurricanes

Inside a storm cloud At a certain height, known as the condensation level, rising air will cool to its dewpoint temperature and water droplets will start to form. This level marks the base of the cloud. The air is still rising strongly, and the release of latent heat warms it and makes it rise even faster. It is mainly the release of the latent heat of condensation in the towering clouds that form in very warm, very moist air that drives the low-latitude Hadley cells (see sidebar, page 35). Inside a storm cloud air is rising by convection and cooling adiabatically, its water vapor is condensing, and cloud droplets are evaporating. Eventually, the rising air cools so much that its water droplets freeze, releasing still more latent heat. As they grow larger and heavier, ice crystals start to fall, colliding with one another and splintering as they do so. At a lower level they melt, but are then carried aloft again in rising air currents, so droplets freeze, fall to a level at which they start to melt, then freeze again, collecting more water all the time. This is how hailstones form. If you could cut through a big one, you would see it is layered, like an onion. When a hailstone grows too heavy for the upcurrents to lift, it falls from the cloud, and the bigger the stones that reach the ground in a hailstorm, the stronger the upcurrents in the cloud above, and the taller the cloud. Inside the storm cloud upcurrents and downcurrents form side by side, and water is constantly changing from vapor to liquid to ice and back again. Ice particles are shattered by collisions, and large liquid droplets disintegrate. The small ice fragments and droplets acquire positive charge and drift to the top of the cloud. The large ones acquire negative charge and sink to the bottom of the cloud. Before long this starts to distribute electrical charge inside the cloud. As the diagram on page 55 shows, the upper part of the cloud becomes positively charged and the lower part negatively charged. Electricity flows to the surface constantly from the ionosphere. In that part of the atmosphere, at a height of about 55–125 miles (90–200 km), molecules absorb shortwave solar radiation, which supplies the energy to strip electrons from atomic nuclei. An atom that has lost one or more electrons is an ion and is said to have been ionized, hence the name ionosphere for the region of the atmosphere in which a significant proportion of the air is ionized. The electrons, carrying negative charge, flow downward, leaving the ionized nuclei with a positive charge and the Earth’s surface with a negative charge. Inside a storm cloud this electrical polarization becomes much stronger, and the powerful negative charge at the base of the cloud can induce local areas of positive charge on the ground.

Thunder and lightning Air is a good electrical insulator, but locally the strength of the electrical field can exceed about 300,000 volts per foot (1 million volts per meter). When this happens, a spark flies from positive to negative. This is light-

Storm clouds ning, leaping from place to place along a forking path. The lightning spark heats the air around so suddenly the air explodes. Thunder is the noise of that explosion. As the illustration on page 56 shows, lightning will spark from one place in a cloud to another in the same cloud. From the ground this appears as a white flash, or sheet lightning. It can also spark from one cloud to another and, of course, lightning can flash between a cloud and the ground. There are several stages to a flash of “forked lightning.” The first, called the stepped leader, carries negative charge—in fact, a stream of electrons— downward from the base of the cloud, jumping from place to place wherever it finds the least resistance. As it moves, it ionizes the air adjacent to it, creating an ionized path about 8 inches (20 cm) in diameter—the diameter of a lightning stroke. As it approaches the accumulated positive charge on the surface, the stepped leader is met by a return stroke, carrying positive charge upward along the same ionized path. This is the main part of the flash. The stepped leader and return stroke only partially neutralize the charge in the cloud. They are followed by a dart leader, starting at a higher

55

Charge separation in a storm cloud. Small ice fragments and water droplets carry positive charge upward. Bigger fragments and droplets carry negative charge downward. This separates charge inside the cloud. Eventually, lightning sparks between positive and negative.

56

Types of lightning. Lightning may spark between two points inside the same cloud, between one cloud and another, or between a cloud and the ground.

Hurricanes

level inside the cloud, and a second stroke. This also triggers a return stroke. Further flashes follow until the cloud is completely neutralized. A lightning stroke typically consists of three or four flashes, about 50 milliseconds apart, and the entire flash lasts about 0.2 second. The flash carries a current of about 10,000 amperes. Thunderstorms can develop only in really huge, towering clouds. Often they extend from a height of 1,000 feet or so all the way to the tropopause. This much height is needed for water to exist as liquid in some places and ice at others and to produce strong vertical currents. Such (cumulonimbus) clouds form in unstable air or in air that becomes unstable once something has forced it to start rising (it is conditionally unstable). There are several ways in which air can be made to rise. These account for the formation of most clouds of the cumuliform (heaped) type (see sidebar, page 47), including most cumulonimbus thunderstorm clouds. Over the tropical seas, however, the convective forces inside a weather system that produces cumulonimbus may sometimes be so strong that the system as a whole becomes much more violent even than the largest thunderstorm. It may grow into a tropical storm and then into a full hurricane.

How a hurricane begins

INSIDE THE STORM How a hurricane begins On Saturday, October 6, 2001, Hurricane Iris killed three people as it crossed the Dominican Republic. The following day it lashed the coast of Jamaica, its 85-MPH (137-km/h) winds ripping trees from the ground and tearing roofs from houses. Then, as it crossed the open sea, Iris strengthened to a category 4 hurricane, with sustained winds of 140 MPH (225 km/h), and headed for the Central American mainland. It struck Belize on Monday, making landfall about 80 miles (129 km) south-southwest of Belize City, and sending people hurrying to find shelter. Then the storm died down rapidly. It was bad, and very frightening, but it could have been worse. As the winds drop, the rain eases, and the clouds start to break overhead, it is clear that the storm is dying—and when it dies, it often dies quickly. After an hour or two only the recollections of shocked survivors and the sad wreckage of property, homes, and lives remain to bear witness to what happened. That is how it dies. But how was it born?

Easterly waves Where the trade winds meet, there is a permanent area of low pressure, called the equatorial trough. This is where winds from the northeast in the Northern Hemisphere and southeast in the Southern Hemisphere converge and air rises strongly (see sidebar, page 11). The trough moves north and south with the seasons, but in a very complicated way that scientists do not fully understand. Hurricanes form either along the equatorial trough or close to it. Hurricanes are able to form only over very warm water. The sea-surface temperature must be at least 80°F (27°C) over a large area. This means a hurricane can begin only in the Tropics in late summer or autumn. Air at the surface is warmed by contact with the sea, and so it expands and rises. If the sea is warmer in a particular place than it is elsewhere, the air above that place will rise farther and faster than the surrounding air. This will cause a local area of low atmospheric pressure to develop. The difference in temperature is quite small. This is because the Tropics receive so much solar energy that air moves very quickly and temperatures and pressures are much the same throughout the region. Consequently, the effect of small, local differences tends to be much greater than it would be in middle latitudes, where there is much more variation. Nevertheless, local areas of slightly warmer water and lower pressure are common. Some are caused where the southern trade winds are stronger

57

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Hurricanes than elsewhere and push a section of the equatorial trough northward, forming a wave or “kink” in it. Others begin as tropical thunderstorms over land and then move westward. Over the ocean, the remains of the thunderstorm survive as an isolated, high-altitude tongue, or trough, of low pressure. The remains of a vigorous weather system in middle latitudes can also survive as a high-level trough “pointing” toward the equator. The resulting “kinks” are called easterly waves or tropical waves and, as the diagram on page 61 shows, they cause a slight distortion in the pattern made by the trade winds. Notice that the distortion makes the air follow a path that starts to curve in a counterclockwise direction. The diagram does not show an easterly wave in the Southern Hemisphere, but these curve in a clockwise direction. This direction of flow is called cyclonic, because it is the way air circulates around a cyclone, or area of low pressure (see sidebar on opposite page). The air temperature in the center of the trough is lower than that of the surrounding air, because it consists of air carried toward the equator from a higher latitude. This makes the environmental lapse rate steeper and increases the instability of the air below the trough (see sidebar, page 6). Air is accelerated upward, producing low pressure at the surface. Air is removed at high level, because of divergence—an outward flow of air—from the eastern side of the high-level trough. At first the drop in pressure is strong enough to cause only thunderstorms behind the advancing wave. If the low pressure persists for several days, it is known as a tropical disturbance. Most tropical disturbances are harmless, but a few strengthen as they move west, growing into tropical depressions, with winds up to 38 MPH (60 km/h), then into tropical storms, with winds up to 73 MPH (117 km/h). Now and again one grows further, into a full-scale hurricane.

Rising air For a tropical disturbance to grow, air within it must be rising very vigorously. In the Tropics, warm air is continuously rising high into the atmosphere, cooling, sinking, and returning toward the equator as the trade winds. It begins to sink, however, as soon as it reaches its maximum height and starts to cool. As it sinks, it warms adiabatically. This produces a layer of high-level air that is warmer than the air below it. This is an inversion— a layer of warm air overlying cooler air. Because it is associated with the trade winds, this one is known as the trade wind inversion, and it is present for most of the time. Ordinarily, rising warm air is trapped beneath it, and this limits the height to which storm clouds can grow. If they are growing very vigorously, however, driven by unusually strong upcurrents, they may penetrate the high-level inversion. When this happens, the clouds become towering giants up to 40,000 feet (12 km) tall, producing the fierce storms of a tropical depression.

How a hurricane begins

59

Christoph Buys Ballot and his law In 1857, the Dutch meteorologist Christoph Buys Ballot (1817–90) published a summary of his observations on the relationship between atmospheric pressure and wind. He had concluded that in the Northern Hemisphere winds flow counterclockwise around areas of low pressure and clockwise around areas of high pressure. In the Southern Hemisphere these directions are reversed. Unknown to Buys Ballot, a few months earlier the American meteorologist William Ferrel (1817–91) had applied the laws of physics and calculated that this would be the case. As soon as he learned of it, Buys Ballot acknowledged Ferrel’s prior claim to the discovery, but despite this, the phenomenon is now known as Buys Ballot’s law. This states that, in the Northern Hemisphere, if you stand with your back to the wind, the area of low pressure is to your left and the area of high pressure to your right. In the Southern Hemisphere, if you stand with your back to the wind, the area of low pressure is to your right and the area of high pressure to your left. (The law does

not apply very close to the equator.) The diagram below illustrates this. The law is a consequence of the combined effect of the pressure-gradient force (PGF) and the Coriolis effect, sometimes incorrectly (because no force is involved) called the Coriolis force, and always abbreviated as CorF. Air flows from an area of high pressure to one of low pressure, like water flowing downhill. Just as the speed of flowing water depends on the steepness of the slope (the gradient), so the speed of flowing air depends on the difference in pressure between high and low—the pressure gradient. Gravity is the force that makes water flow downhill. The force making air flow across a pressure gradient is the pressure-gradient force. As the air flows at right angles to the pressure gradient, the CorF, acting at right angles to the direction of flow, swings it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. As it starts to swing to the right, the CorF decreases, until the CorF and PGF produce a resultant force that accelerates the moving air. (continues)

HIGH

W

wind direction

Cyclonic and anticyclonic flow. In the Northern Hemisphere, winds flow in a clockwise direction around a center of high pressure. This is anticyclonic flow. The winds flow counterclockwise around a center of low pressure. This is cyclonic flow. These directions are reversed in the Southern Hemisphere.

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Hurricanes

(continued) CorF is proportional to the speed of the moving air, so it increases, swinging the air still more to the right. This continues until the air is flowing parallel to the isobars (at right angles to the pressure gradient). At this point, the PGF and CorF are acting in opposite directions, but with equal magnitude, so they are in balance. If the PGF were the stronger force, the air would swing to the left and accelerate. This would increase the CorF, swinging it back to the right again. If the CorF were the stronger, the air would swing farther to the right, the PGF acting in the opposite direction would slow it, the CorF would decrease, and the air would swing to the left again.

The eventual result is to make the air flow parallel to the isobars (pressure gradient) rather than across them. The diagram below illustrates this. Near the ground, friction with the surface and objects on it slows the air. This reduces the magnitude of the CorF (which is proportional to wind speed), altering the balance in favor of the PGF and deflecting the air so it flows at an angle to the isobars, rather than parallel to them. Over land, where the surface is uneven and so friction is greatest, the wind usually blows across the isobars at an angle of about 45°. Over the ocean it crosses at about 30°. Clear of the surface, the air does flow parallel to the isobars. This is called the geostrophic wind.

LOW PRESSURE

PGF

PGF isobar PGF w in d d

i r e c ti o n isobar

The geostrophic wind. The balance between the pressure-gradient force (PGF) and Coriolis effect (CorF) causes the wind to flow parallel to the isobars.

CorF

CorF isobar

isobar HIGH PRESSURE

If a tropical storm is to grow still further, into a hurricane, the pressure in the high-level trough must continue to fall until there is a clearly defined cyclone at an altitude of about 56,000 feet (17 km), near the tropopause. The deepening low pressure draws more air upward, accelerating the winds feeding air into the column of rising air. Air flows from an area of high pressure to one of low pressure, but not in a straight line. Influenced by the Coriolis effect (see sidebar, page 69), the air swings to the right in the Northern Hemisphere until it flows around the high- or low-pressure center. Air circulates clockwise around

How a hurricane begins areas of high pressure and counterclockwise around areas of low pressure (these directions are reversed in the Southern Hemisphere). This was discovered in 1857 by the Dutch meteorologist Christoph Buys Ballot and is known as Buys Ballot’s law (see sidebar, page 57). Air drawn into the low-pressure area near the surface flows around it counterclockwise (in the Northern Hemisphere), strengthening the cyclonic circulation. The wind speed increases until the difference in pressure between the high- and low-pressure areas can accelerate it no further. As it approaches the center, it rises, spiraling upward to fill the high-level cyclone. Air flows into the low-pressure core at every level up to a height of 33,000 feet (10 km) or more. The wind speed decreases at higher levels, and the spiral spreads outward. When the diameter of the spiral is greater than about 125 miles (200 km), the air is rotating more slowly than the surface of the Earth beneath it. Its circulation then becomes anticyclonic, as the widening spiral carries air away from the core. The diagram on page 62 compares the wind and cloud patterns at low and high level. Alternatively, air may be swept away at high level if the surface lowpressure area forms near the subtropical jet stream, but not directly beneath it (see sidebar, page 00). The subtropical jet stream is usually at about latitude 30°, and it can blow at 100 MPH (160 km/h) or more, the wind speed being proportional to the difference in temperature of the air to either side. It is farther from the equator in summer than in winter, but waves develop in it, some of which may take it deep into the Tropics. Should a tropical depression intensify close to the jet stream, the high-level wind will carry away rising air, adding to the upper-level divergence.

