Drought

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Drought

The African Sahel: Devastated by drought
Recent events: Major droughts since 2000
What causes a drought?
Aftermath: The effects of drought
The human factor
The technology connection
For More Information

A drought (pronounced DROWT) is an extended period where the amount of rain or snow that falls on an area is much lower than usual. A drought usually lasts at least one season and sometimes continues for years. It may affect an area the size of several states or greater. During a drought, the amount of water used by plants, animals, and people is much greater than the precipitation, which is the amount of water that falls to the ground in the form of rain or snow. Rivers and lakes may dry up. Droughts happen almost everywhere in the world, although some areas experience drought more frequently than others.

There is no universal definition of drought based on rainfall amounts, since what is considered usual varies from region to region and climate to climate. In Australia, which is a dry climate, a drought is defined as a year in which precipitation is less than 10 percent of average. In the United States—most of which has a temperate, humid climate—drought is defined as a period at least twenty-one days long during which rainfall over an extensive area is less than 30 percent of average. In places with distinct wet and dry seasons, such as India, a drought is defined as a year in which precipitation is less than 75 percent of average. The severity of a drought is determined by the amount of rainfall, the water levels in rivers and lakes, the duration of the dry spell, and the size of the area affected.

Droughts lead to crop losses, soil erosion, death of livestock, and even famine (long-term shortages of food that can lead to hunger, malnutrition, and death). Human beings sometimes make drought conditions worse by stripping the land of vegetation and placing heavy demands on the water supply.

The African Sahel: Devastated by drought

The longest-lasting and most devastating drought in recent times occurred in the Sahel region of Africa in the last half of the twentieth century. The Sahel (Arabic for "margin" or "shore") is a strip of semiarid land in northern Africa, sandwiched between the Sahara to the north and the wetter grasslands to the south. It extends from the continent's west

WORDS TO KNOW

air mass:: a large quantity of air throughout which temperature and moisture content is fairly constant.
air pressure:: the pressure exerted by the weight of air over a given area of Earth's surface. Also called atmospheric pressure or barometric pressure.
aquifer:: an underground layer of spongy rock, gravel, or sand in which water collects.
arid:: describes a climate in which almost no rain or snow falls.
blocking system:: a whirling air mass containing either a high-pressure system (a blocking high) or a low-pressure system (a blocking low), that gets cut off from the main flow of upper-air westerlies.
climate:: the weather experienced by a given location, averaged over several decades.
desert climate:: the world's driest climate type, with less than 10 inches of rainfall annually.
desertification:: the process by which semiarid lands turn into desert (also called land degradation). It is caused by prolonged drought, during which time the top layers of soil dry out and blow away.
drought:: an extended period where the amount of rain or snow that falls on an area is much lower than usual.
ecosystem:: a community of plants and animals, including humans, and their physical surroundings.
El Niño:: means "the Christ child" in Spanish. A period of unusual warming of the Pacific Ocean waters off the coast of Peru and Ecuador. It usually starts around Christmas, which is how it got its name.
erosion:: the wearing away of a surface by the action of wind, water, or ice.
global water budget:: the balance of the volume of water coming and going between the oceans, atmosphere, and continental landmasses.
heat cramps:: muscle cramps or spasms, usually afflicting the abdomen or legs, that may occur during exercise in hot weather.
Heat exhaustion:: a form of mild shock that results when fluid and salt are lost through heavy perspiration.
heat stroke:: a life-threatening condition that sets in when heat exhaustion is left untreated and the body has exhausted its efforts to cool itself. Also called sunstroke.
heat wave:: an extended period of high heat and humidity.
La Niña:: Spanish for "little girl," a period of unusual cooling of the Pacific Ocean waters off the coast of Peru and Ecuador. It often follows an El Niño.
semiarid:: a climate in which very little rain or snow falls.
transpiration:: the process by which plants emit water through tiny pores in the underside of their leaves.
upper-air westerlies:: global-scale, upper-air winds that flow in waves heading west to east (but also shifting north and south) through the middle latitudes of the Northern Hemisphere.

coast almost to its east coast. The Sahel ranges in width from 200 to 700 miles (320 to 1130 kilometers) and covers more than 2.5 million square miles (5.1 million square kilometers). It runs through the countries of Senegal, Mauritania, Mali, Burkina Faso, Niger, Nigeria, Chad, and Sudan. A similar semiarid belt exists in Ethiopia and Somalia. The Sahel region constitutes about 20 percent of the landmass of Africa, and the nations within this region are among the poorest in the world.

