Monsoon

Atypical monsoon climate (climate is the weather experienced by a given location, averaged over several decades) is warm year-round with very rainy summers and relatively dry winters. The change from dry season to rainy season is abrupt, and the rainy season is marked by heavy rainfall. For many people, the monsoon represents only the anxiously awaited rains of summer, which provide a break from the oppressive heat and mark the beginning of the growing season.

While most people associate the term monsoon with seasonal rains, meteorologists (scientists who study weather and climate) use the term specifically to refer to a relatively abrupt seasonal wind shift: the wind blows from land to sea during the winter and from sea to land during the summer. The term "monsoon" comes from the Arabic word mausim, meaning "season."

The monsoon climate is most pronounced in India and in parts of Southeast Asia. Monsoon climates also exist along the Atlantic coastal regions of northern South America, on the coasts of central Africa, and in other warm regions of the world where large landmasses meet ocean basins.

Monsoon rains can mean salvation or disaster to the areas where they fall. Half the world's population relies on the seasonal rains to water their crops (principally rice, which requires very wet conditions) and support livestock, as well as to bring relief from the hot, dry winter and spring. If there isn't enough rain, crops will die and people may face food shortages and famine. Too much rainfall, on the other hand, may bring flooding, destruction, and death.

Bangladesh's monsoon floods of 1998

In the summer of 1998, monsoon rains in the south Asian republic of Bangladesh caused the worst flooding in that nation's history. While in an average rainy season Bangladesh has one-third of its land submerged, the rains of 1998 covered three-fourths of the country. While the flooding typically lasts one or two weeks, in 1998 it lasted two months (three times longer than the next longest flood in Bangladeshi history). The flooding began in late July, when the Ganges and other rivers overflowed their banks. The rivers continued to rise until mid-September, only starting to recede at the end of that month.

WORDS TO KNOW

air pressure:

the pressure exerted by the weight of air over a given area of Earth's surface.

climate:

the weather experienced by a given location, averaged over several decades.

deforestation:

the removal of all or most of the trees from a region.

drought:

an extended period during which the amount of rain or snow that falls on an area is much lower than usual.

El Niño:

Spanish for "the Christ child"; an extraordinarily strong episode (occurring every two to seven years) of the annual warming of the Pacific waters off the coast of Peru and Ecuador.

erosion:

the wearing away of a surface by the action of wind, water, or ice.

flood:

the overflow of water onto normally dry land.

meteorologist:

a scientist who studies weather and climate.

monsoon:

seasonal wind that blows from land to sea during the winter and from sea to land during the summer; also, more commonly, a seasonal period of heavy rainfall.

monsoon climate:

a climate that is warm year-round with very rainy (flood-prone) summers and relatively dry winters. It encompasses much of southern and southeastern Asia, the Philippines, coastal regions of northern South America, and slices of central Africa.

orographic uplift:

the forcing of air upward, caused by the movement of air masses over mountains.

pressure gradient:

the rate at which air pressure decreases with horizontal distance.

Southern Oscillation:

shifting patterns of air pressure at sea level, between the eastern and western edges of the Pacific Ocean.

More than fifteen hundred people lost their lives in the floods, and about twenty-five million were left homeless. In many villages, residents spent two months living on the tin roofs of their homes. There was widespread loss of livestock and crops in this primarily agricultural nation. Countless roads, bridges, and power lines were damaged or destroyed, and half of the nation's rice seedlings, waiting to be planted, were washed away.

Background on Bangladesh

Bangladesh is a small, flood-prone country situated just east of India on the Bay of Bengal. It is the size of the state of Wisconsin, but holds more than 125 million people (about half the population of the United States), making it the most densely populated nation in the world. It is also one of the poorest nations in the world; its citizens earn an average of only $200 a year.

Bangladesh is in the path of waters that flow down the slopes of the Himalayas. Those waters course through India before entering Bangladesh. Since Bangladesh sits at a lower elevation than India, it receives the runoff from India's floods.

Most of the land of Bangladesh is in the flat, low-lying Plain of Bengal. Roughly 80 percent of Bangladesh is less than 30 feet (10 meters) in elevation. Near the coast, the land is practically at sea level with an elevation less than 3 feet (1 meter).

Bangladesh is crisscrossed by a complex and interconnected system of rivers and tributaries. The situation is further confused because the same river is known by different names in different portions of its course. The four major rivers are the Ganges, the Jamuna (also known as the Brahmaputra where it crosses India), the Padma, and the Meghna. These rivers also have hundreds of tributaries. The Ganges, Jamuna, and Padma combine to form the Meghna which enters the Bay of Bengal in a wide delta system. During the rainy season, the multiple channels of the Meghna merge to form a vast river of water and silt.

Much of the land exists as islands, called "chars" locally, that are constantly appearing and disappearing as the rivers change course. The chars' soil is made extremely fertile by the silt deposited by the rivers. Individual chars, however, rarely exist for more than ten years. Most homes in this fertile region are built on stilts to keep them above the flood waters.

Bangladesh has an extremely rainy climate, and flooding is common during the monsoon season. That flooding becomes disastrous if some or all of the major rivers flood, and worse yet if it coincides with high tides in the Bay of Bengal. The flooding can put nearly all of the country under water if a tropical cyclone strikes at the same time. The floodwaters run swiftly, sweeping away many homes—even those built on stilts. As dry ground disappears and villagers can no longer find safety from the flood-waters, the number of deaths from drowning increases. In the worst years, tens of thousands of people in Bangladesh have died, and millions have been made homeless.

Another consequence of flooding in Bangladesh is the spread of disease. Disease is common because of the mixing of sewage with drinking water and the increase of illness-spreading insects. In a country that has only one medical doctor for every forty-five hundred people, there is little chance that sick people will receive the necessary treatment.

Torrential rains swell rivers

The primary source of Bangladesh's 1998 floods was torrential rains that pelted the nation every day for a month. Adding to the rising floods was runoff from rain in the Himalayas and India. Floodwaters took at least one thousand lives in India before continuing into Bangladesh. Bangladeshi rivers overflowed their banks and spread out over the land. In early September the Ganges River reached its highest level in recorded history.

Typically, Bangladesh's rivers empty into the Bay of Bengal. In the late summer of 1998, however, high sea levels blocked the seaward-flowing rivers. The high sea levels were believed to be the result of numerous minor undersea earthquakes in the region as well as runoff from melting glaciers in the Himalayas.

More than 10,000 miles (16,000 kilometers) of roads, fourteen thousand schools, and millions of homes were partially or totally submerged. Flooding was made worse by faulty levees (high walls built along the banks of a river to prevent flooding) that had been put in place in a failed attempt to tame the river. That effort, which was coordinated by the World Bank at a cost of $150 million and called the Flood Action Plan, was declared a failure in 1996.

"I can trace the orange clay lines of major roads, which are built on artificially raised ground … then, again and again, I see these roads disappear under water," wrote journalist Mark Levine in a November 1998 article in Outside Magazine, while surveying Bangladesh from an airplane. "I see floating clusters of trees and houses, isolated from one another like islands. I see the tips of power lines poking through the surface of the floodwater, and small herds of cattle—four or five heads apiece—huddling on tiny dots of land."

Crops destroyed by flooding

More than 2 million tons (1.8 million metric tons) of rice crops were destroyed by the floods, reducing rice production in the nation by about 75 percent. Replanting of rice was impossible while the floods persisted and had to wait until the next year's growing season. In the winter months, some people were able to plant wheat, spices, and other dry-weather crops.