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Development of a tropical storm. An easterly wave in the trade-wind pattern produces low surface pressure. This deepens, forming a tropical depression and then a tropical storm.

NE trade winds

easterly

tropica

SE trade winds

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Hurricanes

surface wind high-level wind

hurricane path

Wind directions around a hurricane. The surface winds spiral inward and the high-level winds spiral outward, removing rising air and thereby sustaining the vertical flow.

How a hurricane begins By removing the rising air, upper-level divergence draws more air upward. This makes the surface atmospheric pressure fall still further, although air directly below the jet stream is removed so quickly that a large, organized, upward spiral pattern is unable to form and a hurricane cannot develop. The pressure drop is not great. Ordinarily, the average sea-level pressure is 1,016 millibars (mb). At the center of a hurricane it may be no lower than 980 mb, and it rarely falls below 920 mb, although there have been hurricanes with core pressures lower than 900 mb. The lowest pressure ever recorded anywhere on Earth was 870 mb, in the eye of Typhoon Tip on October 12, 1979. This is a drop of only 4–10 percent, but it is enough.

The full-grown cyclone Now there is an area of low surface pressure, 400 miles (645 km) or more across, with a pressure of about 950 mb at the center. This is a very low pressure, and its effect is greater than it would be in middle latitudes because of another important difference between the middle-latitude system and the tropical one—their relative sizes. Big though it sounds, in fact the tropical system is much smaller than a middle-latitude cyclone—which can be more than 1,000 miles (1,600 km) across. Its smaller size means the low-pressure core is also much smaller. Consequently, the pressure gradient is steeper, and so inflowing air is accelerated much more than it would be if it were to flow across a shallower gradient. It is also accelerated more than air flowing into a middle-latitude cyclone because it is being drawn into a smaller area (see sidebar, page 71). Friction between the inflowing air and the ocean surface slows the wind and increases the angle at which it crosses the isobars. Because it crosses a warm ocean, the air feeds warmth and moisture into the core, intensifying convection. At the same time, it pushes the storm clouds closer together around the core, gathering scattered convection cells into an organized cluster. Inside the core, air that has not been swept away at high level is sinking and warming adiabatically as it does so. Consequently, the center of the hurricane—the eye—is markedly warmer than the air surrounding it. Air spiraling inward is already warm, but its temperature rises still further as it nears the warm eye. This increases its buoyancy, intensifying the convection still more. By now, huge storm clouds have developed around the eye. Warm, moist air, drawn toward the center, circulates cyclonically and in an upward spiral all the way to the cloud tops, then disperses outward, drawing more air from below. While it remains a tropical depression, there is cold air at the center, with warmer air circulating around it. There are rain showers at the center and the sky is mainly overcast. When the air in the center becomes warmer than the air flowing around it, the sky clears until only a few small clouds remain, there is no rain, and the wind speed falls to about 10 MPH (16 km/h). This calm center, often 20

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Hurricanes to 30 miles (32 to 48 km) across, is surrounded by what looks like a solid wall of cloud. It is the eye of what has now grown into a hurricane.

Vortices When you pull out the plug from a full bathtub and the water starts flowing away, usually it will soon begin to swirl in a spiral, forming a little whirlpool. If you watch the water each time you do this and note the direction in which it turns, you will find that sometimes it turns to the right and sometimes to the left. If you churn up the water while it is swirling and break up the whirlpool, then leave it to start swirling again, it may turn in the opposite direction to the one in which it turned earlier. The direction it turns is a matter of chance, and occasionally the water does not swirl at all. Some people will tell you that in the Northern Hemisphere the water always flows out of the bathtub counterclockwise and that it invariably flows clockwise in the Southern Hemisphere. They say that if you take a bath on one side of the equator, then cross to the other hemisphere and take a second bath there, the water will turn in different directions on either side. They will also tell you that if you place the bath precisely on the equator the water will not turn at all. What is more, they will usually tell you that they know someone who has tried it personally and can confirm it. This is a popular belief, similar to an urban myth, and very difficult to dislodge, but it is quite wrong—and you can easily show that it is wrong without going to all the trouble and expense of carrying your bathtub to the equator. The confusion is one of scale. A rule that applies to an ocean current or a mass of air hundreds of miles across does not necessarily apply to something as small as a bathtub. To convince yourself that the myth is just that, all you need do is watch what happens to the water every time you empty the bath.

Whirlpools On the small scale of a bathtub, moving water is affected by vorticity. This is the tendency of any moving fluid—a liquid or gas—to turn about an axis. It is what produces the whirlpool, or vortex. The axis may be horizontal or vertical. In the case of the bathtub, where the water is flowing downward, the axis is at right angles to the Earth’s surface. Air is also a fluid, and when it moves it also develops vorticity, and its vorticity about a vertical axis contributes greatly to the development of a hurricane. Vorticity often begins when two streams, of water or air, flow side by side in the same direction, but at different speeds. This produces a shearing force. The faster stream is slowed on one side by friction with the slower, and this makes it start to curve toward the slower. You use something similar to this effect to help you steer a rowboat. To make the boat turn, you row more strongly with one oar than with the other, and the boat follows a path that curves toward the side where you are rowing more slowly.

Vortices You can also watch it happen if you live near a river with a bridge across it. Stand on the bridge and drop a small stick into the river, somewhere near the middle. The stick will float away, of course, but as it does so, it may (or may not) also start to turn around. The stick’s rotational movement is its vorticity. If the stick turns, it possesses vorticity, which is positive or negative depending on the direction it turns; if it does not turn at all it is said to possess zero vorticity. This is just putting a name to the movement of the stick, though. It does not explain why the stick possesses vorticity. In order to understand that, you need to think about the water all around the stick. As you can see from the ripples and little waves on the surface, the water in the river is not really a single mass that all moves together. It is more like a vast number of tiny “particles” of water, all of them moving in their own way, but all carried along in the general movement. Flocks of birds and swarms of locusts move like this. If you film them and slow the film down, you find that although the flock or swarm moves as a whole, like the river, the individual birds or locusts are moving in every direction, including the direction opposite to the overall movement. The stick floats in the water and is carried by it, but the “particles” of water may be moving faster on one side of it than on the other. Water close to the bank moves more slowly than water in the middle of the river, for example, because it is retarded by friction, and islands, big rocks, and other obstructions will also cause friction. These local differences in speed represent the shear in the river current, and they will start the stick turning. Or the river itself may curve. In that case the water has to follow the curve, and water on the outside of the curve usually moves faster than water on the inside of the curve because it has farther to travel. This also produces a shear that will set the stick turning. Often, both of these operate together in imparting vorticity. The diagram on page 66 shows how shearing forces occur. Your twisting stick is caught in a tiny vortex, or whirlpool. Whirlpools in water can also be big and dangerous to small ships. In Greek mythology, the whirlpool in the Strait of Medina, between Sicily and Italy, was called Charybdis. It was described as a monster that lurked below the waves close to the Sicilian shore. Three times a day Charybdis sucks down water and then spews it out again. Sailors who try to avoid falling into her clutches by sailing close to the Italian shore run the risk of being caught by Scylla, another monster. She has 12 feet, six heads each with three rows of sharp teeth, and the barking heads of dogs all around her hips. She will devour any ship that ventures too close. In fact, Scylla was either a submerged rock or reef. There are other famous whirlpools, each with its own legends and stories. Off the north coast of Norway, just to the west of the Lofoten Islands, there is the Lofoten Maelstrom, known in Norwegian as the Moskstraumen. Jules Verne and Edgar Allan Poe wrote stories about the Maelstrom. Another whirlpool, also with its own legend, is located off the western coast of Scotland, in the Gulf of Corryvreckan between the islands of Scarba and Jura. Whirlpools such as these are produced by vorticity generated by obstructions to the flow of strong tidal currents. In the case of

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66 Vorticity. In the top example, the upper stream (a) is moving faster than the lower stream (b), because it must cover a greater distance in the same length of time. In the bottom example, suppose that stream (a) is moving faster than (b). In both A and B the difference in speed produces a shearing force.

Hurricanes

a b

a b

the Corryvreckan, for example, the tide flows northward very strongly through the narrow Sound of Jura. As part of it turns to the left and enters the Gulf of Corryvreckan, it encounters first a deep hole in the seabed and then a tall pinnacle of rock. Together, these produce the shearing force that sets the current turning. Whirlpools form only in water, of course, and they stay in the same place. There are atmospheric equivalents, however, and these can form anywhere the necessary conditions occur. We know these atmospheric “whirlpools” as depressions or cyclones—and as hurricanes and tornadoes.

Vorticity in moving air Air also moves and it, too, can be pictured as a vast number of minute “particles,” all twisting and turning and moving in all directions like birds in a flock or locusts in a swarm. Each “particle” possesses vorticity and vorticity itself has three components: magnitude, direction, and sense. The magnitude is equal to twice the angular velocity (conventionally symbolized by the Greek letter omega, ω), the direction refers to the horizontal or vertical orientation of the axis about which the particles rotate, and the sense is whether the rotation is clockwise or counterclockwise when viewed from above. The rotation of a large mass of air is the sum of the rotations of all its component “particles.” It is like the movement of the flock or swarm. In the Northern Hemisphere, air that turns counterclockwise is turning in the same direction as the Earth’s rotation. This is termed positive vorticity, and the rotation of the air is said to be cyclonic, because it is the direction in which air turns around a center of low atmospheric pressure, or cyclone. Clockwise rotation is termed negative vorticity, and the rotation is anticyclonic—the direction air circulates around an anticyclone, or center of high pressure. These are reversed in the Southern Hemisphere. The diagram below illustrates this. Buys Ballot’s law describes this kind of air movement and the reason for it (see sidebar, page 59).

Vortices

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There are also types of vorticity. Shear causes air to rotate about its own axis—giving it its shear vorticity. It also rotates about a center of high or low atmospheric pressure—its curvature vorticity. Together, these the relative vorticity of the air (symbolized by the Greek letter zeta, ζ). Air also rotates because the entire atmosphere is moving due to the rotation of the Earth. This is its Earth vorticity, and it varies according to the latitude. Its value is zero at the equator, where the local vertical axis is at right angles to the Earth’s rotational axis, and attains a maximum at the North and South Poles, where the local vertical and Earth axes are aligned. The sum of the relative and Earth vorticities is known as the absolute vorticity.

Vorticity and bad weather Curvature vorticity is the movement of air about a center of high or low pressure (see sidebar, page 73). The air spirals inward toward a center of low pressure and outward away from a center of high pressure. The speed of the moving air—the wind speed—is proportional to the difference in pressure between the center and outside of the system—the pressure gradient. This is another way of saying that the relative vorticity changes according to the pressure gradient. When pressure at a center is low and the vorticity of air circulating around it is high, the weather is often foul. When air spirals inward, its convergence near the center causes it to rise. This is due to another fundamental physical property: the conservation of mass in a fluid that is free to move in three dimensions. Convergence

Northern Hemisphere

negative anticyclonic

positive cyclonic

Southern Hemisphere

positive cyclonic

negative anticyclonic

Directions of positive, negative, cyclonic, and anticyclonic vorticity in the Northern and Southern Hemispheres

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Hurricanes reduces the horizontal dimensions of a volume of air. This would increase the mass of the converging air, but in fact the mass and volume remain constant because the air expands vertically. Divergence, when air flows away from an area, has the opposite effect of making air contract vertically and subside, but for the same reason. Rising air cools, often producing cloud and precipitation. If vorticity increases in the air high in the troposphere, the result is often a decrease in the surface pressure directly below, with consequent convergence. Weather forecasters pay close attention to the movement of air near the tropopause, for this reason. If they detect an increase in upper-level vorticity, they anticipate a drop in surface pressure and bad weather.

The Coriolis effect Buys Ballot’s law explains why, in the Northern Hemisphere, air moves counterclockwise around a center of low pressure and clockwise around a center of high pressure. This is what leads people to suppose that in the Northern Hemisphere water leaving a bathtub will invariably spiral clockwise around the low pressure at the center of the whirlpool. This does not happen, but not because the effect is unreal. It is real enough, but it applies only to masses of air or water that are very much bigger than the contents of a bathtub and that are moving across the surface of the Earth. When we observe air movements, we do so from a fixed position, standing (or sitting) in a particular place. It is easy to forget that the Earth itself is rotating and we are traveling with it. When you lie on your back in the grass, gazing at the peaceful summer sky, there is nothing to indicate that you are traveling eastward at around 800 MPH (1,287 km/h)—and far less that the Earth is carrying you around the Sun at about 48,000 MPH (72,000 km/h)! Air also travels with the Earth, but it is not attached firmly to the surface, and so it may be traveling at a different speed. Indeed, if a mass of air moves from one latitude to another it will start by traveling at approximately the same speed as the surface in its first latitude. This speed will be different from the speed the surface is moving in its new latitude because the Earth is a sphere. It takes 24 hours for the Earth to complete one turn about its axis, but places in different latitudes move at different speeds because they have different distances to cover. Consequently, air that changes latitude is bound to be traveling at a different speed from the surface. People realized long ago that the rotation of the Earth affects the way air moves over its surface, but it was not until 1835 that the French physicist and engineer Gaspard Gustave de Coriolis (1792–1843) found out why. Today this is known as the Coriolis effect (see sidebar, page 69). Its strength ranges from zero at the equator to a maximum at the poles.