The ecosystem of the Sahel is the most fragile in all of Africa. The primary vegetation includes low-growing grass, thorny shrubs, and a few varieties of trees.

During the last thirty years of the twentieth century, the Sahel suffered from desertification—the process of turning the land into desert. In the beginning of desertification, the soil slowly loses its ability to grow plants. Next comes a period of rapid soil breakdown. In the final stage of desertification, the soil becomes unable to hold nutrients or retain enough water to support plant life. Desertification is a long process and is difficult to reverse.

The major cause of desertification in the Sahel has been a series of droughts that lasted for years. Human activity, such as intensive farming and grazing large numbers of livestock, has also contributed to desertification.

Drought is common

Throughout history the Sahel has experienced frequent droughts. The region receives an average of just 4 to 20 inches (10 to 50 centimeters) of rain per year, mostly in the southern areas during the months of June, July, and August. Because the area is also hot during this period, about 90 percent of the water that falls to the ground returns to the atmosphere through evaporation and transpiration. With the water balance so unsteady in normal times, even a small decrease in rainfall over a season may result in water shortages and crop failures.

After a relatively rainy period in the 1950s and early-to-mid 1960s, a drought started in 1968 and lasted until 1973. During that period, crops died throughout the region, and a famine took the lives of some 250,000 people and 3.5 million grazing animals. Many more people would have died if not for a massive amount of help from other countries.

After a few years of normal rainfall, relatively dry conditions prevailed from 1976 through 1981. Then drought struck the region again from 1983 through 1985 and in the early 1990s. Throughout the dry years, desertification continued its march southward from the Sahara. Overall, between the years 1935 and 1985, a portion of the Sahel, roughly equal in size to the combined size of France and Austria, turned into desert.

By the end of the 1990s, however, normal precipitation levels had returned to the Sahel. While desertification was continuing in some places, much of the Sahel had recovered to the point that local food production needs were being met.

The influence of the people

Although the problem of desertification in the Sahel started because of the long periods of drought, population growth and bad use of the land intensified the problem. Because of the lack of rainfall and high temperatures in the region, the soil is thin, sandy, and fragile. Most of the land is covered with grass and is best used by people who move from one area to another with small herds of animals, or by small-scale farmers.

In years past, Sahel-dwellers grew crops only on a particular piece of land one season at a time. They would then leave that piece of land undisturbed for several years so that the soil could regain its nutrients. During that time, cattle would graze on the crop stubble and fertilize the soil with their wastes.

In the latter half of the 1900s, however, the population of the Sahel increased greatly. Increasingly more land was used for growing crops, which thinned the soil and reduced its fertility. As a result, farmers could not let part of the land sit undisturbed for several years; instead, they had to plant every available piece of land every year in order to produce enough food to survive.

A series of unusually rainy years in the 1950s and 1960s also encouraged more farming in the Sahel. Shortsighted government officials, not anticipating that a drought might return, urged farmers to cultivate greater areas of land. Land previously undisturbed or used for grazing was cleared. The trees and brush were taken for firewood, and the soil was plowed for crops.

Changes in methods of grazing livestock also contributed to desertification. The traditional practice of moving livestock from one area to another to eat the grass changed as small towns were built up around water wells. Thus, the livestock would eat grass only in one area and the land in the surrounding area was stripped bare of grass and other plants. As a result, it became hard and barren within just a few years.

Dry lands that have been stripped of vegetation take a long time to recover. Even when rains do fall, very little water penetrates the earth. Most of the rain runs off the ground, washing away the topsoil and leaving a wasteland. Moreover, the wind causes more erosion when there are no plants and their roots to hold the soil in place.