Some 80 percent of Bangladeshis make their living by farming. "I don't know how God will save us," stated Mohammad Harunuddin Sheik, a villager from Chor Shibola who was camped out on the roof of his shack with his wife and daughters, in a September 7, 1998, New York Times article. "All my crops were destroyed. Where will I get seeds to plant next year?"

Causes of death

At the end of November 1998, the Bangladeshi death toll was estimated to be more than fifteen hundred people. The main causes of death were drowning, diarrhea, cobra bites, and being crushed by collapsing houses. Disease and hunger posed grave threats in a nation where two-thirds of children are seriously malnourished even in the best of times.

Sickness spread through the foul floodwaters, which served as both garbage dump and toilet in a nation where indoor plumbing is a rarity, and the landfills were now underwater. People living in slums had no choice but to use the thick, black water around them for cleaning, cooking, and in some cases for drinking.

By mid-October, more than one million people were suffering from diarrhea. While the government had ample supplies of anti-diarrheal medication, it was difficult to deliver the medicine to sufferers.

As the floods receded, they left in their wake a thick layer of black sludge. The sludge, like the floodwaters, contained contaminants that spread disease and provided a breeding ground for malaria-carrying mosquitoes.

Difficulties faced by relief missions

The floods and resulting food shortages left Bangladeshis, especially those without grain reserves from the previous year's harvest, in desperate need of international aid. The United Nations responded to the crisis by raising about $200 million from international donors. The distribution of food was difficult, however, given that millions of hungry people were distributed on numerous strips of land that stood out in the rising water. Dropping supplies from airplanes was not feasible, since packages would likely fall into the water. Some aid agencies chartered boats to traverse the floodwaters, however many crews refused to work because they feared capsizing in the dangerously fast currents. As floodwaters receded, relief agencies faced new obstacles to food delivery: water was too shallow for boats yet the roads were too muddy for cars.

There were also political obstacles to aid distribution. Some residents of Dhaka, the capital of Bangladesh, accused the government of withholding food from people in regions known to be opposition-party strongholds.

As the fall wore on, food distribution became more reliable. According to the program set up by the government, each family in need received 31 pounds (14 kilograms) of rice and wheat every month. That quantity was said to be adequate for at least one meal a day. Programs were also initiated that paid Bangladeshis to rebuild roads, schools, and hospitals. In that way people were given the opportunity to buy their food rather than relying on handouts.

Moneylenders make it worse

Many Bangladeshis, desperate to make ends meet, sold their farm animals for discouragingly low prices. Such a move meant they would have no animals with which to till their land the following year. Many were also left with no choice but to sell their chickens and milk cows—for about half of the pre-flood prices.

Experiencing the monsoon: An eyewitness account

The following passages are from a book titled Chasing the Monsoon by English journalist Alexander Frater. Frater, who was in Trivandrum, India (at the country's southern tip), when the rains came, described the scene as he joined a group of Indians gathered on the seashore. His account demonstrates how quickly the monsoon season can start, and how its arrival is anxiously awaited by many people.

A line of spectators had formed behind the Kovalam beach road. They were dressed with surprising formality, many of the men wearing ties and the women fine saris…. Their excitement was shared and sharply focused, like that of a committee preparing to greet a celebrated spiritual leader…. As I joined them they greeted me with smiles…. The sky was black, the sea white. Foaming like champagne it surged over the road to within a few feet of where we stood….

More holiday-makers were joining the line. The imbroglio of inky cloud swirling overhead contained nimbostratus, cumulonimbus and Lord knows what else…. Thunder boomed. Lightning went zapping into the sea…. Then … we saw a broad, ragged ban of luminous indigo heading slowly inshore. Lesser clouds suspended beneath it like flapping curtains reached right down to the sea.

'The rains!' everyone sang.

The wind struck us with a force that made our line bend and waver….

The deluge began….

The rain hissed on the sea and fell on us with a buzzing, swarming noise….

Water sheeted off the hillside, a rippling red tide carrying the summer's dust down to the sea….

Even after selling their possessions, cash-strapped Bangladeshis had difficulty meeting their basic needs. Moneylenders took advantage of the situation. These businessmen lent villagers small amounts of money (about $125 maximum per loan) at interest rates as high as 100 percent. For some borrowers, such an equation would leave them in debt the rest of their lives. "I borrowed 1,000 taka ($21.00) at 100 taka ($2.10) interest per month," stated one villager in a 1998 CNN report. "They (private lenders) squeeze us to the bones to get their money back."

Money matters were made worse by the financial crisis that struck Asia at the same time as Bangladesh's floods. Many Bangladeshis who had been working in other countries in Southeast Asia were sent home. Prior to the flooding, money sent to Bangladeshis by family members working abroad totaled almost $1.5 billion per year. That was second only to exports for the nation's largest source of foreign income.

Bangladesh floods in 2002 and 2004

Bangladesh suffered devastating floods again during the monsoon rains of 2002. While not as severe as the 1998 floods, 2.3 million people were left stranded or homeless from floodwaters and two hundred thousand homes were submerged. Fortunately, the death toll was relatively low. Official accounts place the toll at nineteen people dead, sixteen of them from waterborne diseases. Other news accounts place the death toll at around 157 people. Hundreds of homes and farms were destroyed.

Flood problems were worse in 2004 due to flooding caused by monsoon rains and high tides in the Bay of Bengal. Millions of people were left without adequate food or clean water as the floods submerged two-thirds of the country. According to some news reports, the death toll was as high as one thousand persons. Annual floods during the monsoon rains are likely to continue to plague Bangladesh for years to come, especially when combined with high tides and tropical cyclones.

Dangerous science: Monsoon

A monsoon is a huge, natural weather event that exchanges heat and moisture between land and sea. It is driven by the temperature differences between the two environments.

The monsoon climate is most pronounced in the Indian subcontinent and southeast Asia—home to a full 50 percent of the world's population. The monsoon zone includes India, the Indus Valley of Pakistan, portions of Nepal, Bhutan, Bangladesh, Burma, Malaysia, Vietnam, Thailand, Laos, and the Philippines, and the southeast tip of China. Some Atlantic coastal regions of northern South America also experience monsoon climate, as do slices of central Africa. Recently monsoon winds have been shown to exist in the southwestern United States, Japan, and portions of southern Europe.

The monsoon-driven rainy season typically lasts for four months, from June through September. India, for instance, gets 80 percent of its yearly rainfall during that period. The start and end dates of the monsoon rains vary from place to place and from year to year, as do the rainfall amounts.

A seasonal wind pattern

Throughout the winter, the region affected by the monsoon is tilted away from the sun. During that time the land is cooler than the ocean. (Land heats up and cools down more quickly than water. Water heats up more slowly than land in the summer—which is why we go swimming to cool off—but water retains its heat longer than land.) The cooler air, which is denser, sinks. As the air sinks, the moisture within it evaporates and prevents the formation of clouds. At the same time, the relatively warm air above the ocean expands and rises, because it is less dense. The air over the sea is replaced by cooler air from far inland, due to the difference in air pressure (the different pressures exerted by the weight of the air). The air travels from a region of higher air pressure, over land, to a region of lower air pressure, over sea.