Angular momentum When air moves, its relative vorticity causes it to turn more and more, until it moves in a circle. It then possesses angular momentum, which is the

Vortices

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The Coriolis effect Any object moving toward or away from the equator and not firmly attached to the surface does not travel in a straight line. As the diagram illustrates, it is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Moving air and water tend to follow a clockwise path in the Northern Hemisphere and a counterclockwise path in the Southern Hemisphere. The French physicist Gaspard Gustave de Coriolis (1792–1843) discovered the reason for this in 1835, and it is called the Coriolis effect. It happens because the Earth is a rotating sphere, and as an object moves above the surface, the Earth below is also moving. The effect used to be called the Coriolis “force,” and it

is still abbreviated as CorF, but it is not a force. It simply results from the fact that we observe motion in relation to fixed points on the surface. The Earth makes one complete turn on its axis every 24 hours. This means every point on the surface is constantly moving and returns to its original position (relative to the Sun) every 24 hours, but because the Earth is a sphere, different points on the surface travel different distances to do so. If you find it difficult to imagine that New York and Bogotá—or any other two places in different latitudes—are moving through space at different speeds, consider what would happen if this were not so: the world would tear itself apart. (continues)

direction of Earth’s rotation N

initial direction actual path S

Coriolis deflection. Moving air masses, winds, and ocean currents are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis effect is at a maximum at the poles, and it does not exist at the equator.

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(continued) New York

Madrid

The Coriolis effect. The track of an aircraft flying from the equator to a destination in a higher latitude appears to be deflected to the east.

Consider two points on the surface, one at the equator and the other at 40° N, which is the approximate latitude of New York and Madrid. The equator, latitude 0°, is about 24,881 miles (40,033 km) long. That is how far a point on the equator must travel in 24 hours, which means it moves at about 1,037 MPH (1,668 km/h). At 40° N, the circumference parallel to the equator is about 19,057 miles (30,663 km). The point there has less distance to travel and so it moves at about 794 MPH (1,277 km/h). Suppose you planned to fly an aircraft to New York from the point on the equator due south of New York (and could ignore the winds). If you headed due north, you would not reach New York. At the equator you are already traveling eastward at 1,037 MPH (1,668 km/h). As you fly north, the surface beneath you is also traveling east, but at a slower speed the farther you travel. If the journey from 0° to 40° N took you 6 hours, in that time you would also move about 6,000 miles (9,654 km) to the east,

relative to the position of the surface beneath you, but the surface itself would also move, at New York by about 4,700 miles (7,562 km). Consequently, you would end not at New York, but (6,000 – 4,700 =) 1,300 miles (2,092 km) to the east of New York, way out over the Atlantic, somewhere due south of Greenland. The diagram illustrates this. The size of the Coriolis effect is directly proportional to the speed at which the body moves and the sine of its latitude. For each unit of its mass, a body is deflected by –2ω V sin ϕ, where ω is the angular velocity, V is the velocity of the mass, and ϕ is the latitude. The angular velocity of the Earth is 15° hr–1, which can also be expressed as 2π/24 rad hr–1 or 7.29 × 10–5 rad s–1. The effect on a body moving at 100 MPH (160 km/h) is 10 times greater than that on one moving at 10 MPH (16 km/h). Sin 0° = 0 (the equator) and sin 90° = 1 (the poles), so the Coriolis effect is greatest at the poles and zero at the equator.

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product of the mass of the air, its angular velocity, and the radius of the circle that its path describes. Angular momentum is another physical property that is conserved (see sidebar, page 78). If the circle it describes becomes smaller, the conservation of its angular momentum will make the air move faster. Around a hurricane, where air is spiraling inward, the wind speed increases the closer it comes to the central eye.

Conservation of angular momentum gram, her fingertips begin by describing the outer circle, and when she has withdrawn her arms she describes the inner circle. She has reduced one of the three variables and so one or both of the others must increase in order to compensate. Her mass cannot change (she cannot suddenly become heavier), and so the remaining variable, her angular velocity, has to change. It increases as her radius of spin decreases. In other words, she spins faster—but without making any additional effort beyond withdrawing her arms.

us

1

di

ra

radius 2

dir

ec n tio

Imagine a body that is spinning about its own axis. You can measure the mass of the body, the radius of the circle it describes, and the speed of its rotation. Its speed of rotation is known as its angular velocity and is measured as the number of degrees through which it turns in a given time. The Earth, for example, completes one revolution in 24 hours. One full turn takes it through 360°, so the Earth’s angular velocity is (360 ÷ 24 =) 15° per hour. Angular velocity is usually expressed in radians per hour or per second (rad hr–1 or rad s–1). A radian is the angle between two radii of a circle that marks out on the circumference an arc that is equal in length to the radius. Therefore the circumference of a circle is 2 π radians and 1 rad = 57.296°. Multiply these three values together and the product, called angular momentum, is a constant. Call the mass M, the radius R, and the angular velocity V, and M × R × V = a constant. M, R, and V are variables. They can be altered, but the constant must remain the same. This is called the conservation of angular momentum, and it means that if one of the variables changes, one or two of the others must also change in order that the constant remains the same. No one needs to do anything to make this happen: it is entirely automatic. Dancers and ice skaters make use of the conservation of angular momentum when they perform pirouettes. The dancer starts spinning with her arms fully outstretched. The distance from the center of her body (the axis of her rotation) to her fingertips is the diameter of the circle her body describes; the radius is half of this. Then she slowly draws her arms inward to her body. This reduces her radius of spin. In the dia-

of

ro

tat

io n

Conservation of angular momentum. If the radius of turn (radius 1) is reduced (radius 2), the mass of the rotating body and/or its angular velocity must increase in proportion.

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Hurricanes A tropical cyclone, with a diameter of about 400 miles (644 km), is only about half the size of a middle-latitude depression, but the pressure at its center is very much lower. The wild ferocity of the storm, compared with the merely drab wetness of a depression, results from its much steeper pressure gradient. This accelerates the air spiraling into the eyewall, so it is already moving fast when it joins the system. Its radius of rotation then shortens rapidly as the air approaches the center. The conservation of its angular momentum then increases the angular velocity—the wind speed. This is why the wind speed around a hurricane is greatest around the eyewall and decreases with distance from the eye. Vorticity and the conservation of angular momentum account for the ferocity of the winds around the eye of a hurricane, and the Coriolis effect explains the direction in which those winds blow. The Coriolis effect also explains why hurricanes do not form very close to the equator, and it contributes to the path they follow once they have formed.

What happens inside a hurricane As air flows in from surrounding regions of higher atmospheric pressure to fill the deepening tropical depression and the system starts rotating, a distinct structure begins to develop. By the time the tropical depression has grown into a tropical storm and then into a hurricane, it is this structure that sustains its force. Its energy is immense. An average hurricane possesses 10 times more energy than was released in the 1883 eruption of Krakatau (Krakatoa) and about 10,000 times more than the daily output of the Hoover Dam. If you wished to release a hurricane’s worth of energy by burning coal, you would need to shovel approximately 70 million tons (63.6 million tonnes) of it into your furnace. Alternatively, you could detonate around 210 thermonuclear weapons each with a one-megaton yield. A one-megaton bomb explodes with a force equal to that produced by detonating 1 million tons of TNT (conventional high explosive). An average hurricane packs enough energy to work all the streetlights in New York City for more than 27,000 years. Unfortunately, no one has yet figured a way to make hurricanes do useful work.

The structure of a hurricane The Coriolis effect (see sidebar, page 69) swings moving air along a circular path around the low-pressure area, but this movement is countered by friction between the wind and the sea. Friction slows the wind, reducing the Coriolis effect (which is proportional to wind speed), so the pressure-gradient force is slightly greater than the Coriolis effect and the

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Why the wind blows If you have ever sat on a cold winter’s day in a room heated only by a coal fire, you will know that coal fires look great, but they cause drafts. Hot air from the fire goes up the chimney—taking most of the heat with it—and this draws in air at floor level to replace it. The air at floor level is cold. That is the draft. It is also a wind. This is easier to picture if you imagine that the fire is outdoors, like the one in the drawing. Anyone standing a little way from the fire will feel the wind blowing toward the fire. It is why a fire warms your face and the front of your body, but makes your back colder. Wind blows toward places where air is rising. When air rises, it produces a region of low atmospheric pressure at the surface, so you can also think of wind as air moving toward a region of low pressure from an area where the pressure is higher. Air moves from areas of high pressure to areas of low pressure at a speed that is proportional to the difference in pressure. This difference produces a pressure gradient. On a weather map, where lines called isobars join places where the pressure is the

same, the distance between the isobars indicates the steepness of the gradient, just like the contour lines on an ordinary map. The “slope” down the pressure gradient from high to low exerts a pressure-gradient force (PGF). If this were all that happened, pressure gradients would disappear almost the moment they appeared and winds would amount to nothing more than brief gusts as air rushed to even out pressure differences. It is not what happens, however. The rotation of the Earth produces the Coriolis effect (CorF). This deflects the moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, acting at right angles to the PGF. CorF is proportional to the speed of the moving air. If the air accelerates, the CorF increases. In the absence of friction, the PGF and CorF strike a balance that results in the wind blowing parallel to the isobars. Friction with the land or sea surface makes a difference, however. It slows the moving air, but without altering the PGF. The frictional force acts in (continues)

rising air

What makes the wind blow

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(continued) low pressure isobar

Surface wind. The pressure-gradient force moves at right angles to the isobars. The moving air is deflected by the Coriolis effect (CorF) and is slowed by friction. The result is that the wind blows across the isobars.

pressure-gradien force

isobar

isobar

isobar rF isobar high pressure

the opposite direction to that of the wind. As the wind slows due to friction, the CorF decreases, so that the PGF is greater than the CorF. The wind then moves at an angle to the isobars, as shown in the diagram, and flows toward the low-pressure center. Depending on the magnitude of the PGF and the roughness of the surface, the wind crosses the isobars at between 10° and 30°. Air is now spiraling toward the center of low pressure. Because it follows a circular path, it possesses angular momentum, a property that is con-

served. With each turn of the spiral, the radius of the circle decreases and therefore, to conserve angular momentum, the speed of the wind increases. Wind blows because air is driven by the pressure-gradient force to move from an area of high atmospheric pressure to an area where the pressure is lower. The Coriolis effect deflects the moving air, and friction retards it. The balance of these forces produces a wind that spirals inward. The conservation of angular momentum then accelerates the wind as it approaches the center.

wind spirals inward to the low-pressure area (see sidebar, page 73). Friction with the sea also produces vast quantities of spray. Droplets of seawater evaporate in the warm air, adding to the water evaporated from the sea surface. Spiraling inward, the air is warmed by contact with the sea. This makes it expand and rise. The air is moist to a considerable height, because of the large amounts of water that evaporate into it from the tropical ocean and through the addition of spray. The rising air cools adiabatically (see sidebar, page 41), and its water vapor starts to condense. Condensation releases latent heat (see sidebar, page 50), warming the air again and making it continue to rise. Still spiraling inward and moving with increasing speed as it approaches the center because of the partial conservation of its angular momentum (see sidebar, page 71), the air continues to rise and its water vapor continues to condense. This vigorous convection, with upcurrents rising at up to 30 MPH (48 km/h), produces cumulonimbus (storm) clouds that sometimes tower to a

What happens inside a hurricane height of 50,000 feet (15.25 km). Violent thunderstorms develop and beneath the clouds the rainfall is torrential and often accompanied by hail. Wind speeds fall off rapidly near the top, until the air is moving more slowly than the rotating surface of the Earth. When this happens, again because of the Coriolis effect, the winds begin to move anticyclonically (clockwise in the Northern Hemisphere), producing the wide, curving trails of cloud that make a hurricane photographed from space look very like a spiral galaxy. This anticyclonic circulation carries away and disperses much of the rising air, but not quite all of it. Some sinks again, from the high-level region of high pressure. It descends at the center of the vortex, warming adiabatically as it does so. Its rising temperature increases its waterholding capacity. If fragments of the surrounding cloud are swept into it, most of the water droplets in them evaporate. The warm air produces almost clear skies and warm air at the eye. The warmth is very noticeable, and after the raging wind and rain have passed, the comparatively still air at the eye, several degrees warmer than the air outside the hurricane, can seem oppressive.

The eye and the eyewall In the middle and upper regions of the storm there is a large difference in temperature between the air inside the eye and that outside it. This difference in temperature produces a correspondingly large difference in air pressure, adding to the energy of the hurricane. Despite the sinking air, it is in the eye that the surface atmospheric pressure is at its lowest. It is often more than 50 millibars lower than the pressure outside the hurricane. In the most severe category of hurricanes the pressure in the eye is 920 mb or less. Taking the average sea-level pressure as 1,013 mb, this is a drop of 96 mb, and as a hurricane approaches, atmospheric pressure may fall at a rate of 3 mb per mile (1.9 mb km–1). If the storm travels at, say, 20 MPH (32 km/h), this means the pressure will fall about 1 mb every minute. Reduced pressure in the eye means a smaller weight of air is pressing down on the surface of the sea. This allows the sea surface to rise up to 3 feet (1 m) higher than the surrounding sea level. The power of a hurricane is often measured by comparing the temperature and pressure in the eye with those beyond the influence of the system. The outward flow of air, called divergence, at the top of the storm increases the inward flow, or convergence, near the surface. This intensifies the winds and, therefore, strengthens the upward spiral, which leads in turn to the condensation of water vapor and release of latent heat, and the rising air adds to that already at the top, sustaining the high pressure. At this stage the storm is feeding on itself, and it will continue to do so for as long as it has an ample supply of water and is being warmed from below. Towering cumulonimbus clouds form a circle surrounding the eye and extending almost from sea level all the way to the tropopause. This is the

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Height (feet) 65,000

direction of air movement/wind cirrus

cumulonimbus

cumulonimbus 30,000 towering cumulus

scattered small cumulus

towering cumulus

eye

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Cross section of a hurricane

0 Distance (miles)

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dark eyewall of the hurricane, often with tattered fragments of cloud falling down its face, and it is where conditions are most severe. As the drawing of a cross section through a hurricane shows, at the top of the eyewall, the remaining water vapor in the air that is carried away by divergence freezes to form cirrus and cirrostratus clouds, made from ice crystals. In satellite photographs of hurricanes, these thin, high-level clouds often show clearly, spiraling out from the edge of the main cloud mass.