Millions of people forced to move

During the periods of drought from the late 1960s through the mid 1980s, more than ten million farmers and shepherds from the Sahel moved to cities and towns. They built shanties on the outskirts of already overpopulated urban areas and tried to earn a living. For example, in Mauritania, one of the countries hit hardest by the drought, refugees increased the population of the town of Nuakchott from 20,000 in 1960 to 350,000 in 1987.

The migration of people from country to city has done more than create population pressures on cities; it has disrupted centuries-old ways of life and tribal bonds. Omar Mahmoud, a former herdsman who moved to an urban shantytown in the early 1970s, was trying to live by raising vegetables on a small, dusty plot of land. He told a National Geographic reporter, "I don't know much about this work. My life is being with my animals, but now they are all gone. Forty head of cattle, forty sheep. Sixty goats."

Foreign aid has mixed results

Foreign governments and international relief agencies began providing emergency aid to the Sahel in the late 1960s. While this assistance prevented mass starvation in many areas, and a few projects helped slow the pace of desertification, many of the projects were costly and poorly managed; they also did not have much effect on the problems.

Many relief-agency projects involved the planting of trees or grasses in an effort to stabilize the soil and prevent further erosion. In some cases planners planted too few trees, which did not provide enough stability for the soil. In other cases, the planting of trees was not coupled with instructions about the importance of the trees to the people's survival. As a result, many people cut down the trees for firewood. Some agencies made the mistake of providing seeds for crops such as eggplant that were unfamiliar and distasteful to the people living in the region.

In one notable success story, however, the relief agency CARE began a project in the Majja Valley in Niger. They planted a double row of trees along a 230-mile-long (370-kilometer-long) stretch of land. The trees, which have been maintained by local villagers, stop the wind from eroding the soil during dry times and protect crops from the wind during the rainy season.

What can be done in the Sahel?

Scientists have different opinions about what can be done to save the Sahel from drought in the future. Some feel the people must be moved out of the area and the farmlands replaced with forests to prevent future problems. Others feel the people themselves can solve the problems without being moved by exchanging ideas, building water supplies, and planting trees. Everyone seems to agree that a big part of the solution is to develop a society in the Sahel where there are fewer people dependent on the land for their survival and more people doing other jobs.

Recent events: Major droughts since 2000

Between 1999 and 2002, the United States experienced a drought that was among the worst of the past 40 years. During these years of drought, parts of the United States experienced the driest conditions in more than 100 years of record keeping. The year 2001 was the second-hottest on record in the United States, second only to 1998, which saw record temperatures due to El Niño (an extraordinarily strong episode of the annual warming of the Pacific waters off the coast of Peru and Equador).

The consequences of the 1999 to 2002 drought were significant and far reaching. Heat and water restrictions hampered U.S. crop production. Rivers lost water, and fish died by the thousands. People, especially elderly people living in cities, died in their homes from the heat. Wildfires raged and caused millions of dollars worth of damage.

Drought is defined as an extended period of unusually dry conditions. However, drought conditions can and do exist even in years not officially termed "drought" years. In August 2006, for example, the National Weather Service published drought statements for thirty American cities from Talahassee, Florida, to Denver, Colorado, to Duluth, Minnesota.

Even in the absence of an official drought, hot and dry conditions create the risk of wildfires, crop destruction and river evaporation. Many cities throughout the United States experience water shortages during the summer months. City governments sometimes respond by imposing bans on water use for irrigation purposes in order to conserve precious water supplies.

What causes a drought?

Droughts occur when rain-producing clouds fail to form, and there is little, if any, rainfall. Several factors can bring on those conditions. In temperate regions, including much of the United States, the primary cause of drought is the prolonged presence of a high-pressure system called a blocking high. In many parts of the world, the weather condition known as El Niño (and its sister, La Niña), which occurs about every five to seven years, also influences rainfall amounts. The removal of vegetation can also contribute to drought in some regions.

Blocking highs

The main cause of drought in the United States is the prolonged presence of a high-pressure system over a given area. High-pressure systems generally produce clear skies and little rain. As air descends beneath a high-pressure system, the moisture within it evaporates. Low-pressure systems, in contrast, are associated with cloudiness. The air beneath them rises, and the moisture within the air condenses and forms clouds.