When spring arrives, the monsoon-prone area moves to a position almost directly beneath the sun and the winds shift direction. The land heats up, becoming warmer than the ocean. The surface air over land expands and rises, thus lowering the air pressure. Relatively cool air from over the ocean, made moist by evaporated ocean water, flows in and equalizes the pressure. Once the ocean air is heated over the land, it also rises. As it rises, the moisture within it condenses and forms clouds. Those clouds yield heavy rains throughout the summer months. When temperatures on land in the summer are exceptionally hot, the rains are very strong and cause widespread flooding.

The monsoon climate

In the monsoon climate, the year is divided into dry and rainy seasons. While monsoon regions generally receive as much total annual rainfall as do rain forests, the precipitation is concentrated in the summer months. It is typical for around 120 inches (300 centimeters) of rain to fall during the rainy season, causing destructive flooding. Rainfall in the spring and fall is moderate while the months of December, January, and February are dry. The official designation of a monsoon climate is one in which monthly precipitation drops below 2.3 inches (6 centimeters) for one or two months of the year.

The total annual rainfall in monsoon regions is generally sufficient to sustain forests, even through the dry period. In some areas this forest resembles a rain forest while in others it is less dense and merges into scrublands. The land is primarily cultivated with rice, a crop that thrives on flooding.

Monsoon in India

The cycle of life in India is closely related to the seasonal rain pattern. In India, the shape of the land intensifies the monsoon; it strengthens the flow of the wind out to sea, as well as back to land. India is shaped like a huge triangle. It is surrounded by ocean on two sides and very tall mountains on the third. The Bay of Bengal lies to the east of India, the Arabian Sea lies to the west, and the Himalayan Mountains are situated to the north.

Experiment: Convection currents in the atmosphere

A monsoon is driven by solar energy working through convection. When air is heated, it becomes less dense and rises. Cooler air flows in to take its place. Likewise, cooler air is more dense and sinks, displacing warmer air.

For this experiment you will need a large clear, plastic box. A plastic shoe box works well. You will also need a cup full of ice cubes, a small candle in a candle holder (such as a tea-light), and some black or dark-colored construction paper. Ask a teacher or parent to help you with the lit candle.

First, attach black paper to the back side of the plastic box, so the convection currents will be visible. Next, place the cup of ice in one end of the shoe box. Then have your parent or a teacher carefully light the candle and place it at the opposite end of the shoe box. Allow the candle to sit for a few moments. Then gently blow out the candle and place the lid on the shoe box. The smoke from the candle will follow the convection current inside the box.

This experiment reproduces the circumstances under which monsoon winds are formed. Earth's surface is warmed by sunlight and the less dense air rises. Cooler, denser air from over the ocean flows in to take the place of the warmer, rising air. The cooler, denser air brings moisture from the ocean to the land and the rains come.

In the winter months, the land is at its coolest and the sea is warmer than the land. The relatively cool air sinks down from the mountains, pulled by the force of gravity, and moves across the land. It then moves out over the water. Precipitation during the winter months is scarce.

In the spring the land heats up. In May and June, daytime air temperatures are typically more than 104°F (40°C). By June the land becomes warmer than the sea and the wind reverses direction. Cool, moist air flows onto land from both the Indian Ocean and the Bay of Bengal, forming eastern and western arms of the monsoon. That humid air abruptly triggers torrential rainfalls. The air is forced upward, in a process called orographic uplift, by the hills that occupy the center of the Indian peninsula. This contributes to the rainfall along the coasts. In a typical year, most of India has rain every day in June.

The rainfall is particularly heavy in the foothills of the Himalayas. When the moist air reaches the mountains, it is forced upward sharply. That motion causes the formation of towering storm clouds that produce abundant rain. One of the wettest places on Earth is the town of Cherrapunji. This town sits on the slopes of the Himalayas, just north of the Bay of Bengal. Cherrapunji receives more than 400 inches (1,100 centimeters) of rain per year.

Consequences of monsoons

Monsoons are complex phenomena that, unlike tornadoes or avalanches, are not always disastrous. Yes, some monsoons, those that bring far too

The Arizona monsoon

In recent decades, meteorologists have recognized that the late summer thunderstorms that occur over the mountains of the southwestern United States are a monsoon, at least marginally. While the wind does seasonally shift from the west and northwest in winter to the south and southeast in late summer, the shift is not as strong or abrupt as that experienced in India and Southeast Asia.

The mechanics that drive the southwestern monsoon are similar to those that drive other monsoons. During the winter months the air pressure is higher over land than it is over bodies of water to the south. In the spring, as the air warms considerably and the pressure over land lowers, moist air from over the Pacific Ocean, the Sea of Cortez (the water separating the Baja Peninsula from the Mexican mainland), and the Gulf of Mexico flows northward. As a result, in mid-to-late summer and early fall the deserts and dry plateaus of Arizona and New Mexico experience numerous thunderstorms. The thunderstorms that accompany the monsoon are caused by a combination of orographic uplift, daytime heating from the sun, and weak upper-level disturbances moving across the region.

The summer rains supply a significant portion of the region's annual precipitation. Arizona, for instance, receives only 6 percent of its annual rainfall in April, May, and June. In July, August, and September—the period considered the wet monsoon—the state gets 32 percent of its annual rainfall.

much or far too little rain, fit the definition of "natural disaster." In general however, monsoons are a seasonal rhythm. The billion people who live in monsoon climates have adapted their lives to the seasonal changes.

Flooding

Many parts of India, Bangladesh, and certain Southeast Asian nations become flooded every rainy season. The poor generally feel the effects of the flooding the most, as their homes are built on land most prone to floods, especially in shantytowns surrounding major cities. Some people have standing water in their houses for weeks at a time every year. Particularly large floods cause drownings and carry away people's meager possessions—and sometimes even entire homes. Floodwaters often mix with sewage and other refuse, creating health hazards for people reliant on river water for drinking, cooking, and washing.

The collapse of the Manchu River Dam

On August 11, 1979, excessively heavy monsoon rains caused the collapse of the Manchu River Dam near Morvi, India. Morvi is located 300 miles (480 kilometers) northwest of Mumbai (formerly Bombay). Before the dambreak, Morvi was a thriving industrial town of seventy thousand people. The collapse of the dam unleashed a 20-foot (6-meter) wall of water that killed some fifteen thousand of Morvi's citizens, as well as countless others from thirty small villages between Morvi and the dam.

The Manchu River Dam, built in 1972, had been designed to withstand an average yearly rainfall of 22 inches (56 centimeters). The 197-foot-high (60-meter-high) dam protected Morvi and other downstream settlements from flooding. The heavy monsoon rains of the summer of 1979 had already been placing stress on the dam when a severe storm dumped 28 inches (71 centimeters) of rain on the region in just twenty-four hours. Engineers tried to open the sluice gates to allow more water to flow out of the dam and relieve the pressure, but they found the gates had been rusted shut. When water levels behind the dam had risen to 20 feet (6 meters) above normal, the dam gave way.

The water rushed forward, destroying everything in its path within fifteen minutes. In Morvi, people were knocked over and drowned, and mud homes were washed away. The floodwaters reached the second stories of tall buildings. After the flood passed, bodies were strewn everywhere, and a thick layer of mud covered everything.