Bands of cloud In all, there may be up to 200 towers of cumulonimbus. These are storm clouds, and they produce storms. There is lightning and thunder, with hail and torrential rain. Some of the storms are so violent that they trigger tornadoes—the vicious companions of hurricanes. The cloud forms in bands. Outside the eyewall cloud there is a cloudfree band. This is surrounded in its turn by a second circular band of cloud, then another cloud-free band, and further alternating bands of cloud and clear air, with the clouds becoming smaller and more scattered the farther they are from the center. At any one time, it will be raining or hailing in about 15 percent of the area covered by bands of cloud. The weather in the cloud-free bands is dry. The sky in the cloud-free bands is not blue, however. Divergence at the top of the storm spreads enough cloud to cover the sky. Within the area of the hurricane, often more than 400 miles (644 km) across, the “cloud towers” cover no more than about 1 percent of the sky, yet it is they that form the heart of the storm. They are its “engine,” releasing latent heat and producing the warm center without which a hurricane

What happens inside a hurricane cannot develop. In the cloud-free bands between the cloud bands, some of the air that was carried aloft is descending, warming adiabatically, and feeding back into the system at the surface.

Hurricane winds Rain, hail, and thunderstorms are common enough. Think of a hurricane, though, and the image that springs to mind is of the wind. That, after all, is its most important and obvious characteristic. Wind force is still measured by a scale devised at the beginning of the 19th century by a British naval officer (see sidebar). In the days of sailing ships, the Royal Navy thought it advisable to supply British naval commanders with a simple method for estimating the strength of the wind so they could know how much sail to set. A sailing ship could be severely damaged or even sunk if it carried too much sail for the wind conditions, and if it carried too little it might fail in its pursuit of the enemy. In those days ships did not carry instruments (called anemometers) for measuring wind speed. Even if they had, it is doubtful whether an ordinary anemometer could have withstood winds of hurricane force. The wind scale also provided a way to check on the performance of the captain, because everything had to be recorded in his daily log, which was handed in when the ship returned to port. By checking the sail that was set against the reported state of the wind, an experienced sailor back at the Admiralty could tell whether the warship was being operated correctly. The Beaufort scale is straightforward and easy for anyone to use. You need only look from your window to estimate the Beaufort wind force. That is its great advantage. Its disadvantage is that it classes as a hurricane, rated as Force 12, any wind stronger than 75 MPH (121 km/h), and this is the minimum wind speed for a hurricane. If the wind speed is less than 75 MPH, it is not a hurricane at all, and in the most severe hurricanes it is more than 155 MPH (249 km/h). For this reason, the U.S. Weather Bureau has added a further five categories to describe hurricanes with winds from 75 MPH, called Category 1, to Category 5, with winds of more than 155 MPH (see sidebar, page 114).

Storms at sea At sea, the winds raise huge waves then whip away their tops, so it becomes impossible to see through the spray where the sea ends and the sky begins. Spray that is blown across the sea surface is called spindrift. The wind speed in a hurricane is not constant. Like most winds, a hurricane produces gusts much stronger than the main wind. A sustained wind of 110 MPH (177 km/h) will raise 30-foot (9-m) waves, and most hurricanes can produce gusts much stronger than this. Not even the largest, most powerful ships sail through such a storm deliberately—but occasionally they do so by accident. In 1944, a U.S. battle fleet in the Philippine Sea was misinformed about the location and movement of a small but extremely violent typhoon

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Wind force and Admiral Beaufort In 1806, the Royal Navy issued a scale by which the commanders of its warships could estimate the strength of the wind by observing its effects. In its early versions (it was revised several times) no mention was made of the actual speed of the wind. Instead, the wind was defined in terms of the speed at which it would propel a man-of-war carrying a specified amount of sail. In a gentle breeze, for example, a ship under full sail and in smooth water would move at 3–4 knots (3.45–4.6 MPH; 5.5–7.4 km/h). Wind speeds were agreed in 1926 and finally added to the scale in 1939. The scale had been devised by Commander Francis Beaufort (1774–1857), and is still known as the Beaufort scale. It was adopted internationally at the International Meteorological Conference, held in Brussels in 1874. The Beaufort scale classifies winds into 13 named “forces” (in 1955 meteorologists at the U.S. Weather Bureau added five more to describe hurricane-force winds). Wind speeds were originally given in knots, the unit that is still often used by ships and aircraft. In the scale given here, knots have been converted to miles per hour and rounded to the nearest whole number. (1 knot = 1 nautical mile per hour = 1.15 MPH = 1.85 km/h.) Force 0. 1 MPH (1.6 km/h) or less. Calm. The air feels still and smoke rises vertically. Force 1. 1–3 MPH (1.6–4.8 km/h). Light air. Wind vanes and flags do not move, but rising smoke drifts.

Force 2. 4–7 MPH (6.4–11.3 km/h). Light breeze. Drifting smoke indicates the wind direction. Force 3. 8–12 MPH (12.9–19.3 km/h). Gentle breeze. Leaves rustle, small twigs move, and flags made from lightweight material stir gently. Force 4. 13–18 MPH (20.9–29.0 km/h). Moderate breeze. Loose leaves and pieces of paper blow about. Force 5. 19–24 MPH (30.6–38.6 km/h). Fresh breeze. Small trees that are in full leaf wave in the wind. Force 6. 25–31 MPH (40.2–49.9 km/h). Strong breeze. It becomes difficult to use an open umbrella. Force 7. 32–38 MPH (51.5–61.1 km/h). Moderate gale. The wind exerts strong pressure on people walking into it. Force 8. 39–46 MPH (62.7–74.0 km/h). Fresh gale. Small twigs are torn from trees. Force 9. 47–54 MPH (75.6–86.9 km/h). Strong gale. Chimneys blown down, slates and tiles torn from roofs. Force 10. 55–63 MPH (88.5–101.4 km/h). Whole gale. Trees are broken or uprooted. Force 11. 64–75 MPH (103.0–120.7 km/h). Storm. Trees are uprooted and blown some distance. Cars are overturned. Force 12. More than 75 MPH (120.7 km/h). Hurricane. Devastation is widespread. Buildings are destroyed, many trees uprooted. In the original instruction: “Or that which no canvas could withstand.”

(the name given to a hurricane in that part of the world) and sailed directly toward its center. Sailors know, from Buys Ballot’s law, that in the Northern Hemisphere, if they stand with their backs to the wind, the storm center will be to the left. In this case, however, the meteorological officer was confident he knew exactly where the storm was and that it was moving away from the fleet. By the time the commanding admiral realized a mistake had been made it was too late. His ships were sailing into the storm rather than away from it and already they were struggling through 70-foot

What happens inside a hurricane (21-m) waves and winds of more than 115 MPH (185 km/h). By the time the fleet was clear, three destroyers had sunk, 146 aircraft were destroyed on board carriers, the remaining ships were too severely damaged to continue with their mission, and 790 sailors had lost their lives. If a ship has no choice but to sail close to a hurricane, it is safer to pass on the side of its path nearest to the equator (in either hemisphere). That is the side where the winds—circling the eye counterclockwise in the Northern Hemisphere, clockwise in the Southern—will blow the ship behind the storm. It is known as the navigable semicircle. The winds on the other side, which in any case are stronger because they blow in the same direction as that in which the storm is moving, will carry it in front of the storm, with a serious risk of being drawn into it. This is the dangerous semicircle. Waves generated by the wind move outward from the center of the storm. As the storm moves over the ocean, it continues to produce waves. These mix with earlier waves, but the movement of the hurricane means the direction of the waves is changing constantly, so waves may cross one another. The sea looks as though it is boiling, especially behind the eye of the hurricane, and the disturbance affects the sea over vast distances.

Death of the storm With so much energy being released so violently, a hurricane cannot survive for long. Most last no more than two or three days before their wind speeds decrease. A dying hurricane may continue as a deep depression that brings gales strong enough to cause considerable damage, and may even keep the name it was given during its life as a hurricane, but by this time it is not really a hurricane anymore. Its structure, of concentric spirals of cloud around a very warm center, allows it to sustain itself during its brief life, but that structure results from three conditions which the storm does not produce for itself and over which it has no control. The sea-surface temperature must be higher than 80°F (27°C). Should it fall below 76°F (24°C), there will be insufficient heat to maintain the convective currents that generate the towering cumulonimbus clouds. As the hurricane moves into a higher latitude, it will cross cooler water and lose its source of heat. There must be abundant water vapor to release latent heat as it condenses. Should the storm move over land, it will lose its supply of water. Finally, there must be a high-level anticyclone to disperse the rising air and thereby draw more air from below. The anticyclone may weaken, or drift away, or the hurricane may move from beneath it. It is pure coincidence if all three of these necessary conditions are present at the same time and in the same place while a tropical storm is strengthening, and the coincidence cannot last. After a few days one or other condition will be lost. Then, quite quickly, the hurricane will weaken and disappear.

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HURRICANES, TYPHOONS, AND CYCLONES Hurricanes in the United States and the Caribbean In an average year, 10 tropical storms develop over the North Atlantic, of which six grow into hurricanes. Two of those hurricanes are likely to be major—category 3 or higher on the Saffir/Simpson scale (see sidebar, page 114). The year 2001 was not average. It produced 15 tropical storms, of which nine became hurricanes and four of those hurricanes were major. Most of the hurricanes occurred between September and November, toward the end of the hurricane season. For the first time ever recorded, there were three hurricanes in November. None of the 2001 hurricanes crossed the coast of the United States, although Juliette came very close, but two of them—called Iris and Michelle—struck islands in the Caribbean. Allison, a tropical storm that was almost a hurricane, did cross into the United States, where it caused devastating flooding.

Allison Allison was unusual in that it formed over the eastern North Pacific early in June and then crossed Central America and entered the Gulf of Mexico. Its winds increased to 60 MPH (96 km/h) before it crossed into Texas on June 5 with winds of about 50 MPH (80 km/h). It weakened over eastern Texas, crossed back over the Gulf and strengthened once more, and then crossed Louisiana on June 11. It entered North Carolina on June 14 and remained stationary there for three days. Then it crossed back over the Atlantic and headed northward, finally dissipating off Nova Scotia. Allison generated fairly weak winds, but it caused the worst flooding ever associated with a tropical storm. More than 30 inches (762 mm) of rain fell in several places around Houston, Texas. Rain fell very heavily everywhere from eastern Texas, across the Gulf States, and along the Atlantic coast. The damage was estimated to cost at least $5 billion. Tropical storms rarely kill people, but floods are the most dangerous of all weather disasters. The floods that Allison caused cost 41 lives—23 in Texas, eight in Florida, seven in Pennsylvania, and one each in Louisiana, Mississippi, and Virginia.

Hurricanes in the United States and the Caribbean

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Juliette Juliette formed on September 21 and crossed over Baja California, Mexico, on September 30. It then entered the northern end of the Gulf of California. The storm finally disappeared on October 3. At its peak, Juliette had one of the lowest eye pressures ever recorded, of 923 mb. Like most hurricanes, Juliette began as an easterly wave that grew into a tropical depression. It became a hurricane on September 23. At that time it was intensifying rapidly. By the following day its winds had reached 132 MPH (218 km/h) and on the 25th they reached 144 MPH (231 km/h). Juliette was then a category 4 hurricane, but it was already weakening. When the hurricane passed to the west of the tourist resort of Cabo San Lucas, at the southern tip of Baja California, on September 28, its winds had dropped to about 90 MPH (145 km/h). Juliette moved parallel to the coast at about 10 MPH (16 km/h), then moved inland on the 30th, passing close to the town of San Carlos. Its winds had then fallen to 40 MPH (65 km/h). The map shows the track the storm followed. Heavy rains flooded more than 200 homes on the mainland of Mexico and the storm cost two lives. One victim was a vacationer from Denver, Colorado, who drowned while surfing at Cabo San Lucas. The other was a fisherman whose boat capsized near Acapulco.

Hurricane Juliette September 21 to October 3, 2001 hurricane tropical storm

2 3 1 22

midnight position/date

30 29

27 26 923 mb

25 24

23 22 21

Track of Hurricane Juliette, September – October 2001

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Iris Iris was a small hurricane, but a fierce one, that struck Belize in October. It began as an easterly wave that grew into a tropical depression on October 4, when it was close to the Windward Islands. It became a hurricane on October 7 as it passed to the south of the Dominican Republic, where three people were killed. It passed to the south of Jamaica, uprooting trees, ripping the roofs from buildings and still intensifying. Iris reached Monkey River Town, Belize, on October 8. By then it had become a category 4 hurricane, with sustained winds of 145 MPH (233 km/h). The pressure in the eye was 948 mb; it produced a storm surge of 13–18 feet (4–5.5 m). Its fiercest winds were felt in an area that was no more than about 15 miles (24 km) across and all of the damage was contained in an area only 60 miles (96 km) across. Winds of up to 70 MPH (113 km/h) affected some places as much as 145 miles (233 km) from the eye, however, and people were evacuated from their homes in parts of Belize City, where streets were flooded. A boat, the Wave Dancer, capsized in the Monkey River, near Belize City, killing 18 people. They were all members of the Richmond Dive Club, from Richmond, Virginia, and Wave Dancer was one of two boats they had hired for a weeklong fishing trip. Including the victims of the Wave Dancer, Iris killed a total of 31 people.