Systems of high and low pressure typically alternate over a given location as they are pushed along by upper-air winds. Those winds circle the globe along a wavy, west-to-east path. Sometimes, however, an air mass (a large quantity of air throughout which pressure, temperature, and moisture content is fairly constant) gets cut off from the main flow of upper-air winds. If that air mass has high pressure, it is called a blocking high; if it has low pressure, it is called a blocking low. The presence of a blocking system typically brings about an extended period of one type of weather.

A blocking high may remain stationary for several weeks. In addition to producing dry conditions, the blocking high may produce exceedingly hot weather. The surface air heats up because the upper-air circulation, which moderates temperatures by alternating warm and cold air masses, passes around the area.

El Niño and La Niña

In some parts of the world, drought is driven by El Niño (pronounced el NEE-nyo). El Niño is a current of warm water in the tropical Pacific Ocean. Every few years, the water that flows from the western Pacific and settles off the coast of Peru and Ecuador is unusually warm and remains for an extended period of time.

Air pressure across the Pacific Ocean is linked to El Niño. In normal conditions, air pressure is higher over the eastern Pacific, near South America, and lower over the western Pacific, near Australia. (High pressure is typically associated with clear skies, while low pressure is typically associated with cloudy skies and rain.) The difference in pressure between the two areas makes the winds blow from east to west, and toward the equator. The winds carry warmth, moisture, and rainmaking clouds toward Australia and Indonesia.

During an El Niño year, the air pressure in the western Pacific rises while the air pressure in the eastern Pacific lowers. As the warm water in the eastern Pacific evaporates into the air and forms clouds, the normally dry coastal South American nations get more-than-average rainfall, causing flooding and erosion. At the same time, Australia, Indonesia, the Philippines, and other lands of the western Pacific experience less-than-average rainfall and sometimes drought.

Question: Is global warming causing droughts?

As Earth's climate changes, drought becomes a more regular part of human existence. Many U.S. states now experience drought conditions regularly, and the U.S. experienced its worst drought in almost half a century between 1999 and 2002. Scientists believe that global temperatures are on the rise, but could this be contributing to the increase in drought conditions?

Recent scientific data indicates that the global average temperature increased by about 1°F in the last century. Scientists refer to this process as global warming, which is partly caused by increase of certain gases, called greenhouse gases, in Earth's atmosphere. Many scientists think that temperatures will rise even more in the decades to come, and some are worried that increased temperature will cause more frequent, severe, and prolonged droughts.

There are many factors that cause drought, and droughts have plagued humanity for centuries. Certainly global warming is not the only factor that contributes to drought. However, warmer temperatures increase the evaporation of water from Earth's surface, which contributes to and worsens drought conditions.

Often in the wake of El Niño comes La Niña—a period of unusual cooling of the ocean water off the coast of Peru and Ecuador. La Niña's effects on rainfall patterns are practically the opposite of El Niño's. One consequence of La Niña is that drier than usual conditions prevail in many parts of the United States. La Niña also means drought for the South American coast and flooding for the western Pacific region.

Aftermath: The effects of drought

While drought does not produce the sensational pictures of destruction seen in other natural disasters, its economic impact is often greater and longer lasting. According to the Nebraska-based National Drought Mitigation Center, the average yearly economic damage done by droughts in the United States is greater than the economic damage done by floods and hurricanes combined. Droughts cost the United States between $6 billion and $8 billion per year, while the annual cost of floods and hurricanes are $2.4 billion and $1.2 billion, respectively.

The water cycle

The water cycle (also called the hydrologic cycle) is the continuous movement of water between the atmosphere and the Earth's surface (oceans and landmasses). On one side of the equation is precipitation—rain and snow—and on the other side is evaporation—the process by which liquid water at the surface converts to a gas and enters the atmosphere.

From 85 to 90 percent of the moisture that evaporates into the atmosphere comes from the oceans. The rest evaporates from the soil, lakes, and rivers that make up the continental landmasses. Even plants emit water through tiny pores in the underside of their leaves in a process called transpiration.