Drought

For regions dependent on the monsoon rains for their livelihood, when a monsoon does not come it can be deadly as well. An example of this tragedy can be found in the African Sahel—the strip of semi-arid land south of the Sahara Desert. The Sahel depends on annual seasonal rains to support grasslands for grazing livestock and to grow crops. The monsoon was weak or virtually nonexistent in the Sahel from 1968 through 1973, and again during several periods throughout the 1970s, 1980s, and early 1990s. This caused extensive drought (pronounced DROWT), an extended period during which the amount of precipitation is much lower than usual. Due to the lack of rainfall, large portions of the Sahel have been turning to desert.

Edmund Halley studies monsoons

English astronomer Edmund Halley (1656–1742) is best known for his calculations involving a comet that passed close to Earth in 1682, and which he (correctly) predicted would return again in 1758. Today, we know it as Halley's comet, and it crosses our skies about every seventy-five years. Halley was a friend of English physicist Isaac Newton (1642–1727), and it was Halley who provided the money to publish Newton's famous Principia Mathematica in 1687, which many people feel is the most important document in the history of science.

What most people do not know is that Halley also made one of the first studies of monsoon wind patterns. He conducted his observations while at sea and during a two-year stay on the island of St. Helena in the Atlantic Ocean off the western coast of Africa. He combined his observations with information obtained from sailors who had traveled throughout the world. As a result of these studies, Halley established that the difference between the temperatures of the air over land and over the adjacent ocean produced different air densities, which were the driving force behind seasonal monsoon winds. He published his findings in 1686 along with one of the first meteorological maps, which showed wind patterns on the world's oceans.

Positive aspects of monsoons

Monsoon rains nourish crops and support people and livestock. For instance rice, the number-one crop in monsoon regions, requires very wet conditions to grow. There are even positive aspects of floods caused by the monsoon. The floodwaters deposit a layer of nutrient-rich soil on farmland, increasing its fertility. In some areas, along with the floodwaters come fish and other edible aquatic life. Fish provide a needed source of protein when crops are submerged and other food is scarce.

The human factor

The level of flooding caused by monsoon rains in India and Bangladesh is made worse by erosion (wearing away) of the Himalayan mountainsides. Massive deforestation, the removal of most or all of the trees, on the slopes, has left the soil exposed and vulnerable to being washed away by heavy rains. The sediments get swept downstream and are deposited in riverbeds and in the Bay of Bengal. The sediments raise the levels of the riverbeds, leaving less room for water and contributing to the incidence and severity of flooding.

Deforestation also contributes to the severity of drought when rainfall is scarce. The lack of plants reduces the soil's ability to retain moisture. Erosion creates hard-packed ground that is unable to absorb any rain that does fall.

Another way that human activity magnifies the negative effects of monsoons is through the concentration of settlements in monsoon regions. Indeed, the locations affected by monsoons have the highest population density in the world. This fact means that when natural disasters do occur there, large numbers of people suffer the consequences.

The effect of El Niño on India's monsoons

In recent years, scientists have discovered a link between El Niño and the failure of India's monsoon. El Niño is an extraordinarily strong episode (occurring every two to seven years) of the annual warming of the Pacific waters off the coast of Peru and Ecuador. In 1982–1983, in conjunction with one of history's strongest El Niños, India's monsoon rains never came.

Scientists linked El Niño and the monsoon by discovering that at the same time the waters grow warm off the coast of Peru, a warming also occurs in the Indian Ocean. That warming causes a reversal of the pressure gradient in the Indian Ocean. The surface winds that usually blow to the northwest, toward the coast of Africa and India, change direction. They blow toward the southeast, where temperatures are warmer and pressure is lower, bringing warmth and moisture to western Australia. At the same time they leave India and southern Africa drier than normal.

Another human factor when talking about monsoons is that aid meant for victims of floods or drought is often mishandled. Distribution of food, water and supplies may be hampered by inefficiency on the part of government officials or may be deliberately withheld from certain areas because of political considerations, such as support for an opposition political movement.

Technology connection

The behavior of monsoons has been the subject of intense study by meteorologists for the last century. Nonetheless, monsoons, and especially what causes them to vary from year to year, are not entirely understood. This lack of knowledge makes monsoon prediction very difficult.

Efforts at predicting the arrival dates of monsoons are largely based on readings of the Southern Oscillation—changes in patterns of air pressure at sea level between the eastern and western edges of the Pacific Ocean. The rising and falling of the pressure gradient (the rate at which air pressure changes with horizontal distance) across the ocean may foretell changing wind patterns and the arrival of monsoon rains. Attempts at predicting rainfall amounts for specific locations involve the imperfect method of searching for patterns in past rainfall records.

"Though the monsoon winds constitute one of the greatest weather systems on earth, and an enormous amount of research has been carried out …, many questions remain shrouded in mystery," stated Julius Joseph, Monsoon Officer of the Indian Meteorological Center, in the book Chasing the Monsoon. "It's like the human brain. We know it but we don't know it."

[See AlsoClimate; Drought; El Niño; Flood; Weather: An Introduction ]

For More Information

BOOKS

De Villiers, Marq. Windswept: The Story of Wind and Weather. New York: Walker, 2006.

Frater, Alexander. Chasing the Monsoon. New York: Picador, 2005.

Hyndman, Donald, and David Hyndman. Natural Hazards and Disasters. New York: Brooks Cole, 2006.

WEB SITES

"The Monsoon." National Weather Service. 〈http://www.wrh.noaa.gov/fgz/science/monsoon.php?wfo=fgz〉 (accessed March 23, 2007).

"North American Monsoon Experiment (NAME)." NASA Earth Observatory. 〈http://earthobservatory.nasa.gov/Newsroom/Campaigns/NAME_Mission.html〉 (accessed March 23, 2007).

Monsoon

General monsoon circulation

The Asian monsoons

The monsoon of India

The monsoons of South China and Japan

When the monsoon fails

Resources

A monsoon is a seasonal change in the direction of the prevailing wind that typically brings about a marked change in local weather. Monsoons are often associated with rainy seasons in the tropics (the areas of Earth within 23.5° latitude of the equator) and the subtropics (areas between 23.5° and about 35° latitude, both north and south). In these areas, life depends on the monsoon rains. A weak monsoon rainy season may cause drought, crop failures, and hardship for people and wildlife. The central role that monsoons play in determining climates around the world has made their study a high priority for meteorologists.

Many parts of the world experience monsoons. Probably the most famous are the Asian monsoons, which include the distinctly different monsoons that affect India, north China, and Japan, and south China and Southeast Asia. Monsoons also affect portions of central Africa, where their rain is critical to supporting life in the area south of the Sahara Desert. Lesser monsoon circulations affect parts of the southwestern United States. These summer rainy periods bring much needed rain to the dry plateaus of Arizona and New Mexico.

General monsoon circulation

Monsoons, like most other winds, occur in response to the sun heating the atmosphere. In their simplest form, monsoons are caused by differences in solar heating between the oceans and continents, and they are most likely to form where a large continental land mass meets a major ocean basin. During the early summer, the increasing solar energy heats up the land surfaces fairly quickly. Water, on the other hand, heats much more slowly in response to the sun. The large amount of water in the oceans guarantees they will remain cooler than the nearby continents during the early summer. The relatively warm land surface will heat the air over it, causing it to rise, or convect. The convection of warm air produces an area of low pressure near the land surface. Meanwhile, the air over the cooler ocean will be more dense and tend to stay at the surface or sink downwards from aloft. Thus during the summer, oceanic air flows onshore toward the low pressure over land. This onshore flow is continually supplied by cooler oceanic air sinking from higher levels in the atmosphere.