Michelle On October 29 another monster appeared in the form of a tropical depression near the eastern coast of Nicaragua. It remained there for two days, dumping enough rain to cause flooding in Nicaragua and Honduras. Late on October 31 it moved away over the Caribbean and began to intensify. By November 3 it had become Hurricane Michelle and it was heading for Cuba. Later that day its winds increased to 140 MPH (225 km/h), and Michelle was classified as category 4. It weakened fairly quickly and was downgraded to category 3 by the time it struck Cuba on the afternoon of Sunday November 4—although this still exposed Cubans to winds of up to 135 MPH (217 km/h). As it crossed the island, its winds decreased to 110 MPH (175 km/h), and by Sunday evening it was a category 2 hurricane and still weakening. Michelle left Cuba on Sunday night. It crossed the Bahamas and then moved out over the Atlantic. It was the fiercest hurricane to strike Cuba for 50 years and caused widespread damage. In Soplillar, a town of 500 people close to where Michelle crossed the coast, 100 of the 156 homes were destroyed. Five people died in Cuba, but Cuba was not the only country to suffer. Homes were destroyed, roads and bridges wrecked, and six people lost their lives in Honduras, four in Nicaragua, and two in Jamaica.

Hurricanes in the United States and the Caribbean

Opal The United States escaped serious hurricane damage in both 2000 and 2001. This was the result of chance. There were plenty of hurricanes, but they failed to cross the coast. In other years the nation has not been so lucky. Opal touched the mainland in 1995, but a few years earlier, in 1989, Hugo moved overland from South Carolina to Labrador. The encounter with Opal began at around 6 P.M. on the evening of October 5, when screaming wind and torrential, lashing rains crossed the coast near Panama City, Florida, between Pensacola and Tallahassee. Opal, the second hurricane to assault the area in two months, was moving north from the Gulf of Mexico. Anticipating its arrival, the governors of Florida, Alabama, and Mississippi had declared states of emergency. People were compulsorily evacuated from coastal areas and offshore islands close to the hurricane’s predicted path. All public buildings were closed in Pensacola and Mobile, Alabama. Staff controlling the emergency services moved into an underground bunker, and the U.S. Navy removed all the aircraft from its Pensacola airbase. By the time Opal reached Florida, its wind speed was falling. At its worst, when it was over the Yucatán Peninsula, in Mexico, the wind speed had risen to 150 MPH (241 km/h), with gusts stronger than that, but then they fell to below 130 MPH (209 km/h). Even so, it was ferocious enough. Five inches (127 mm) of rain fell during the night of October 5. That is considerably more than the 3.5 inches (89 mm) of rain Pensacola and Tallahassee ordinarily expect to receive in the whole of October. It was not so much the wind or rain people feared, however, but the sea. The hurricane brought huge breakers, and as the wind drove water toward the coast, the sea rose 12 feet (3.7 m) above its usual high-tide level. Homes near beaches were washed away and small boats were plucked from their moorings and hurled ashore. The storm-driven sea and wind killed 13 people in the United States, having already killed 50 in Mexico and Guatemala. The damage it caused in Florida and Alabama was estimated to have cost about $4 billion. Opal continued to move north at about 25 MPH (40 km/h), but weakening all the time. Some nine hours after it crossed the Florida coast it was about 55 miles (88 km) east of Huntsville, Alabama, and had been reclassified as a tropical storm, although it was still bringing heavy rain and galeforce winds. This hurricane was the 15th storm of the 1995 season (not all were hurricanes) and the ninth to strike the Gulf coast. Several storms faded harmlessly far from land, but had Opal not weakened so quickly it might well have proved one of the most violent of the century. In other respects it was fairly typical. It formed over Yucatán on September 27 and by the end of the month had intensified sufficiently to be classed as a tropical storm. It continued to grow fiercer, and on October 2 it was reclassified as a hurricane. Four

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days later it was already dying. Four days is the average life expectancy for an Atlantic hurricane. During its brief life, Opal moved almost directly north. Had it lasted longer, by then reduced to a tropical storm rather than being a full hurricane, probably its track would have curved to the east, much like Hurricane Hugo, which crossed the United States and Canada in 1989. Hugo lasted much longer and followed a more complete track, shown on the map.

Hugo Hugo began to form on September 11 in the eastern North Atlantic, not far from the African coast (the numbers on the map are the dates in September and the location of the storm on those dates). By the next day it had become a tropical depression. It was a tropical storm by the 14th and a hurricane by the 19th. Typically, its track had carried it westward and as it developed into a hurricane it swung northwest, crossing the coast at Charleston, South Carolina, at about 10 P.M. on the night of the 21st. Moving at an average speed of 29 MPH (47 km/h), by the 23rd its track

Hurricanes in the United States and the Caribbean was curving toward the east, and on that day and the next it crossed the northeastern United States. Its passage over land had isolated it from the warm water needed to sustain it, so by the time it reached North Carolina it had weakened to a tropical storm. It moved through Québec and Labrador, Canada, on the 24th, and on the 25th it crossed the coast again, moving out into the North Atlantic and heading northeast, now as an extratropical storm. Throughout its journey across eastern North America, the eye of Hugo had a diameter of 30 miles (48 km), which is large. Some are no more than five miles (8 km) across. A large eye means the entire storm is large, because the hurricane forms as circles around the eye, and the bigger the storm the fiercer are its winds (see sidebar, page 71). Hugo covered a vast area within which it caused a great deal of damage, estimated to have cost about $10.5 billion in the United States. This made it possibly the most destructive hurricane in American history in financial terms, though fortunately few lives were lost. Hurricanes develop quickly, but nowadays satellite monitoring gives the U.S. authorities several days to prepare for their arrival. As Hugo approached, about 12,000 people in Charleston were evacuated into shelters, although some who sought safety in Charlotte, North Carolina, found they had placed themselves directly in the hurricane’s path. There were only four deaths in the continental United States: one in Charleston, one in Charlotte, and two in Virginia. In fact, Charleston was lucky. The eye of Hugo passed to the east, exposing the city to the less violent winds on the left side of the storm (where the winds blow in the opposite direction to that in which the hurricane is traveling, reducing their speed). Had the eye crossed the coast just 20 miles (32 km) farther south, Charleston would have experienced the full force of the much more dangerous right side of the storm (where the 29 MPH (47 km/h) at which the storm was moving would have added to the speed of the spiraling wind). The Caribbean islands fared worse. They are more exposed and have less time to prepare, because the storm is already close to them when it first forms. Hugo reached first Guadeloupe and then Dominica, in the Leeward Islands, on September 17, and the U.S. Virgin Islands and Puerto Rico on the 19th. There was time in Puerto Rico to warn shipping to clear the area, close the airport, and organize street patrols to prevent the looting that took place in the wake of the storm in St. Thomas and St. Croix, in the Virgin Islands, but the death toll was high. Eleven people were killed in Guadeloupe, 10 in Montserrat, six in the Virgin Islands, and 12 in Puerto Rico.

Luis Atlantic hurricanes travel west, then north, and finally east, but not all of them reach the United States. In 1988, for example, 11 tropical storms developed in the Atlantic, including five hurricanes, three of which generated winds of more than 131 MPH (211 km/h). Only four of them

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Hurricanes crossed the United States coast, although one of the hurricanes, Gilbert, came close enough for its outer margins to cause some damage in Texas. If a hurricane develops far enough to the east, its curving track may carry it parallel to the coast of Florida and the Carolinas, but it will remain over the ocean. Communities on land will then escape its full force. This is partly because they are well clear of the eye and partly because at all times they are to the left of the eye, in the quarter where the wind speed is lower (see the figure on page ix). That is how the United States escaped Luis, which struck the U.S. Virgin Islands and Puerto Rico on September 6, 1995. With an eye 60 miles (96 km) across and a total diameter of about 700 miles (1,126 km), it covered a surface area of about 385,000 square miles (997,150 km2), Luis was even bigger than Hugo in 1989. It was not quite so fierce, though, with winds gusting up to 140 MPH (225 km/h). Traveling west at about 12 MPH (19 km/h), it struck Guadeloupe, the U.S. Virgin Islands, and Puerto Rico. As is usually the case, the sea did as much damage as the wind and torrential rain, rising up to 9 feet (2.7 m) above its usual high-tide level. In Guadeloupe, a huge wave swept a French tourist to his death as he was trying to photograph the sea. The Caribbean islands are popular tourist destinations, and airlines, tour operators, and foreign governments all issued warnings to their customers or citizens. In this case, Thomas Cook, the British tour operator, flew its customers to Barbados or Miami and the British Foreign Office advised travelers to avoid the area or, if they were there already, to remain in secure accommodations and follow the instructions of the local authorities. Puerto Rico, the U.S. Virgin Islands, and the countries of Central America suffer more from hurricanes than most countries. In September 1995, for example, hurricane Marilyn destroyed four-fifths of the houses on St. Thomas, in the Virgin Islands, although the only casualties occurred among people on boats when the storm struck. Of those, three died and 100 were injured or missing. The team arriving by air with emergency supplies said none of the buildings on St. Thomas appeared to have roofs. Hurricane Roxanne, which brought 115-MPH (185-km/h) winds to Mexico in October 1995, killed at least 14 people and forced tens of thousands to leave their homes.

Gilbert From September 12–17, 1988, Gilbert, one of the worst of all hurricanes in recent years, ravaged Jamaica and Mexico. Then it moved into Texas and then into Oklahoma. It triggered more than 29 tornadoes, which caused property damage costing an estimated $40–$50 million, although by the time it reached Texas its force had largely abated and farmers in Texas welcomed the rain it brought. At its peak it was a category 5 hurricane. It killed a total of 318 people. A minor deflection in the trade winds making up the easterly wave that would develop into Gilbert was first noticed on September 3 as a

Hurricanes in the United States and the Caribbean group of clouds moving westward over the ocean, away from the coast of West Africa. It was designated a tropical storm on September 9, when its wind speeds exceeded 39 MPH (63 km/h). That was the day it moved through the Lesser Antilles. A day later, with wind speeds exceeding 74 MPH (119 km/h), it officially became a hurricane, and within about 12 hours its winds had increased to more than 96 MPH (154 km/h). On the 11th the National Hurricane Center in the United States warned the Jamaican authorities to expect a hurricane with winds in excess of 100 MPH (160 km/h) in the next 12 to 24 hours. When it reached Jamaica, passing over Kingston around noon on September 12, the winds were more than 111 MPH (178 km/h) and the hurricane was still intensifying. When its center passed about 20 miles (32 km) to the south of Grand Cayman Islands, at 9 A.M. on September 13, its winds (on the dangerous, northern side) were blowing at more than 131 MPH (211 km/h), and two hours later they exceeded 155 MPH (249 km/h). It had reached category 5. By 6 P.M. on September 13 the atmospheric pressure in the eye was 888 mb. That is the lowest surface pressure ever recorded in the Western Hemisphere. The storm reached the Mexican coast on September 14. By then it had weakened to category 3, but nevertheless a Cuban ship several miles out at sea was thrown onto the shore as the sea rose by 20 feet (6 m). Gilbert’s eyewall began weakening as the hurricane moved inland, but away from the center the winds retained their force for some time longer. Once the eyewall disappeared, however, it did not form again.

Lives and property When a hurricane does reach the United States coast, adequate early warning and efficient, experienced emergency services minimize casualties. Property is not so easily protected, however, and over the years in which they have killed and injured ever fewer people, the cost of hurricane damage has increased. This is due to the popularity of vulnerable areas as places to live and to the increase in the amount of property that is insured. The cost that is quoted for hurricane damage—as well as damage from any other cause— is based on the insurance cost. Hurricanes are likely to cross the coast of the United States anywhere along a stretch of about 2,000 miles (3,200 km) from Texas on the Gulf of Mexico, around Florida, to Virginia on the Atlantic. The number of people living along this coastline has roughly doubled since the 1930s, and there are now more people living in the Miami and Fort Lauderdale areas of Florida than lived along the entire stretch of coast in the first half of the 20th century. To make matters worse, many of them live in mobile homes. Not only can a hurricane demolish such dwellings entirely, but in doing so it can hurl debris from them against other buildings to cause further damage. According to Lloyds of London, the biggest insurance organization in the world, between 1966 and 1987 there was no single natural disaster costing more than $1 billion

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Hurricanes in insurance claims, but between 1987 and 1992 there were 10 wind storms with a combined cost of more than $15 billion. Camille, a hurricane that struck Mississippi and Louisiana in 1969, illustrates this. Of comparable strength to Gilbert, in 1988, but with a higher eye pressure (905 mb), it killed some 250 people living along the coast and a further 125 died in the flooding it caused. The cost of the damage it did to property came to $1.42 billion. In 1989, however, Hugo caused 43 deaths and damage in the U.S. costing $10.5 billion, while in 1995 Opal caused 13 U.S. (and 50 Central American) deaths and property damage costing $4 billion. Hurricanes are becoming less deadly, but more costly. Hurricane Camille illustrates the further point that hurricanes may interact with other weather systems. Camille was very strong, but it had weakened to a tropical storm when it turned east and crossed the Blue Ridge Mountains, Virginia. There, its winds gathered moist air that was moving inland from the Atlantic and funneled it through two narrow valleys, of the Rockfish and Tye Rivers. Then this moist air met an advancing cold front that was already producing thunderstorms. The moist, low-pressure air rose up the front (see sidebar, page 19) and produced 18 inches (457 mm) of rain in a matter of hours. Most of the damage Camille caused was due to the resulting floods. Ordinarily, in recent years there have been an average of nine or 10 named storms and six hurricanes. There were 11 Atlantic hurricanes in 1995, compared with only three in 1994. When tropical storms that failed to develop into full hurricanes are included, the 1995 season was more active than any for more than 60 years, although this is not the case when only named hurricanes are counted. As the graph shows, there were 12 hurricanes in 1969 and 11 in 1950. All this illustrates is the extent to which the number of hurricanes can vary from one year to another. Some scientists believe there have been more hurricanes in recent years and that they are becoming more frequent. In fact, though, as the graph illustrates, the number each year has changed little since 1945, although there have been particular years with more—for example, 1950, 1969, 1995, and 1998. It is also true that hurricanes were more frequent during the first half of the 20th century than they are now. Indeed, it is the long period during which hurricanes were fairly uncommon that lulled so many people into moving into hurricane-prone areas believing they were safe. Some scientists suspect that hurricanes may be becoming more frequent, but there is no clear evidence for this. There was a sharp increase of about 50 percent in hurricane frequency from 1994 to 1999, but it appears to have leveled off, and five years is far too short a period to reveal a long-term trend. The trend from 1945 to the present day shows no increase in frequency. Certainly there is no evidence that hurricanes are becoming more violent. In fact, the average sustained wind speed in hurricanes has been decreasing slowly but steadily since 1940. On the present evidence, there is no reason to fear that hurricanes in the Atlantic and Caribbean are becoming more frequent or more violent.