Some of the moist air above oceans is carried over land by the wind. Clouds form and drop rain and snow on the ground. When precipitation hits the ground, it either sinks in or runs off, depending on the surface composition. On soft ground, most of the water sinks into the soil to be absorbed by plant roots or to seep down into underground aquifers. Some of it runs off into streams and rivers. If the water strikes a hard surface, like rock or pavement, most of it runs directly into streams or artificial drains. Eventually this water also flows into rivers.

The oceans lose water in this portion of the cycle—more water evaporates from them than returns as precipitation. The oceans get this water back when the rivers empty their water back into the oceans. Thus the global water budget—the volume of water coming and going from the oceans, atmosphere, and continental landmasses—is kept in balance.

If the water cycle is kept in balance, that means that global precipitation levels remain fairly constant. So why do droughts occur? The answer is that rain and snow do not consistently fall in equal amounts in any given place. Moisture may evaporate from one place, travel through the atmosphere, and fall to the ground as rain in another. It is possible, then, for a given location to get lots of rainfall one year and almost no rainfall the next.

The impact of droughts varies widely, depending on the severity and length of the drought and its location. Among the consequences of drought are shortages of water, livestock and wildlife death, crop loss, soil erosion, wildfires, and dust storms. Famine is the most drastic consequence of drought and is most likely to occur in poor nations where food supplies are already scarce and governments have little money for importing food or drilling wells.

Reduction in water levels

One direct consequence of drought is a shortage of water. Wells run dry, and water levels drop in lakes and rivers. The first water supply to be affected is aquifers, which are underground water reservoirs made of porous rock, gravel, or sand. Aquifers feed lakes and rivers; the top of an aquifer is higher than the bed of the lake or river it feeds. When water is plentiful and an aquifer is full, water runs over into a surface body of water. During drought, however, the level of the aquifer falls below the floor of the lake or river, and the aquifer ceases to replenish the surface body of water. In severe drought, aquifers go dry, and lake and river levels become very low. Wells (holes drilled into aquifers that provide people with water) also may run dry, in which case it becomes necessary to continue drilling to tap sources of water deeper underground.

The lowering of water levels in lakes and rivers has several consequences. For one, fish and other forms of aquatic life may die. If bodies of water become too shallow for boats to operate, the water recreation industry suffers economic harm. If barges and ships cannot carry goods down waterways, the increased cost of shipping materials over land gets passed on to consumers. In addition, the amount of electricity produced by hydroelectric power stations (facilities at which the energy of the moving water is converted into electricity) decreases when rivers flow less forcefully.

The reduction in water levels that results from a drought are felt long after the rains resume. First, enough rain has to fall to recharge dried-out aquifers. This process typically takes a year or more. Only when the aquifers are full will the water begin its slow journey to rivers and lakes.

Migration and famine

When drought is serious enough to affect food and water supplies, people in the affected region are forced to move to another area or subsist on donated food. In Africa, Asia, and Australia—regions where there have been droughts lasting several years—the loss of crops and cattle has caused widespread human suffering. A severe drought in northern China in 1876, for instance, caused crop failures that led to the deaths of millions of people. As many as five million people died in Russia as a result of a drought along the Volga River basin in 1921.

Experiment: How rainfall affects soil

During a drought, rainfall is extremely scarce. To understand some of rain's effects, try this experiment. Get two large disposable aluminum baking pans. Fill one with some dirt from a garden or yard. Use a pair of scissors or an awl to poke six or eight small holes in one end of the pan that is full of dirt. Put the empty pan under the side of the dirt-filled pan that has holes in it. Lift up the opposite end of the pan (you can use books to hold it up), and then use a watering can to pour water onto the dirt at the elevated end of the pan. Notice what happens to the dirt when the water comes into contact with it, and pay attention to the color of the water as it runs into the second pan.

Secondary impacts

Droughts also have indirect impacts, such as increased prices for food, energy, lumber, and other products; unemployment (in the agricultural, forestry, and fishing industries); and the closing of businesses in the affected area. The loss of crops that results from drought does not just affect the pocketbooks of farmers. It also means a reduction in revenues for merchants who provide goods and services to farmers.

The human factor

While drought on Earth is older than human existence, human activity in recent years has contributed to the frequency and severity of droughts. The primary way in which people bring about drought is by stripping the land of vegetation. Denuded land not only suffers more than covered land during times of drought, but the lack of vegetation also contributes to the occurrence of drought. The major ways in which people encourage drought are through farming intensively, overgrazing cattle, and cutting down trees for timber or fuel.