In the upper atmosphere, the rising continental air is drawn outward over the oceans to replace the sinking oceanic air, thus completing the cycle. In this way a large vertical circulation cell is set up, driven by solar heating. At the surface, the result is a constant wind flowing from sea to land.

The oceanic air moving onto the land is usually humid because of its prolonged contact with the sea surface. As it flows on shore the moist marine air is pulled upward as part of the convecting half of the circulation cell. The rising air cools and eventually forms rain clouds. Rain clouds are especially likely when the continental areas have higher elevations (mountains, plateaus, etc.), because the humid sea air is forced upward over these barriers, causing widespread cloud formation and heavy rains. This is the reason why the summer monsoon forms the rainy season in many tropical areas.

In the late fall and early winter, the situation is reversed. Land surfaces cool off quickly in response to cooler weather, but the same property of water that makes it slow to absorb heat, called heat capacity, also causes it to cool slowly. As a result, continents are usually cooler than the oceans surrounding them during the winter. This sets up a new circulation in the reverse direction: air over the sea, now warmer than that over the land, rises and is replaced by winds flowing off the continent. The continental winds are supplied by cooler air sinking from aloft. At upper atmospheric levels the rising oceanic air moves over the land to replace the sinking continental air. Sinking air prevents the development of clouds and rain, so during the winter monsoon continental areas are typically very dry. This winter circulation causes a prevailing land to sea wind until it collapses with the coming of spring.

The Asian monsoons

While the thermal circulation described previously is a central part of monsoons, it is not sufficient to explain the world’s most pronounced monsoons, those of Asia. Because they cover such a large area and affect over a billion people, the Asian monsoons have been studied for over a century to determine their causes and reasons for their variation. Although scientific understanding is not complete, it is clear that the monsoons of Asia are a complicated set of circulations, which combine the sun-driven winds with large scale circulations that span the entire planet. The extremely high mountains of the Himalayas also play a role in determining monsoon behavior.

An important factor in the development of the Asian monsoons is the existence of jet streams. These are great rivers of air, that ring Earth at levels in the atmosphere ranging from 7-12 mi (12-20 km) above the surface. Jet streams are part of the global wind circulation, brought about by the large differences in temperature between the equator and the poles. Jet stream winds blow at several different latitudes, and play a major role in determining the weather beneath them. The monsoons of southern Asia are affected by two jet streams called the subtropical and the tropical jets. The subtropical jet is a permanent feature, flowing westerly (from west to east) at an altitude of about 7 mi (12 km). It migrates over the year in response to the seasons, moving northward to higher latitudes in the summer and southward in the winter. It occasionally splits in two. The tropical jet is a weaker easterly (east to west) flow that forms near the equator at a height of nearly 12 mi (20 km). It is found only in the summer months.

The monsoon of India

The interplay of jet streams and monsoon winds is well illustrated in the Indian monsoon. During the early summer months, increased solar heating begins to heat the Indian subcontinent, which would tend to set up a monsoon circulation cell between southern Asia and the Indian Ocean. However, the subtropical jet stream occupies its winter position at about 30° north latitude, south of the Himalayan Mountains. As long as the subtropical jet blows over India, it inhibits the development of summer monsoon. As summer progresses, the subtropical jet slides northward. The high Himalayas present an obstacle for the jet, which must flow over the mountains and reform over central Asia. When it finally does so, a summer monsoon cell develops, supported by the tropical jet stream overhead. The transition can be very fast—the Indian monsoon has a reputation for appearing suddenly, as soon as the subtropical jet is out of the way. The retarding effect of the subtropical jet delays the Indian monsoon by up to one month compared with the rest of Asia.

During the Indian summer monsoon, winds blow from the southwest, bringing moist air on shore. As the air is forced to rise over the foothills of the Himalayas, it causes constant and frequently heavy rains. The town of Cherrapunji, India, located on the Himalayan slopes, receives an annual rainfall of over 36 ft (11 m), making it one of the wettest places on Earth.

The summer monsoon ends as the highlands of Tibet begin to cool in the fall. The cooling atmosphere over Tibet allows the subtropical jet to reappear south of the Himalayas, bringing the rainy summer monsoon to an end. The circulation shifts to a winter monsoon cell, with sinking air over India and surface winds that blow out to sea. The resulting winter weather is dry.

The monsoons of South China and Japan

The monsoons of China and Japan are strongly affected by the huge land mass of Siberia. During the winter, the interior of Siberia becomes extremely cold. Cold air is dense, so a cold area of high pressure forms, where the air sinks from aloft. When it reaches the surface, the air spreads outward in all directions. The result is a dry winter monsoon that blows from the north

KEY TERMS

Circulation cell— A circular path of air, in which warm air rises from the surface, moves to cooler areas, sinks back down to the surface, then moves back to near where it began. The air circulation sets up prevailing (constant) winds at the surface and aloft.

Convection— The rising of warm air from the surface of Earth.

Heat capacity— The amount of heat required to raise the temperature of a substance; water has a high heat capacity, while land surfaces (soil, rock, etc.) and air have much lower heat capacities.

Jet stream— High speed winds that circulate around Earth at altitudes of 7-12 mi (12-20 km) and affect weather patterns at the surface. Two important jet streams are the subtropical jet, flowing from west to east, and the weaker tropical jet, which flows from east to west.

Monsoon— A seasonal change in the direction of the prevailing wind, often associated with a rainy or a dry season.

Subtropics— Regions between 23.5 and about 35 degrees latitude, in both the northern and southern hemispheres, which surround the tropics.

Tropics— The region around Earth’s equator spanning 23.5° north latitude to 23.5° south latitude.

through south China and Southeast Asia. The same circulation affects northern China, Japan and Taiwan, where the prevailing wind is from the northwest. Although originally dry, the northwest winds pass over the sea and pick up moisture. When the winds arrive at the islands of Japan and Taiwan, they are forced up over the land. As a result, the western slopes of these islands experience a rainy winter monsoon.

During the summer, strong heating in the interior of Siberia sets up the summer circulation, giving south China, Indonesia, and Southeast Asia humid southerly winds from the equator. Japan experiences winds from the south or southeast, which bring in moisture from the north Pacific Ocean.

When the monsoon fails

The importance of monsoons is demonstrated by the experience of the Sahel, a band of land on the southern fringe of the Sahara Desert. This area would also be arid if it were not for the seasonal monsoon, the rains from which normally transform it to a grassland suitable for grazing livestock. The wetter southern Sahel can support agriculture, and many residents migrated to the area during the years of strong monsoons. Beginning in the late sixties, however, the annual monsoons began to fail. The pasture areas in the northern Sahel dried up, forcing nomadic herders and their livestock southward in search of pasture and water. The monsoon rains did not return until 1974. In the intervening six years, the area suffered devastating famines and loss of life, both human and animal, and placed extreme stress on the countries of central Africa.

Because of their tremendous effect on many tropical areas, atmospheric scientists continue to study the formation and variability of monsoons. Monsoon variations are still not entirely understood, making the prediction of the monsoon a distant goal. However, researchers have shown that monsoons are affected by El Nião, the changes in winds and seawater that occur in the tropical Pacific Ocean. Research continues into modeling monsoon circulations with complicated computer simulations.