Hurricanes that reach Europe

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Hurricanes that reach Europe Western Europe enjoys an equable climate. True, it rains a lot, but extremes are fairly uncommon. It is not a part of the world where people expect to be scorched or frozen, or to watch windstorms demolish their homes. These are not what Europeans expect. But occasionally it is what happens. At least 17 people died in January 2002 in storms that lashed northern Europe from Scotland to Poland. Five of them were drivers of trucks that were blown over by winds gusting to 100 MPH (160 km/h). A state of emergency was declared in Kaliningrad, a piece of Russia bordering Lithuania and Poland, when the winds reached 74 MPH (120 km/h). The sea level rose more than 13 feet (4 m) above its usual level along the Danish North Sea coast and residents were evacuated. It happened around Christmastime in 1999—and the weather struck not once, but three times. By the time the storms had passed, nearly 200 persons had lost their lives and the number of trees that were blown down over three days was equal to six months’ European timber harvest. France was the country that suffered most. The streets of Paris were littered with fallen trees, and in the country as a whole about 100 square miles (259 km2) of forest was flattened—more than 2.5 times the amount of timber harvested annually. More than 400,000 French homes lost their telephone service, more than 2 million were without power, and in some rural areas the water supply failed. Insurance companies estimated the cost of the damage at about $385 million. When it was over, the French government declared a national emergency covering two-thirds of the country and

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Number of Atlantic hurricanes each year 1944 –2000 and the fiveyear running mean

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Hurricanes mobilized 6,000 troops to clear debris and help restore supplies of drinking water. The first storm occurred on December 11. It affected Denmark and Sweden, bringing the fiercest gales for more than a century to Denmark and causing severe damage to forests. This storm was but a prelude to the monsters that would soon assault the lands farther south. A very deep depression formed on December 23 over the western North Atlantic, a little to the south of the latitude of Cape Hatteras, then moved along a track that carried it northeastward to the latitude of Newfoundland and then eastward. It passed to the south of Ireland and reached the French coast in the early hours of December 26. The storm generated winds gusting to 107 MPH (173 km/h) at Orly Airport, Paris, and in places reaching 136 MPH (219 km/h). By noon the storm had reached Germany. It reached Poland by evening, but by then it was weakening. People were still reeling from this storm when the second arrived. Its life began on Christmas Day, as a depression that formed at about 38° N 65° W—over the North Atlantic due south of Nova Scotia. It crossed the ocean, moving northward as it did so, and reached the Britanny coast of France on the afternoon of December 27. This storm produced winds just as violent as those of the first, but the storm track lay farther to the south. It affected northern Spain and after crossing central Germany, the storm moved through Switzerland and into Italy. It killed 122 people, 70 of them in France. As well as the wind, the second storm caused flooding in parts of Belgium, Germany, Switzerland, and Spain, and heavy snowfalls in the mountains, leading to avalanche alerts being issued for the French Alps. A group of nine German tourists celebrating the millennium in a rented mountain hut were among the 12 people killed by avalanches in Austria. The combination of wind, rain, and snow brought down trees and power lines in Transylvania, about 280 miles (450 km) from Bucharest, Romania, cutting the power supply to more than 100 villages. Both storms generated winds of hurricane strength, but the storms themselves were not hurricanes. They formed outside the Tropics, and so failed to qualify as “tropical cyclones,” and they lacked the warm center that is a diagnostic feature of true hurricanes. Charley, on the other hand, was a hurricane.

Gentle Charley As hurricanes go, Charley was nothing special. It reached the coast of North Carolina on August 17, 1986, moved northward along the coast of Maryland, then turned east and headed out over the Atlantic. It caused little damage. Indeed, it brought welcome rain to farmers in the Carolinas, Virginia, and Maryland. There were only three Atlantic hurricanes in that year, and none was particularly strong. Charley, the gentle hurricane, might have been forgotten, only it persisted. Its track carried it right across the North Atlantic and a few days later it reached Britain. By this time it was classified as only a storm. Its warm eye had cooled and its winds weakened. Nevertheless, it retained some of its old

Hurricanes that reach Europe vigor, and people living in southwestern Wales were unprepared for its arrival. It took them by surprise and caused a great deal of damage. Although officially it was no longer a hurricane, it retained the essential structure of one. It still had the remains of an eyewall of cloud bringing gales and torrential rain and, behind that, the eye in which the sky was clear and the air still. As the storm passed overhead, the eye moved away to be followed by the second side of the eyewall, and even fiercer winds. It took two days and the night between to pass. The combination of rain and high seas caused widespread flooding. The Tenby inshore lifeboat had to rescue holidaymakers from one inland trailer park. Dyfed, the northern part of which took the full force of the storm, is the county occupying the peninsula of southwest Wales. It is mainly rural, a sparsely populated place of farms, hills, and small villages. This limited the damage to property—there was not much property for the storm to wreck. The next hurricane to strike Britain did so in England, and the consequences were much worse.

Floyd Hurricanes can cross the Atlantic and occasionally they do, arriving with enough power to wreak considerable havoc. Floyd, the last hurricane of the 1987 season, formed on October 9, and on the 12th its winds reached more than 75 MPH (121 km/h), the lowest speed to qualify it as a hurricane. It passed through the Florida Keys, but within 12 hours it had weakened to less than hurricane force. Then it headed out over the ocean. European meteorologists saw it coming, but miscalculated its track. This is easily done, because a degree or two can make a great deal of difference. The meteorologists believed Floyd would pass through the English Channel, well south of the British coast, then head into the North Sea, weakening all the time. Obviously, it would endanger shipping, and the English Channel is one of the busiest sea-lanes in the world, but they said people on shore had nothing to fear. It arrived on the night of October 15, traveling a few degrees to the left of its predicted path, and caused havoc throughout the densely populated towns and villages of southern England. Though barely a hurricane in the strict sense, its winds gusted to more than 80 MPH (129 km/h) and by dawn on the 16th, 19 million trees had been uprooted. In Kent, to the southeast of London, there is a small town called Sevenoaks. Its name refers to seven oak trees growing there. What came to be called “The October Storm” reached Sevenoaks in the early hours of Friday, October 16, and it brought down six of the seven trees. People joked about the need to rename the town, but it was unnecessary, because the trees were soon replaced. The storm killed 19 people, and the damage cost about $2.25 billion (about £1.5 billion) in 1987 money.

Burns Night For Scots people everywhere, the evening of January 25 is Burns Night, when the celebration meal is paraded into the dining room to the sound of

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Hurricanes the bagpipes, haggis is eaten and a certain amount of whisky is drunk, and the national poet, Robert Burns, is honored by reading aloud from his works. On that day in 1990, Britain, along with much of northwestern Europe, suffered the fiercest storm in many years. This time, although it was technically a storm rather than a hurricane, winds reached more than 100 MPH (160 km/h). Its track was predicted accurately, but there was nothing anyone could do to protect property. Roofs were torn from buildings, trees uprooted, power lines brought down, and transport and communications seriously disrupted. Most of Britain was affected and about 47 people died. The storm then moved into continental Europe, killing 19 people in the Netherlands, 10 in Belgium, eight in France, seven in Germany, and four in Denmark. Soon after that, on February 3, 29 people died when winds of hurricane force struck France and Germany, and a month later, on February 26, another windstorm killed at least 51 people in Britain, the Netherlands, Germany, France, Belgium, Switzerland, Ireland, and Italy. Winds strong enough to qualify a storm as a hurricane occur in Europe every few years. There was one in December 1993, for example, which killed 12 people in Britain, and on January 25 and 26, 1989 hurricane-force winds killed at least 12 people in Spain. The middle 1980s was a time of little hurricane activity. The Atlantic was quiet. Few tropical storms developed, few of those grew into hurricanes, and those that did were generally weak. Weak storms are just as likely to cross the ocean as strong ones, however, because what matters is not the size or power of the storm, but the distance it travels over land. Hurricanes are sustained by a limitless supply of water. It is evaporating seawater and sea-spray that releases the latent heat to feed the vigorous convection needed to build the warm, towering clouds of the eyewall. If, like Charley and Floyd, a hurricane turns north without penetrating the American mainland, there is a chance it will turn east before it begins to weaken significantly. It may then encounter weather systems that reinvigorate it, so what seemed to be a dying hurricane strikes Europe with formidable strength.

Frontal storms When steady wind speeds exceed 75 MPH (121 km/h), they are rated as “hurricane force” on the Beaufort scale (see sidebar, page 78). Winds of this force in Europe are not really hurricanes, however, even if that is how they began, far away in the tropical Atlantic. They lack the extreme violence of a tropical cyclone, because they are not fed energy by a very warm sea, and most European “hurricanes” are associated with frontal systems. Tropical cyclones are not frontal. Over the Tropics, the air temperature and pressure is fairly constant throughout vast areas, and a quite small disturbance is enough to trigger

Air masses and the weather they bring As air moves slowly across the surface, it is sometimes warmed, sometimes cooled, in some places water evaporates into it, and in others it loses moisture. Its characteristics change. When it crosses a very large region, such as a continent or ocean, its principal characteristics are evened out, and all the air is at much the same temperature and pressure over a vast area and is equally moist or dry. Such a body of air is called an air mass. Air masses are warm, cool, moist, or dry according to the region over which they formed and they are named accordingly. The names and their abbreviations are straightforward. Continental (c) air masses form over continents, maritime (m) ones over oceans. Depending on the latitude in which they form, air masses may be arctic (A), polar (P), tropical (T), or equatorial (E). Except in the case of equatorial air, these categories are then combined to give continental arctic (cA), maritime arctic (mA), continental polar (cP), maritime polar (mP), conti-

nental tropical (cT), and maritime tropical (mT). Equatorial air is always maritime (mE), because oceans cover most of the equatorial region. North America is affected by mP, cP, cT, and mT air, the maritime air masses originating over the Pacific, Atlantic, or Gulf. These are shown on the map. As they move from where they formed (called their source regions) air masses change, but they do so slowly and at first they bring with them the weather conditions that produced them. As their names suggest, maritime air is moist, continental air is dry, polar air is cool, and tropical air is warm. At the surface there is little difference between polar and arctic air, but there are differences in the upper atmosphere. It is cP air spilling south when the cT and mT move toward the equator in the fall that brings cold, dry winters to the central United States. It is the meeting of mT air from the Gulf and cT air from inland that produces fierce storms in the southeast of the country.

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Hurricanes the development of a tropical cyclone. In middle latitudes the situation is very different. Air masses with quite different characteristics cross constantly, mainly from west to east (see sidebar). Where two air masses meet while moving at different speeds a front develops (see sidebar, page 19). Where a warm and cold front meet at surface level, there is often a region of low pressure, called a depression. Depressions also move, generally from west to east, dragged along by waves that move through the jet stream overhead (see sidebar below). Air flowing into a depression is deflected by the Coriolis effect into a spiraling path around it and the greater the difference in pressure between the center of the depression and the air surrounding it, the stronger the winds will be. As they approach the center, the conservation of their angular momentum (see sidebar, page 71), causes the winds to increase in speed. Depressions in middle latitudes can be deep and, therefore, the winds around them can be strong. Winds often reach gale force, and at sea they

Depressions and the jet stream A front is the boundary between two air masses. An air mass is a large volume of air with distinctive characteristics of temperature, pressure, and humidity. These characteristics are fairly constant throughout the air mass at any given height. Adjacent air masses do not mix readily, because the air in them is at different densities. The sharp difference in temperature to either side of a front produces the difference in pressure, a difference that increases with height. This is because cold air is more compressed than warm air is. Consequently, air pressure decreases with height more rapidly in cold air than it does in warm air, so that the difference in pressure on either side of the front increases with height, reaching a maximum at the tropopause. The pressure difference generates a wind, called a thermal wind because of its association with a temperature difference. The polar fronts between polar and tropical air are permanent in both hemispheres. They are located in the middle latitudes, although their positions shift with the seasons. These are the fronts that produce the strongest thermal winds. At the tropopause, where the thermal wind speed reaches

a maximum, they are known as the polar front jet streams. The jet stream is usually located in the cold air, and it flows with the cold air to its left in the Northern Hemisphere and to its right in the Southern Hemisphere—resulting in a west-to-east direction of flow in both hemispheres. There is also a subtropical jet stream. It lies at about 30° N and 30° S throughout the year; the pressure difference generating it occurs only in the upper troposphere. The polar front jet stream does not blow in a straight line. There are waves in it. Waves that carry the jet stream toward the pole are known as ridges, and those that carry it toward the equator are troughs. Downwind of each ridge, air is being drawn into the jet stream and air is subsiding. This produces a region of high surface pressure. Air is leaving the jet stream downwind of each trough. This draws more air upward, producing a region of low surface pressure. The diagram shows the relative positions of the ridges, troughs, and surface highs and lows. A center of low surface pressure is known as a depression. It appears on weather maps at the crest of a frontal wave, with a warm front on one side and a cold front on the other. Between the fronts there is a

Hurricanes that reach Europe

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can sometimes reach hurricane force, of more than 75 MPH (121 km/h). Winds this strong are uncommon over land, because their passage over uneven ground generates friction, which slows them, but now and then one crosses a coast and may even penetrate deep inland. To a meteorologist, tropical and midlatitude depressions are cyclones, regions of low pressure around which air flows cyclonically (counterclockwise in the Northern Hemisphere), and any cyclone can produce strong winds. A tropical cyclone forms in the absence of fronts, whereas most midlatitude cyclones are frontal, but the principal difference is found at the center. A tropical cyclone forms over very warm water and turns into a hurricane when it develops an eye and eyewall at a markedly higher temperature than the surrounding air. A midlatitude cyclone lacks this warm core. It is the warmth that drives the hurricane, and because they lack this huge source of energy, midlatitude depressions may produce fierce storms, but they can never grow into full-scale hurricanes.

wedge of warm air—this area is known as the warm sector. It is from the warm sector that air is rising. The waves in the jet stream move from west to east (in both hemispheres). As they do so, the surface pressure pattern moves with them, highs (anti-

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cyclones) always ahead of each ridge and lows (cyclones) ahead of each trough. This is why depressions travel from west to east in middle latitudes, and why the weather in this part of the world is so changeable.