The concept of surviving a drought has different meanings depending on the location and severity of the drought. For some people, especially those in poor nations, surviving a drought means subsisting on donated food or migrating to wetter regions. In wealthier nations, where food is more abundant, people learn to change their behaviors to adapt to chronic drought rather than aiming merely to survive it. Coping with drought may entail conserving water and keeping cool in hot weather.

Desertification

Desertification, also called land degradation, is the process of turning an area into desert. Approximately 25,000 square miles (64,750 square kilometers) of Earth's surface is turned into desert each year. Reversing desertification is a lengthy and difficult process.

While drought is the primary cause of desertification, humans can make the problem worse by removing a land's natural vegetation. Exposed soil is susceptible to erosion by wind and rain. In the absence of roots to bind the soil together and trees and shrubs to slow the wind, heavy rains create deep ravines and gullies in the land, and the wind blows away the topsoil.

The enforcement of travel restrictions across political boundaries in Africa and Asia in the twentieth century has been one of the leading causes of human-induced land degradation in those areas. Prior to the restriction of travel across national borders, nomads roamed with their herds in search of vegetation. New policies forced herdsmen to settle in particular areas, and wells were drilled to meet their water needs. As a result, the settled areas have been overgrazed and stripped of vegetation, and the water levels of the reservoirs feeding those wells have greatly decreased.

Vegetation loss and the cycle of drought

The clearing of vegetation also acts as a deterrent against cloud formation and rain—in essence creating a cycle of drought. When water evaporates from leaves, that water condenses to form clouds, which can cause precipitation. In the absence of plants, the air is dry and does not form clouds. When it does rain, the lack of vegetation also means the surface is less able to retain the water. The water runs off and erodes the soil, which means even fewer plants can grow—making the cycle repeat and grow worse.

Overuse of groundwater

Another practice that creates many problems during droughts is the overuse of groundwater. Groundwater is water from rain or melted snow that seeps into underground aquifers. Pumps are used to tap groundwater for community needs. If the rate of ground-water usage is not reduced during dry periods, aquifers can become depleted, thus contributing to the lowering of lake and river levels.

When aquifers become depleted they may collapse, causing the ground around them to sink—thus forming sinkholes. A sinkhole is a large depression in the Earth's surface; often shaped like a funnel, it is typically 50 feet (15.3 meters) in diameter and up to 100 feet (30.5 meters) deep.

Conserving water

In the United States, water conservation measures are typically required in times of drought; nonessential uses of water, such as car-washing and lawn-watering, are banned. Some communities have programs in place to recycle wastewater (from sources such as bathtubs and dishwashers) for use on small farms and domestic gardens.

There are many ways people can reduce water consumption. For example, people can take showers instead of baths. A bath typically uses 30 gallons (114 liters) of water, whereas a five-minute shower, at a flow of 5 gallons (19 liters) per minute, uses 25 gallons (95 liters) of water. An economy showerhead reduces the flow to 2.5 gallons (9.5 liters) per minute and would use only 12.5 gallons (47.5 liters) for a five-minute shower. If each person in a family of four takes a bath every day for a week, the family uses 840 gallons (3,180 liters) of water. If the family installs an economy showerhead, and each person takes a five-minute shower each day instead of a bath, however, they will use just 280 gallons (1,060 liters) of water. This would save 560 gallons (2,120 liters) of water a week.

Another water-saving device is a low-flow toilet. This type of toilet uses less than 2 gallons (7.6 liters) per flush, as compared to the 5 gallons (19 liters) per flush used by a conventional toilet. Some states in drier regions of the United States have laws requiring the installation of reduced-flow showerheads and toilets in new construction.

Some people conserve water by collecting rainwater and using it for watering the garden. When rainfall is minimal, bath water can be used on the garden. Another method of saving water is installing plumbing that routes bath water or water that has been used for washing dishes to the toilet, for flushing.