See also Atmospheric circulation; Storm.

Resources

BOOKS

Ahrens, Donald C. Meteorology Today. Pacific Grove, CA: Brooks Cole, 2006.

Palmer, Tim and Renate Hagedorn, eds. Predictability of Weather and Climate. New York: Cambridge University Press, 2006.

James Marti

Monsoon

A monsoon is a seasonal change in the direction of the prevailing wind . This wind shift typically brings about a marked change in local weather . Monsoons are often associated with rainy seasons in the tropics (the areas of Earth within 23.5° latitude of the equator) and the subtropics (areas between 23.5° and about 35° latitude, both north and south). In these areas, life is critically dependent on the monsoon rains. A weak monsoon rainy season may cause drought , crop failures, and hardship for people and wildlife . The central role that monsoons play in determining climates around the world has made their study a high priority for meteorologists.

Many parts of the world experience monsoons to some extent. Probably the most famous are the Asian monsoons, which include the distinctly different monsoons that affect India, north China, and Japan, and south China and southeast Asia . Monsoons also affect portions of central Africa , where their rain is critical to supporting life in the area south of the Sahara Desert. Lesser monsoon circulations affect parts of the southwestern United States. These summer rainy periods bring much needed rain to the dry plateaus of Arizona and New Mexico.

General monsoon circulation

Monsoons, like most other winds, occur in response to the sun heating the atmosphere. In their simplest form, monsoons are caused by differences in solar heating between the oceans and continents, and they are most likely to form where a large continental land mass meets a major ocean basin . During the early summer, the increasing solar energy heats up the land surfaces fairly quickly. Water , on the other hand, heats much more slowly in response to the sun. This is one reason why we cool off by swimming in lakes during the summer—the water is still chilled from the recent winter and takes much of the summer to warm up. The enormous quantity of water in the oceans guarantees they will remain cooler than the nearby continents during the early summer. The relatively warm land surface will heat the air over it, causing it to rise, or convect. The convection of warm air produces an area of low pressure near the land surface. Meanwhile, the air over the cooler ocean will be more dense and tend to stay at the surface or sink downwards from aloft. Thus during the summer, oceanic air flows onshore toward the low pressure over land. This onshore flow is continually supplied by cooler oceanic air sinking from higher levels in the atmosphere. In the upper atmosphere, the rising continental air is drawn outward over the oceans to replace the sinking oceanic air, thus completing the cycle. In this way a large vertical circulation cell is set up, driven by solar heating. At the surface, the result is a constant wind flowing from sea to land.

The oceanic air moving onto the land is usually quite humid, due to its prolonged contact with the sea surface. As it flows on shore the moist marine air is pulled upward as part of the convecting half of the circulation cell. The rising air cools and eventually forms rain clouds . Rain clouds are especially likely when the continental areas have higher elevations (mountains , plateaus, etc.), because the humid sea air is forced upward over these barriers, causing widespread cloud formation and heavy rains. This is the reason why the summer monsoon forms the rainy season in many tropical areas.

In the late fall and early winter, the situation is reversed. Land surfaces cool off quickly in response to cooler weather, but the same property of water that makes it slow to absorb heat, called heat capacity , also causes it to cool slowly. As a result, continents are usually cooler than the oceans surrounding them during the winter. This sets up a new circulation in the reverse direction: air over the sea, now warmer than that over the land, rises and is replaced by winds flowing off the continent . The continental winds are supplied by cooler air sinking from aloft. At upper atmospheric levels the rising oceanic air moves over the land to replace the sinking continental air. Sinking air prevents the development of clouds and rain, so during the winter monsoon continental areas are typically very dry. This winter circulation causes a prevailing land to sea wind until it collapses with the coming of spring.

The Asian monsoons

While the thermal circulation described previously is a central part of monsoons, it is not sufficient to explain the world's most pronounced monsoons, those of Asia. Since they span such a huge area and affect over a billion people, the Asian monsoons have been studied for over a century to determine their causes and reasons for their variation. Although our understanding is not complete, it is clear that the monsoons of Asia are a complicated set of circulations, which combine the sun-driven winds with large scale circulations that span the entire planet . The extremely high mountains of the Himalayas also play a role in determining monsoon behavior.

An important factor in the development of the Asian monsoons is the existence of jet streams. These are great rivers of air, that ring the Earth at levels in the atmosphere ranging from 7-12 mi (12–20 km) above the surface. Jet streams are part of the global wind circulation, brought about by the large differences in temperature between the equator and the poles. Jet stream winds blow at several different latitudes, and play a major role in determining the weather beneath them. The monsoons of southern Asia are affected by two jet streams called the subtropical and the tropical jets. The subtropical jet is a permanent feature, flowing westerly (from west to east) at an altitude of about 7 mi (12 km). It migrates over the year in response to the seasons, moving northward to higher latitudes in the summer and southward in the winter. It occasionally splits in two. The tropical jet is a weaker easterly (east to west) flow that forms near the equator at a height of nearly 12 mi (20 km). It is found only in the summer months.

The monsoon of India

The interplay of jet streams and monsoon winds is well illustrated in the Indian monsoon. During the early summer months, increased solar heating begins to heat the Indian subcontinent, which would tend to set up a monsoon circulation cell between southern Asia and the Indian Ocean. However, the subtropical jet stream occupies its winter position at about 30° north latitude, south of the Himalayan Mountains. As long as the subtropical jet blows over India, it inhibits the development of summer monsoon. As summer progresses, the subtropical jet slides northward. The extremely high Himalayas present an obstacle for the jet; it must "jump over" the mountains and reform over central Asia. When it finally does so, a summer monsoon cell develops, supported by the tropical jet stream overhead. The transition can be very fast—the Indian monsoon has a reputation for appearing suddenly as soon as the subtropical jet is out of the way. The retarding effect of the subtropical jet delays the Indian monsoon by up to one month compared with the rest of Asia.

During the Indian summer monsoon, winds blow from the southwest, bringing moist air on shore. As the air is forced to rise over the foothills of the Himalayas, it causes constant and frequently heavy rains. The town of Cherrapunji, India, located on the Himalayan slopes, receives an annual rainfall of over 36 ft (11 m), making it one of the wettest places on Earth.

The summer monsoon ends as the highlands of Tibet begin to cool in the fall. The cooling atmosphere over Tibet allows the subtropical jet to reappear south of the Himalayas, bringing the rainy summer monsoon to an end. The circulation shifts to a winter monsoon cell, with sinking air over India and surface winds that blow out to sea. The resulting winter weather is dry.

The monsoons of South China and Japan

The monsoons of China and Japan are strongly affected by the huge land mass of Siberia. During the winter, the interior of Siberia becomes extremely cold. Cold air is dense, so a cold area of high pressure forms, where the air sinks from aloft. When it reaches the surface, the air spreads outward in all directions. The result is a dry winter monsoon that blows from the north through south China and southeast Asia. The same circulation affects northern China, Japan and Taiwan, where the prevailing wind is from the northwest. Although originally dry, the northwest winds pass over the sea and pick up moisture. When the winds arrive at the islands of Japan and Taiwan, they are forced up over the land. As a result, the western slopes of these islands experience a rainy winter monsoon.

During the summer, strong heating in the interior of Siberia sets up the summer circulation, giving south China, Indonesia, and southeast Asia humid southerly winds from the equator. Japan experiences winds from the south or southeast, which bring in moisture from the north Pacific Ocean.