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Asian typhoons and cyclones The people of southern China, Taiwan, and the Philippines are no strangers to tropical storms and typhoons, and 2001 was a fairly typical year. It is the wind speed that determines whether or not a storm is deemed violent enough to be called a typhoon, but to its victims this is often a distinction without a difference. It is not so much the wind that wrecks homes and destroys lives, but the rain. Typhoon Durian struck all three countries in late June 2001. It began in the Philippines, crossed Taiwan, and arrived in Guangdong Province, China, on Sunday, July 1. Its winds reached 104 MPH (167 km/h), and it delivered up to 12 inches (305 mm) of rain. No deaths were reported, but the Chinese authorities said that repairing the damage in their country would cost at least $446 million. Durian moved next to northern Vietnam, where up to 17 inches (432 mm) of rain fell over three days. Six people died when a landslide buried their homes, and three people were swept away in the floods and drowned. Yutu struck Guangdong in the last week of July with winds of up to 94 MPH (151 km/h). The typhoon destroyed about 2,340 houses around the city of Maoming and damaged thousands more, as well as destroying crops. The total cost of that storm was put at $76 million. In early July 2001 Typhoon Utor generated winds of 80 MPH (129 km/h) gusting to 97 MPH (156 km/h) and rain that triggered flash floods and mudslides. Utor killed 121 people in the Philippines and left more than 40 unaccounted for. Most of the victims died when they were caught in mudslides or landslides. The typhoon merely grazed southern Taiwan. Landslides and floods blocked some roads, and one person was swept into a river and drowned. From Taiwan, Utor crossed into Guangdong Province, in southern China, where it dissipated. Toraji arrived in Taiwan early on Monday, July 30. One survivor wept as he described how floodwater had rushed through the living room of his house, carrying away his wife and nine other members of his family so rapidly there was nothing he could do to save them. He was left after the storm, digging through mud and debris in the hope of finding some trace of them. By the time it left Taiwan later that day, Toraji had cost at least 100 lives and many more people were missing. Early on Tuesday morning, Toraji arrived in Fujian Province, in mainland China, but it was weakening and caused no serious damage or loss of life there. Danas was weak. Its wind speeds barely entitled it to be called a typhoon. Nevertheless, it caused havoc when it struck Tokyo in September 2001. Five people died. Four of them were buried by mudslides just to the north of Tokyo. Trees fell, rain lashed down, and Toyota had to close 12 of its factories because it was impossible to move vehicles and supplies. Even as Danas—the name is a Philippine word meaning “to experience”—was moving away, another storm was approaching. Nari—which

Asian typhoons and cyclones means “lily” in Korean—was another weak typhoon. Most of the time it was classed as a tropical storm. Its wind speeds were misleading, however. Between early Sunday and around noon on Monday, September 16 and 17, Nari dumped 32 inches (813 mm) of rain on Taipei, Taiwan, overwhelming the drainage system, filling the streets to the height of car roofs and flooding parts of the subway. Rain rushed down mountainsides. Some people drowned in their beds. Five people were buried in mudslides. Homes were demolished and bridges destroyed in the north of the island, railroads and rail bridges were wrecked, and a total of more than 90 people lost their lives. A week later, people in Taipei were building sandbag walls in preparation for the arrival of Lekima—the name of a Vietnamese fruit tree. Its winds reached only about 74 MPH (119 km/h), but it moved very slowly, dropping around 20 inches (508 mm) of rain over parts of northern Taiwan. Lekima passed close to the Philippines. Tropical storm Lingling struck the islands in early November, killing around 300 people. On the island of Camiguin, about 440 miles (708 km) southeast of Manila, four hours of torrential rain triggered floods that swept away whole houses and sent boulders from the volcano Hibok-Hibok crashing into villages. The storm was so fierce it generated a waterspout. After leaving the Philippines, Lingling moved to Vietnam, destroying houses, uprooting trees, and sinking fishing boats.

The “big wind” In Mandarin Chinese, ta feng means “big wind,” and in some Chinese dialects it is pronounced tai fung. It is the name people give to the ferocious tropical cyclones that form in the South China Sea. These can devastate offshore islands and often cross the mainland coast, sometimes traveling a considerable distance inland and leaving a trail of destruction and human misery. Borrowing the Chinese phrase, we call these cyclones typhoons and extend the name to all tropical cyclones forming over the Pacific. The name is not used consistently, however. Tropical cyclones occurring near Indonesia and the Philippines are sometimes called baguios— Baguio is a town in the Philippines—and those near Australia are often called cyclones. “Cyclone,” in the sense of a tropical cyclone rather than the strict meteorological sense of a low-pressure region around which air moves cyclonically, is the name that used to be reserved for those occurring in the northern Indian Ocean. Nowadays tropical cyclones occurring in the southern Indian Ocean are often called “cyclones” as well. Of all the tropical cyclones that form, almost 90 percent are typhoons or cyclones. Eastern Asia, the Indian subcontinent, Indonesia, the Philippines, and the smaller islands of the tropical Pacific suffer nine such visitations for every one that occurs in the Atlantic and Caribbean, and Pacific typhoons are often fiercer than Atlantic hurricanes. The fiercest hurricane ever recorded was a Pacific typhoon called Tip, in October

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Hurricanes 1979. It had sustained winds of 190 MPH (305 km/h). Their ferocity is because the Pacific is much wider than the Atlantic. Developing storms have much farther to travel before reaching a continent and losing their source of warm water, and so they have more time in which to grow and intensify. For the same reason, a typhoon can cover a much larger area than a hurricane. “Supertyphoons” are rare, but when they occur they can cover 3 million square miles (7.8 million km2), an area equal to that of the mainland United States. Tropical cyclones leave their traces along the Great Barrier Reef, off the Australian coast, and along the eastern coast of the Gulf of Carpentaria, in northern Queensland. Scientists have used these to calculate the frequency of “supercyclones” in that region over the past 5,000 years. “Supercyclones” are those in categories 4 and 5 on the Saffir/Simpson scale (see sidebar, page 114). They have found that a supercyclone occurs every 200–300 years. No category-5 storm struck northeastern Australia during the 20th century, and the one that occurred in 1899 is the only one recorded since European colonization began in the middle of the 19th century, but there were two earlier in that century.

Wind and rain Think of a typhoon, and it is the wind that comes to mind. This is the most important feature of a tropical cyclone, especially while it remains at sea. Typhoons are huge. Winds of more than 75 MPH (121 km/h)—hurricane force—may extend around the eye to a diameter of 300 miles (483 km). They affect an area of more than 70,000 square miles (181,300 km2). These are the hurricane-force winds, but winds of gale force (more than 40 MPH; 64 km/h) can also cause considerable damage in the area surrounding the fiercest winds. In a tropical cyclone with a circle of hurricane-force winds that is 300 miles (483 km) in diameter, there are gales up to a further 240 miles (386 km) from the center. This adds more than 150,000 square miles (388,500 km2) to the total area affected. If a storm this size were centered over Kansas City, Missouri, its gales would be blowing hard as far away as Duluth, Minnesota; Amarillo, Texas; and Jackson, Mississippi. Wind is only one of the hazards, however. Torrential rain is at least as dangerous, and it is not unusual for a tropical cyclone to deliver 20 inches (508 mm) of rain in the space of 48 hours. The record is held by a typhoon that brought 38.5 inches (979 mm) of rain to Manila, in the Philippines, in 24 hours on October 17, 1967. Rainfall in the Philippines is heaviest in summer, between July and October. In the north this is due to the influence of the Asian monsoon (see sidebar), but in the south it is due mainly to typhoons. Even so, the wettest places expect no more than 20 inches (508 mm) of rain in the rainiest month, so the typhoon delivered almost double what is usually the heaviest monthly rainfall.

Monsoon The English word monsoon comes from an Arabic word, mawsim, which means “season.” Monsoons are strongly marked seasons. In Asia the winter monsoon is dry and the summer monsoon brings heavy rain. Monsoons resemble land and sea breezes, but occur on a huge scale. They are due to the different rates at which the land and sea warm and cool. During summer, the land heats much faster than the sea. Warm air rises over the land, forming a large area in which the atmospheric pressure is lower than it is over the cooler sea. This draws in air that is moist from its contact with the ocean surface. In winter, the reverse happens. The land cools rapidly and pressure over it rises. Air flows from land to sea, and, because the air originates deep inside a continent, it is very dry. So there is a dry winter monsoon and a wet summer monsoon.

WINTER

HIGH pressure

Monsoon climates occur over most of the Tropics. They affect much of tropical Africa, the northern part of Madagascar, the southern part of the Arabian peninsula, the Indian subcontinent, southern Asia, eastern Asia as far north as Japan, and northern Australia. Parts of North America also have a monsoon climate. West of the Rockies it brings a dry summer and rainy winter. On the eastern side of the continent the summer is wet and the winter dry. Monsoons are strongest over southern Asia, however, because of other changes in the seasonal distribution of air masses. During the summer, the equatorial trough moves north, intensifying the low pressure over land. At the same time, the Himalayan Mountains are so high they divide the air circulating over Asia into two distinct masses. In winter, the jet stream lies over the mountains and the polar high-pressure area covers most of northern Asia. Air flowing outward from it 60° intensifies the offshore winds. In summer, the jet stream weakens as 50° the continent warms, reducing the difference in air temperature to the 40° north and south of the mountains. The map shows how the changing 30° distribution of pressure alters the 20° direction of the wind. 10°

LOW pressure

0° 10°

SUMMER

60°

50°

LOW pressure 40° 30° 20° 10°

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one (ITCZ)

Winter and summer Asian monsoons. In winter, high pressure over central Asia generates dry winds blowing from land to sea. In summer, pressure is higher over the sea and moist winds blow over the continent.

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Cyclones Tropical cyclones in the Pacific and Indian Oceans are often severe. One that struck the Indian state of Orissa on October 29, 1999, destroyed several villages and caused widespread flooding. After it passed, the government confirmed that 9,463 people had lost their lives and around 8,000 were still missing. The cyclone season in the southern Indian Ocean (in the Southern Hemisphere) lasts from January to March. The season in the Bay of Bengal and Arabian Sea lasts from May to September. There is an average of 9.68 cyclones a year in the southern Indian Ocean and 8.75 in the Bay of Bengal and Arabian Sea. The actual number varies from year to year in both regions, but between 1945 and 2000 the average number decreased slightly in the southern Indian Ocean and very steeply in the Bay of Bengal, where cyclones are now uncommon. Ando was fairly typical of the cyclones that develop in the southern Indian Ocean. It formed on January 2, 2001, about 745 miles (1,200 km) east of the northernmost tip of Madagascar. It moved toward Madagascar and intensified, but on January 5 it turned to the south and passed between Madagascar and Réunion. It was traveling at 8–10 knots (9–11.5 MPH; 15–18.5 kmh) and came within 150 miles (240 km) of Réunion, but without crossing any of the islands in that area. Had it done so it would have caused severe damage, because at its peak intensity, reached on January 6, it was rated category 5. The pressure in its eye fell to 930 mb, its winds reached 140 MPH (225 km/h) with gusts up to 168 MPH (270 km/h), and it generated waves 20 feet (6 m) high. Its eye was well developed and 25 miles (40 km) in diameter. Geralda, a typhoon (or cyclone) that struck Madagascar on February 2–4, 1994, was even worse. It brought torrential rains driven by winds of up to 220 MPH (354 km/h). It left at least 70 people dead and half a million homeless, almost totally destroyed the country’s principal port, and flooded more than two-thirds of its farmland. Geralda was described as the “cyclone of the century,” but there have been others almost as fierce. On May 2, 1994, a cyclone with winds of up to 180 MPH (290 km/h) moving north across the Bay of Bengal crossed islands at the mouth of the Ganges, in Bangladesh. It killed more than 200 people. The death toll would have been even higher but for an early warning system that gave time for a major evacuation. A cyclone that reached the coastal islands of Bangladesh on April 30, 1991, before the early warning system was established, killed at least 131,000 people. Like all tropical cyclones, those of the Eastern Hemisphere form between latitudes 5° and 20°, but unlike the Atlantic, where no tropical cyclones form south of the equator, in the Pacific they develop in both Northern and Southern Hemispheres. They travel west in both hemispheres, then follow tracks carrying them away from the equator, heading north in the Northern Hemisphere and south in the Southern. In the Southern Hemisphere, of course, the Coriolis effect makes their winds spiral clockwise around the eye.

Asian typhoons and cyclones Except for those few that develop off the eastern coast of Central America and usually move out to sea, Pacific storm tracks carry typhoons toward densely populated lands. Cyclones cross the coasts of India, Pakistan, and Bangladesh and some, moving north through the Arabian Sea, reach Oman. South of the equator, they move toward Madagascar, some passing to the west of the island into the Mozambique Channel between Madagascar and Mozambique. In March 1994 a typhoon left 1.5 million people homeless in Mozambique.