Community water-saving initiatives have been shown to work. In Tucson, Arizona, where conservation is mandated, residents use an average of 160 gallons (606 liters) of water per day. In Phoenix, by comparison, where water-conservation measures are not required, residents use an average of 260 gallons (984 liters) per day.

Drought in Australia

Australia experienced a serious drought in the early twenty-first century. The drought, the worst in two decades, began in March 2002. Like the U.S. drought of the same year, the Australian drought produced record-breaking low amounts of rainfall.

The consequences of the Australian drought have been dire. Livestock and crops were particularly hard hit. Because the country had less to export, its trade deficit soared to its highest point in years. Bush fires caused damage and destruction throughout the country.

The heat factor

Heat waves (extended periods of high heat), which are frequently associated with drought and actually worsen the effects of drought, are among the leading causes of death during droughts. Under drought conditions there are few, if any, clouds to block the incoming solar energy. The sunlight heats the land and robs it of existing moisture by causing rapid evaporation. The loss of moisture, in turn, makes the air even hotter. When the ground is dry, all incoming solar energy heats the land (as opposed to being absorbed in the process of evaporation) and is transferred directly into the air.

Hot weather adversely affects human health by raising the body's temperature. High body temperature can lead to dehydration, heat exhaustion (a form of mild shock that results when fluids and salt are lost through heavy perspiration), and heat stroke (a life-threatening condition that sets in when heat exhaustion is left untreated, and the body has used up its efforts to cool itself). People engaged in physical activity in hot conditions, particularly if they don't drink enough water, may fall victim to one or more heat-related illnesses.

How to survive a heat wave

The following suggestions can help you stay healthy during hot weather:

Avoid overexertion, especially during the hottest part of the day. The coolest part of day, and the safest time for strenuous activity, is between 4 am and 7 am.
Wear loose, lightweight, light-colored clothing. This type of clothing allows air to circulate while protecting the skin from the Sun's heat and damaging rays. The light color reflects, rather than absorbs, sunlight.
Remain indoors as much as possible. If you don't have air conditioning, stay on the first floor, out of the sunshine, and keep the air circulating with electric fans. Each day that the air is very hot, try to spend some time in an air-conditioned environment.
Drink plenty of fluids, even if you don't feel thirsty. Water is best. Avoid drinks with caffeine or alcohol, since they actually draw fluids out of the body, which may lead to dehydration.
Eat frequent, small meals. Avoid foods high in protein since the digestion of protein increases your body's temperature.
Avoid overexposure to the Sun. When skin is sunburned, its ability to dissipate heat is hampered.
If you are taking medication that affects your blood circulation, ask your doctor how that medication affects your ability to tolerate heat.
Groups of people most susceptible to heat-related illness include elderly people, small children, people with chronic illnesses, overweight people, and people with alcohol dependency. People in those groups should be especially cautious during a heat wave.

Heat also places added stresses on the body's circulatory system. For elderly people or people with serious illnesses, that added stress can prove fatal. The most common causes of death in hot weather are cardiac arrest, stroke, and respiratory distress.

Heat waves take more lives than any other type of weather calamity. They kill approximately one thousand people per year, on average, in the United States, and a far greater number in other less-developed countries. In contrast, winter storms or cold kill approximately 130 to 200 people per year, on average, in the United States; floods kill 100 to 160; tornadoes kill 80 to 130; and hurricanes kill 40 to 60 people.

The technology connection

Several technological innovations are used to monitor and respond to droughts. Earth survey satellites provide data about water levels in surface bodies of water and the moisture content of the soil. Irrigation systems, dams, drought-resistant crops, and soil management techniques are all used to make the effects of drought less severe.

Drought prediction, however, remains something of a mystery. Droughts are hard to foresee because they are produced by the absence of events in the atmosphere; notably, the absence of rain clouds and precipitation. Other weather phenomena, such as hurricanes, thunderstorms, and blizzards, all have recognizable patterns of clouds and precipitation and show up on weather sensing instruments.

At the end of the twentieth century, researchers were looking at data on the cycles of El Niño and La Niña, as well as trends shown in satellite images, for possible keys to predicting future droughts.