When the monsoon fails

The importance of monsoons is demonstrated by the experience of the Sahel, a band of land on the southern fringe of the Sahara Desert. This area would also be arid if it were not for the seasonal monsoon, whose rains normally transform it to a grassland suitable for grazing livestock . The wetter southern Sahel can support agriculture, and many residents migrated to the area during the years of strong monsoons. Beginning in the late sixties, however, the annual monsoons began to fail. The pasture areas in the northern Sahel dried up, forcing nomadic herders and their livestock southward in search of pasture and water. The monsoon rains did not return until 1974. In the intervening six years, the area suffered devastating famines and loss of life, both human and animal , and placed extreme stress on the countries of central Africa.

Because of their tremendous effect on many tropical areas, atmospheric scientists continue to study the formation and variability of monsoons. Monsoon variations are still not entirely understood, making the prediction of the monsoon a distant goal. However, researchers have shown that monsoons are affected by El Niño , the changes in winds and sea water that occur in the tropical Pacific Ocean. Research continues into modeling monsoon circulations with complicated computer simulations.

See also Atmospheric circulation; Storm

Resources

books

Frater, Alexander. Chasing the Monsoon. New York: Alfred A. Knopf, 1991.

McCurry, Steve. Monsoon. New York: Thames and Hudson, 1984.

Navarra, John G. Atmosphere, Weather and Climate. Philadelphia: W.B. Saunders Co., 1979.

periodicals

Meehi, Gerald A. "Coupled Land-ocean-atmosphere Processes and South Asian Monsoon Variability." Science (October 14, 1994): 263.

James Marti

KEY TERMS

Circulation cell

—A circular path of air, in which warm air rises from the surface, moves to cooler areas, sinks back down to the surface, then moves back to near where it began. The air circulation sets up prevailing (constant) winds at the surface and aloft.

Convection

—The rising of warm air from the surface of the Earth.

Heat capacity

—The amount of heat required to raise the temperature of a substance; water has a high heat capacity, while land surfaces (soil, rock, etc.) and air have much lower heat capacities.

Jet stream

—High speed winds that circulate around the Earth at altitudes of 7–12 mi (12–20 km) and affect weather patterns at the surface. Two important jet streams are the subtropical jet, flowing from west to east, and the weaker tropical jet, which flows from east to west.

Monsoon

—A seasonal change in the direction of the prevailing wind, often associated with a rainy or a dry season.

Subtropics

—Regions between 23.5 and about 35 degrees latitude, in both the northern and southern hemispheres, which surround the tropics.

Tropics

—The region around Earth's equator spanning 23.5° north latitude to 23.5° south latitude.

Monsoon

A monsoon is a regional wind that reverses directions seasonally. In southern Asia, wet, hot monsoon winds blow from the southwest during the summer months and bring heavy rains to a large area that includes India, Bangladesh, Sri Lanka, Pakistan, and Nepal. Northeasterly winds (winds are named by the direction from which they blow) that blow down from the Himalaya Mountains in the winter are cool and dry. Monsoon winds occur in many regions around the world, in Africa, Australia, and in North America, where the Mexican monsoon brings over half of the year's total rain to Northern Mexico, Arizona, and New Mexico each June through August. The Mexican monsoon is a smaller version of the classic and wellknown southern Asian monsoon.

The word monsoon comes from the Arabic word mausim which means "season." In southern Asia, the monsoon controls the seasons: hot and wet in the summer, cool and dry in the winter. Plants and animals of southern Asia have adapted ways to survive the annual cycles of flood and drought (prolonged period of dry weather). Humans depend on the rains to fill storage reservoirs and to water crops, especially water-intensive staples like rice and cotton.

How monsoons work

The Asian monsoon works like a large version of reversing land-sea breezes along coastlines. Water heats and cools more slowly than dry land. On a sunny day at the beach, the air over land heats more quickly than air over the water. Warmer air expands and rises. Cool air moves in from the ocean to replace the rising warm air, and this movement creates a nonshore breeze. When the sun goes down in the evening, the land cools more quickly than the sea, and the wind changes direction.

The Indian peninsula is a piece of low land surrounded on three sides by the waters of the northern Indian Ocean; it separates the Arabian Sea from the Bay of Bengal. The massive Himalaya Mountains to the north isolate it from winds and weather in the rest of Asia. India lies just north of the equator, so it receives very intense sunlight and heats up beginning in April. When the hot air over the peninsula rises, wet air from the tropical Indian Ocean flows onshore to take its place. The wind flows from the southeast, across India and into the Himalayan foothills where it is forced upward. Warm air holds more moisture than cool air. The moist ocean air blown in by these winds cools and condenses (changes into liquid from a gas) as it rises, resulting in heavy rains. Monsoon rains are often hard, sustaining rains.

The rainy season

The "season of the peacock" begins in mid-May as the monsoon wet phase reaches southern India. Peacocks are the symbol of life-giving rains in India. Male peacocks begin courting females by flashing their brilliant tail feathers a few weeks before clouds form and the arrival of downpours that transform the parched, brown landscape to a lush, green paradise. As the rainy season progresses, two arms of wet weather extend across the region: one reaches up from the tear-drop-shaped island of Sri Lanka (formerly Ceylon), to the tip of India, and toward Pakistan; the other comes in from the Bay of Bengal and Bangladesh, and crosses central and northern India. By early July, the two arms have merged, and the torrential monsoon rains extend throughout south Asia. Animals mate and seeds germinate. Rivers, lakes, reservoirs, and wells fill with water. Humans plant and water their crops.

Monsoon rains continue through the summer months and begin to let up by the end of September. During many years, the rains that were so welcome in spring have become " too much of a good thing" by late summer, when flooding threatens crops, buildings, and lives throughout southern Asia. Soil and rock layers that hold water are saturated (completely full of water), and rainwater runs directly off the land surface. Small streams in the Himalayan foothills flow over their banks and flood crops and towns. Small floods run downhill to join others, and then flow together as a very large pulse of water into the mighty Brahmaputra and Ganges Rivers.

The Ganges-Brahmaputra system collects water from most of South Asia; the Indian province of West Bengal and the country of Bangladesh cover its massive delta (the fan-shaped area of land at the river's mouth). The residents of the Ganges Delta have developed some strategies for surviving the annual deluge, but there are years when particularly strong monsoon rains and heavy snows in the Himalayas create huge, uncontrollable floods that devastate the area. During floods in September 1998, almost 70% of Bangladesh, an area about the size of the state of Tennessee, was underwater. The 1998 floods in Bangladesh killed hundreds of people and caused millions to lose their homes. Poor sanitation and ruined crops led to widespread disease and starvation.

The dry season

In the fall, cold air flows down from the high peaks of the Himalayas as the continent begins to cool. A dry northwest wind blows across India and the rain moves offshore into the ocean. Floodwaters recede and leave behind new layers of silt (fine soil particles) and nutrients to fertilize the flooded agricultural lands. By the following July, the land is parched and dry, and south Asians look forward to another drenching rainy season.

Asian monsoon is a blessing and a threat

Plants and animals of South Asia depend on the summer rains and have evolved (changed over time) to survive the floods and droughts of the monsoon. Plentiful rain gives rise to lush forests and grasslands that provide food and shelter for some of Earth's most exotic animal species, including Bengal tigers and Indian elephants. South Asian plants and animals have adapted strategies to reproduce and thrive during the rainy season and then lie dormant (inactive) or survive on stored water during the dry months.