Vulnerability of Bangladesh The Ganges, India’s greatest river, flows sluggishly and its level varies greatly. During the dry winter it carries little water and the level is low, but in spring it rises as meltwater from the Himalayas pours into it. It reaches its highest levels during the heavy monsoon rains of summer. This is also the time of year when cyclones are most likely to form in the Bay of Bengal to the south and move northward. Although the Asian monsoon is strongest in India, it also affects all of southern Asia and, on a smaller scale, there is also a monsoon season in tropical Africa. Most of Bangladesh is low-lying, with fertile plains nourished by silt that is deposited when the rivers flood. It is in Bangladesh that the Ganges, known as the Padma at this point, joins India’s other great river, the Brahmaputra, known locally as the Jamuna, and the combined rivers, known as the Meghna, flow toward the sea, swelled further by the many smaller tributaries that join them. Where the river enters the sea it forms a large delta. The Meghna has no single “mouth,” but many “mouths,” the individual branches of the river weaving their tortuous courses through a maze of islands and, in the south, an almost completely uninhabited region of mangrove swamp called the Sundarbans. The northern part of the delta is inhabited by people who build their homes on earth platforms or embankments to keep them clear of the seasonal floods, when all the river channels merge into a single stream. Bangladesh is one of the most densely populated countries in the world, with an average of nearly 2,000 people to every square mile (772 km–2), and most of them live in the countryside. Fish and other aquatic animals, such as prawns, are their most important source of dietary protein. Most are freshwater species, caught by fishermen who live in the countless villages beside rivers and in the northern part of the delta. When a cyclone strikes, they have little protection. After tropical storms on April 17, 1994, 200 fishermen from the town of Cox’s Bazar were missing and feared drowned, and 5,000 fishermen were missing from coastal islands after a cyclone in 1991, which caused at least 131,000 deaths. Even this was not the most severe storm recorded. A cyclone in November 1970 left some 500,000 Bangladeshi people dead. It was one of the worst natural disasters of the 20th century.

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Pacific typhoons As typhoons move westward across the Pacific, their tracks are likely to bring them close to many small islands and then to the Malay Archipelago, the largest archipelago in the world. Indonesia, occupying the larger part of the archipelago and with an area of about 741,000 square miles (1.92 million km2), is a string of about 3,000 islands straddling the equator and stretching for more than 3,000 miles (4,800 km) from the Malay Peninsula to New Guinea. Geographically, the Indonesian islands are joined to the Philippines. These cover a smaller area, of about 116,000 square miles (300,440 km2), but comprise 1,000 inhabited islands and 6,107 uninhabited ones. Most of Indonesia lies to the west of the usual typhoon tracks, which are turning north by the time they reach the islands, although Indonesia does not escape entirely. Turning north, however, the storms head directly for the islands lying between Indonesia and China—the Philippines.

The Christmas Day storm Much of the northern coast of Australia north of about 20° S is exposed to Pacific typhoons, although severe ones are rare so far south. On Christmas Day in 1974, however, the country experienced its worst natural disaster ever when Cyclone Tracy devastated the northern city of Darwin. The cyclone started to form on December 21 over the Arafura Sea between Australia and New Guinea. In the following days it intensified and moved southwestward on a track carrying it toward the Timor Sea, south of Timor, and from there it was expected to move into the Indian Ocean. Forecasters predicted it would remain at least 60 miles (96.5 km) from the Australian coast. Then, on Christmas Eve, Tracy intensified still further and changed course, now heading southeast and directly toward Australia. It reached Darwin at 4 A.M. on Christmas morning, with winds of up to 150 MPH (241 km/h). The storm lashed the city for about four hours. By the time it moved away, more than 50 people had lost their lives, 48,000 had been evacuated, and 90 percent of the buildings had been destroyed, including 8,000 homes.

Southern and eastern Asia Perhaps the most vulnerable regions of all are in southern and eastern Asia. By the time they reach eastern Indonesia, many typhoons are turning north, toward the Philippines, Vietnam, China, and, beyond them, Korea and Japan. Ten people died in August 1994 in Taiwan in a fairly weak typhoon, with 85-MPH (137-km/h) winds, but typhoons in that part of the East China Sea can be much worse. On August 20–21 of the same year, Typhoon Fred battered Zhejiang Province, China, for 43 hours without a break. It killed about 1,000 people and caused damage costing an

Asian typhoons and cyclones estimated $1.1 billion. The same region suffered twice in August 1990, when Typhoon Yancy killed 216 people in Fujian and Zhejiang Provinces and Typhoon Abe killed 48 people in Zhejiang. The following year Typhoon Amy killed at least 35 people in southern China. It is appropriate that we derive our name for such storms from the Chinese. Taiwan lies at the southern end of the East China Sea, where tropical cyclones formed in the South China Sea are turning north. Still farther north there lie the islands of Japan. Japan, in the East China Sea but between about 30° N and 45° N, might be thought safe from all but occasional typhoons that have weakened to tropical storms by the time they arrive, but typhoons often retain their power long enough for them to travel considerable distances. In 1953 about one-third of the Japanese city of Nagoya, just north of latitude 35° N on the island of Honshu, was destroyed by a typhoon that left 1 million people homeless. This was far from being the most devastating tropical cyclone the Japanese have had to endure. Hokkaido, the northernmost island, was struck in 1954 by a tropical storm that left 1,600 people dead, and in September 1959 Honshu suffered the worst typhoon in modern Japanese history. Called Vera, it killed more than 4,000 people and left 1.5 million homeless.

The Fujiwara effect Occasionally, two typhoons approach within about 900 miles (1,448 km) of each other. This is close enough for them to interact, and they start to turn around their common center, much like two stars orbiting their common center of gravity that lies somewhere between them. If the typhoons are of approximately equal size, they will turn around a point more or less halfway between their two centers. If one is much larger than the other, they will turn about a point closer to the larger one, and in this case the larger one usually absorbs the smaller. This can also happen in the Atlantic. On August 23, 1995, Tropical Storm Iris was approaching the Windward Islands, with Hurricane Humberto close behind. Iris slowed and turned a little to the south, while Humberto moved to the north. As the two began to circle each other, both of them weakened and then separated. About a week later, Iris was to the east of Bermuda and heading northward, now a full hurricane. Tropical Storm Karen moved in behind it, and again they began to circle. This time, though, Karen was much the weaker of the two, and Iris absorbed it. It happened most recently on September 6–7, 2001. Gil and Henriette, two Pacific storms, circled each other. Fortunately, neither of them crossed over land. This circling is called the Fujiwara (or Fujiwhara) effect, after Sakuhei Fujiwara (or Fujiwhara), the meteorologist who first described it. In the years following World War I, Fujiwara was a professor at the University of Tokyo and director of the Japanese Meteorological Society.

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Arctic and Antarctic hurricanes JOIDES Resolution is a research ship that is used to study the ocean floor. On each cruise she usually carries a crew of 52 to operate the ship, as well as about 20 engineers and technicians and 30 scientists. “JOIDES,” pronounced “joy-deez,” stands for Joint Oceanographic Institutions for Deep Earth Sampling, which is the former, rather unwieldy name for what is now known as the Ocean Drilling Program (ODP). The ODP is an international scientific project that involves examining rock and sediment collected in the form of cores that are drilled from the seabed. The work done on JOIDES Resolution forms part of the ODP. She can work in seas up to 27,000 feet (8.2 km) deep and has gathered cores from all of the world’s oceans. Her drilling rig, the pipes carried on her deck, and the 23-feet- (7-m) wide hole—called the “moon pool”—through her hull down which the drill string passes make the ship look a little unwieldy, but JOIDES Resolution is big and tough. Originally built in Halifax, Nova Scotia, for oil exploration, she went into service in 1978, with the name Sedco/BP 471. She was later converted to equip her for scientific research and went into service with the ODP in 1985. JOIDES Resolution measures 469 feet (143 m) from stem to stern, 68.9 feet (21 m) from side to side, and the top of her drilling derrick is 202 feet (61.5 m) above the water. She is owned by a company called Ocean Drilling Ltd. Both oil exploration and scientific exploration demand working in bad weather and JOIDES Resolution is built to withstand anything the sea can throw at her. In the fall of 1995, however, she came close to sinking in an Atlantic storm. The voyage began uneventfully. JOIDES Resolution set sail from Iceland in late September with 120 people on board, a few more than usual, and headed for the Greenland Sea, deep inside the Arctic Circle to the east of Greenland. The weather was not good, but JOIDES Resolution sailed unperturbed through conditions that would have caused difficulties for a smaller vessel. Edwin G. Oonk, the captain, had to maneuver the ship repeatedly to avoid icebergs drifting out from the glaciers of Greenland, to the west. Then the air pressure began falling sharply. There was a severe storm to the east and another to the south. The ship might have sheltered somewhere along the coast of Greenland, but there were icebergs between JOIDES Resolution and the coast, making it impossible to move closer inshore. Captain Oonk decided to sail forward into the weather. His tactic might have worked, but the two storms merged, pressure dropped still further, and the wind speed increased. For two days the storm raged. The dial on the windspeed indicator had a maximum reading of 115 MPH (185 km/h), and that was what it read in the gusts. Breasting waves 70 feet (21 m) high, like solid walls of water, the ship repeatedly rose high into the air and plunged into the troughs. At times the main propellers were lifted clear of the water. Lookouts, securely lashed down, had to be stationed in the stern to watch for

Arctic and Antarctic hurricanes icebergs, because at times the ship was being carried backward at more than 4 MPH (6.4 km/h). After a day and a half of this, Resolution was in danger of sinking. The storm abated at last, and the vessel made its slow way to port for repairs.

An extratropical hurricane This was not a tropical cyclone, but it was as fierce as one. Its winds of more than 115 MPH (185 km/h) were classified as storm force 12+ (12 is hurricane force on the Beaufort scale of wind strength). In lower latitudes it would have been rated a category 3 hurricane, strong enough to uproot large trees and wreck mobile homes. In 1954, the Swedish meteorologist Tor Bergeron (1891–1977) called storms of this force extratropical hurricanes. Unfortunately, this name can cause confusion, because it is used to describe more than one kind of storm. A tropical cyclone (or hurricane) that moves outside the Tropics soon loses some of its most distinctive characteristics. Its thunderstorms die down, its diameter increases considerably, and its strongest winds cease to occur close to the central eye. This formerly tropical storm is then known as an “extratropical hurricane.” The winds in a storm of this kind may be fairly light, but they can also reach hurricane force. They are especially likely to do so if two tropical hurricanes move away from the Tropics independently, both change their character, and then merge because they are traveling at different speeds and the second storm catches up with the first. A merger between two storms usually produces a huge and extremely dangerous monster of a storm. That is what produced the storm that nearly sank JOIDES Resolution. Extratropical hurricanes of this type most often form off the coast of New England or Canada. These are the ones that can cross the Atlantic to cause havoc in Europe. Tropical cyclones that move away from the Tropics can sometimes be revitalized. This is another way extratropical hurricanes develop. When a tropical cyclone moves out of the Tropics, it crosses cooler water. Water evaporates into it more slowly, and this reduces the vigor of its convective clouds, weakening it. Eventually, it will die completely, unless it encounters a cold front. When that happens, the warm air of the cyclone rides up the sloping boundary of the cooler air, and this forced lifting triggers a new bout of convection and convective warming as the rising air cools adiabatically and its water vapor condenses, releasing latent heat. The dying cyclone comes back to life and continues its journey, once more with the power of a hurricane. None of these is what Bergeron meant by an “extratropical hurricane.” He was describing a storm that originates a long way outside the Tropics. It is a storm that has never been tropical. Extratropical hurricanes of this type are very similar in some ways to tropical cyclones, however. They occur in both north and south polar regions. Around Cape Horn, at the southernmost tip of South America, the frequent gales associated with deep cyclonic storms led sailors to name

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Hurricanes the latitudes the roaring forties, furious fifties, and shrieking sixties. This region of frequent gales extends all the way around the world and is not confined to the area south of Cape Horn. It is associated with Cape Horn only because South America projects into the belt, and before the Panama Canal was built ships were compelled to pass around it in order to cross between the Atlantic and Pacific. At Byrd Station, Antarctica, the wind is strong enough to produce severe gales for about two-thirds of the time. Winds over the Southern Ocean are much stronger than those in the Arctic, because there is no large landmass between the southern tip of South America and the northern tip of the Antarctic Peninsula, whereas North America, Greenland, Scandinavia, and Siberia extend into the Arctic. Land slows and deflects the wind. With no land to moderate the wind, it blows more strongly and more frequently. It also blows over a longer uninterrupted distance. This means that it generates much bigger waves than those found in the northern oceans. Occasionally the weather systems that cause extratropical hurricanes can bring severe weather to places in lower latitudes. Around Christmas, 1995, for example, one extended south and brought extreme cold and heavy snow to Scotland and northern England, with snow drifts 30 feet (9 m) deep in the Shetlands, the northernmost group of Scottish islands. It reduced December temperatures over Britain to 4°F (2°C) below the monthly average.

Polar lows Extratropical hurricanes, of the type Bergeron described, usually develop from polar lows. These are areas of relatively low atmospheric pressure that form near the edge of the sea ice. There, the air temperature over the ice can fall locally as low as –40°F (–40°C), while the temperature over the sea is close to freezing (+32°F; 0°C). This means there is a temperature difference of 72°F (40°C) or more between air over the water and air over the ice. When there is also a region of low pressure along the edge of the ice, cold air is drawn into the system from over the ice and warm air from over the ocean. The merging of air at such widely different temperatures generates and then maintains the storm. This is a polar low. Such a sharp temperature difference is quite common in the Arctic. Warm air and ocean currents bring nearly twice as much warmth into the Arctic Ocean as is absorbed from solar radiation, and the seawater never cools below about 29°F (–5.4°C), so there is a constant source of warmth. Polar lows form in winter in the Arctic and throughout the year in the seas around Antarctica. There is also a sharp contrast in temperature between tropical air moving toward the pole and polar air moving away from it. These airflows meet at the polar front, where the warmer air rises as part of the system of convection cells