Earth survey satellites

Starting with the launch of Landsat 1 in 1972, a series of Landsat Earth survey satellites have continually monitored drought conditions by providing information on moisture levels in the soil, as well as the depths of lakes and rivers. Two Landsats continue to transmit information: Landsat 5, launched in 1984, and Landsat 7, launched in April 1999. The satellites orbit Earth about fifteen times every day at an altitude of 438 miles (705 kilometers). Each satellite is capable of observing almost every continental surface on the globe in an eighteen-day period. It can determine moisture content in areas as small as ten football fields.

Moisture content in the soil and evaporation rates (both measured by satellites) are reliable indicators of drought. Researchers combine this information with data on precipitation, temperature, and other factors to predict the length and intensity of a drought.

The images returned by Landsat satellites are also used for monitoring deforestation, receding glaciers, and crop growth. They are used to locate mineral deposits and to observe patterns of strip mining, logging, and damage due to insect infestations and fire.

Agricultural practices

One way that farmers protect the soil from the impact of drought is by planting rows of trees or shrubs, called windbreaks, at intervals throughout their fields. Windbreaks, which run crossways to the direction that the wind usually blows, slow the wind and keep it from blowing away the soil.

In some regions with low levels of rainfall, farmers find success through fallow farming, which is the practice of letting one-half of the land lie fallow, or idle, every other year, while planting the other half. During the growing season, the farmer plows the fallow land to unearth weeds. Plowing also creates spaces in the soil that can trap water and raise the moisture content of the soil. When the previously fallow land is planted the following year, seeds are more likely to germinate rapidly, and crops will have an early growth spurt.

Another way that farmers guard against the effects of drought is by planting drought-resistant crops (plants that thrive even in dry conditions). These crops are relatively small and quick to grow. One crop that is drought-resistant is sorghum (pronounced SOAR-gum). During dry periods, sorghum stops growing and reduces transpiration (the loss of water through its leaves); it resumes growing when rain comes. Another example of a crop that does well in dry conditions is alfalfa, the roots of which extend down 6 feet (1.8 meters) or more and tap into groundwater.

Irrigation

Irrigation is the transportation of water from reservoirs or wells to areas where crops are growing. Since the beginning of civilization, farmers have practiced irrigation as a means of improving their ability to grow crops. Irrigated farmland, in general, is twice as productive as nonirrigated farmland. In 2000, more than 900 million square miles (6 billion square kilometers) of farmland around the world were irrigated. About one-third of the world's food supply comes from irrigated land.

Irrigation, however, has its drawbacks. For one thing, it is expensive; the drilling of wells and purchase of irrigation equipment and fuel, and in some cases the water, is costly. Many farmers relying on irrigation have found themselves buried in debt. Irrigation is also wasteful. In hot, dry weather, up to one-third of water sprayed out of crop sprinklers evaporates before reaching the ground. Irrigation that relies on groundwater also presents the danger of depleting aquifers. This is particularly hazardous in coastal land, because if the water table falls below sea level, seawater will seep into the aquifer and cause the groundwater to become salty, which destroys the soil.

Dams

The construction of river dams is a common method of storing water and lessening the impact of drought (as well as preventing flooding). A dam is a barrier that blocks a river and controls the flow of water. When a river is dammed, the water backs up in the area behind the dam, creating an artificial lake or reservoir (pronounced REH-zer-vwar). Pipes carry the water from the reservoir to factories, homes, and farms. This ready supply of water is especially useful during times of drought. As the 1990s came to a close, there were more than 60,000 dams in use worldwide.

Large dams—defined as those that are more than 492 feet (150 meters) high, or holding back more than 19.6 million cubic yards (15 million cubic meters) of water—have a significant impact on the surrounding community and environment. When a dam is constructed in a valley, it floods the area upstream, putting homes, farmland, and even whole villages underwater. In the process, it displaces the people who inhabit the area. Dams also disrupt a river's natural cycle of flooding. When a river floods a valley, it deposits a layer of silt that enriches the soil. After the river is dammed, the fertile soil is lost. (The soil upstream is buried beneath the reservoir, and the soil downstream remains exposed but becomes less fertile over time.) Dams also disrupt the balance of the river ecosystem, destroying the habitat of birds, fish, and other animals, as well as many species of plants.