Approximately one quarter of the world's population depends on the monsoon rains and is threatened by related floods and droughts. The monsoon counties are very heavily populated. The land area of India is about the same size as the United States but its population is more than three times as large. Bangladesh, a relatively poor country, is one of the most densely settled places on Earth. (Imagine the entire population of the United States living in Oregon.)

Scientists have discovered links between the Asian monsoon and global climate. They worry that natural or human-induced global warming or cooling could affect the monsoon pattern and lead to increased drought or flooding. More than one billion people face starvation, illness, and the loss of their homes during exceptionally rainy or dry monsoon years. Natural ecosystems (interaction of living organisms and their environment in a community) of plants and animals that are suffer as well. The governments of monsoon countries, the United Nations, scientists, and non-profit organizations are working to better understand the monsoon and to develop solutions to economic and environmental problems of South Asia.

Laurie Duncan, Ph.D.

For More Information

Books

Barry, Roger, et al. Atmosphere, Weather, and Climate. 8th ed. New York: Routledge, 2003.

Sudham, Pira. Monsoon Country. London: Breakwater Books, 1990.

Websites

"Bangladesh Floods Maroon Thousands." BBC News.http://news.bbc.co.uk/1/hi/world/south_asia/2116055.stm (accessed on August 17, 2004).

"India Monsoon." Pulse of the Planet Program #1921.http://www.pulseplanet.com/archive/Jul99/1921.html (accessed on August 17, 2004).

"Weather Basics—Monsoon Climate Zone." BBC Weather.http://www.bbc.co.uk/weather/features/weatherbasics/zones_monsoon.shtml (accessed on August 17, 2004).

Monsoon

A monsoon is a seasonal change in the direction of the prevailing wind. This wind shift typically brings about a marked change in local weather. Monsoons are often associated with rainy seasons in the tropics (the areas of Earth within 23.5 degrees latitude of the equator) and the subtropics (areas between 23.5 and about 35 degrees latitude, both north and south). In these areas, life is critically dependent on the monsoon rains. A weak monsoon rainy season may cause drought, crop failures, and hardship for people and wildlife. However, heavy monsoon rains have caused massive floods that have killed thousands of people.

Many parts of the world experience monsoons to some extent. Probably the most famous are the Asian monsoons, which affect India, China, Japan, and Southeast Asia. Monsoons also impact portions of central Africa, where their rain is critical to supporting life in the area south of the Sahara Desert. Lesser monsoon circulations affect parts of the southwestern United States. These summer rainy periods bring much needed rain to the dry plateaus of Arizona and New Mexico.

General monsoon circulation

Monsoons, like most other winds, occur in response to the Sun heating the atmosphere. In their simplest form, monsoons are caused by differences in temperatures between the oceans and continents. They are most likely to form where a large continental landmass meets a major ocean basin. During the early summer, the landmasses heat up more quickly than ocean waters. The relatively warm land surface then heats the air over it, causing the air to convect, or rise. The convection of warm air produces an area of low pressure near the land surface. Meanwhile, air over the cooler ocean waters is humid, more dense, and under higher pressure.

The atmosphere always tries to maintain a balance by having air move into areas of low pressure from surrounding areas of high pressure. This movement is known as wind. Thus during the summer, oceanic air flows toward the low pressure over land. This flow is continually supplied by cooler oceanic air sinking from higher levels in the atmosphere. In the upper atmosphere, the rising continental (landmass) air is drawn outward over the oceans to replace the sinking oceanic air, thus completing the cycle. In this way a large vertical circulation cell is set up, driven by solar heating. At the surface, the result is a constant wind flowing from sea to land.

Words to Know

Circulation cell: A circular path of air, in which warm air rises from the surface, moves to cooler areas, sinks back down to the surface, then moves back to near where it began. The air circulation sets up constant winds at the surface and aloft.

Convection: The rising of warm air from the surface of Earth.

Jet stream: High-speed winds that circulate around Earth at altitudes of 7 to 12 miles (12 to 20 kilometers) and affect weather patterns at the surface.

Subtropics: Regions between 23.5 and about 35 degrees latitude, in both the northern and southern hemispheres, which surround the tropics.

Tropics: Regions of Earth's surface lying within 23.5 degrees latitude of the equator.

As it flows onto shore, the moist ocean air is pulled upward as part of the convecting half of the circulation cell. The rising air cools and soon can no longer contain moisture. Eventually rain clouds form. Rain clouds are especially likely to occur when the continental areas have higher elevations (mountains, plateaus, etc.) because the humid ocean air is forced upward over these barriers, causing widespread cloud formation and heavy rains. This is the reason why the summer monsoon forms the rainy season in many tropical areas.

In the late fall and early winter, the situation is reversed. Land surfaces cool off quickly in response to cooler weather, but the same property of water that makes it slow to absorb heat also causes it to cool slowly. As a result, continents are usually cooler than the oceans surrounding them during the winter. This sets up a new circulation in the reverse direction: air over the sea, now warmer than that over the land, rises and is replaced by winds flowing off the continent. The continental winds are supplied by cooler air sinking from aloft. At upper atmospheric levels, the rising oceanic air moves over the land to replace the sinking continental air. Sinking air (high pressure) prevents the development of clouds and rain, so during the winter monsoon continental areas are typically very dry. This winter circulation causes a prevailing land-to-sea wind until it collapses with the coming of spring.

The monsoon of India

The world's most dramatic monsoon occurs in India. During the early summer months, increased solar heating begins to heat the Indian subcontinent, which would tend to set up a monsoon circulation cell between southern Asia and the Indian Ocean. However, the development of the summer monsoon is delayed by the subtropical jet stream.

Jet streams are great rivers of air that ring Earth at levels in the atmosphere ranging from 7 to 8 miles (11 to 13 kilometers) above the surface. The subtropical jet stream is a permanent feature, flowing westerly (from west to east). It migrates over the year in response to the seasons, moving northward to higher latitudes in the summer and southward in the winter.

As summer progresses, the subtropical jet slides northward. The extremely high Himalayan mountains present an obstacle for the jet; it must "jump over" the mountains and reform over central Asia. When it finally does so, a summer monsoon cell develops. The transition can be very fast: the Indian monsoon has a reputation for appearing suddenly as soon as the subtropical jet stream is out of the way. As the air is forced to rise over the foothills of the Himalayas, it causes constant, heavy rains, often resulting in destructive flooding. The town of Cherrapunji, India, located on the Himalayan slopes, receives an annual rainfall of over 36 feet (11 meters), making it one of the wettest places on Earth.

When the monsoon fails

The importance of monsoons is demonstrated by the experience of the Sahel, a band of land on the southern fringe of Africa's Sahara Desert. The rains of the seasonal monsoon normally transform this arid (dry) area to a grassland suitable for grazing livestock. The wetter southern Sahel can support farming, and many residents migrated to the area during the years of strong monsoons. Beginning in the late 1960s, however, the annual monsoons began to fail. The pasture areas in the northern Sahel dried up, forcing wandering herders and their livestock southward in search of pasture and water. The monsoon rains did not return until 1974. In the intervening six years, the area suffered devastating famines and loss of life, both human and animal.

[See also Atmospheric circulation; El Niño ]