DEFINITION OF DESERT

The driest places on Earth, deserts by definition

receive less than one inch (250 mm) of rain per year. At present

about 30 percent of the global landmass is desert, and

the United States has about 10 percent desert areas. With

changing global climate patterns and shifting climate zones,

much more of the planet is in danger of becoming desert.

Most deserts are hot, with the highest recorded temperature

on record being 136°F (58°C) in the Libyan

Desert. With high temperatures, the evaporation rate is also

high, and, in most cases, deserts are able to evaporate more

than the amount of precipitation that falls as rain. Many

deserts are capable of evaporating 20 times the amount of

rain that falls, and some places, like much of the northern

Sahara, are capable of evaporating 200–300 times the

amount of rain that falls in rare storms. Deserts are also

famous for large variations in the daily temperature, sometimes

changing as much as 50°F–70°F (28°C–39°C)

between day and night (called a diurnal cycle). These large

temperature variations can be enough to shatter boulders.

Deserts are also windy places and are prone to sandstorms

and dust storms. The winds arise primarily because the

heat of the day causes warm air to rise and expand, and

other air must rush in to take its place. Airflow directions

shift frequently between day and night (in response to the

large temperature difference between day and night) and

between any nearby water bodies, which tend to remain at

a constant temperature over a 24-hour period.

There are many different types of deserts located in all

different parts of the world. Some deserts are associated with

patterns of global air circulation, and others form because

they are in continental interiors far from any sources of moisture.

Deserts may form on the “back” or leeward side of

mountain ranges, where downwelling air is typically dry, or

they may form along coasts where cold upwelling ocean currents

lower the air temperature and decrease its ability to

hold moisture. Deserts may also form in polar regions, where

extremely dry and cold air has the ability to evaporate (or

sublimate) much more moisture than falls as snow in any

given year. There are parts of Antarctica that have not had

any significant ice or snow cover for thousands of years.

Deserts have a distinctive set of landforms and hazards

associated with these landforms. The most famous desert

landform is a sand dune, which is a mobile accumulation of

sand that shifts in response to wind. Some of the hazards in

deserts are associated with sand and dust carried by the wind.

Dust eroded from deserts can be carried around the globe and

is a significant factor in global climate and sedimentation.

Some sandstorms can be so fierce that they can remove the

paint from cars, or the skin from an unprotected person.

Other hazards in deserts are associated with flash floods,

debris flows, avalanches, extreme heat, and extreme temperature

fluctuations.

Droughts are different from deserts—a drought is a prolonged

lack of rainfall in a region that typically gets more

rainfall. If a desert normally gets a small amount of rainfall,

and it still is getting little rainfall, then it is not experiencing a

drought. In contrast, a different area that receives more rainfall

than the desert may be experiencing a drought if it normally

receives significantly more rainfall than it does at

present. A drought-plagued area may become a desert if the

drought is prolonged. Droughts can cause widespread

famine, loss of vegetation, loss of life, and eventual death or

mass migrations of entire populations.

Droughts may lead to conversion of previously productive

lands to desert, a process called desertification. Desertification

may occur if the land is stressed prior to or during the

drought, typically from poor agricultural practices, overuse

of ground and surface water resources, and overpopulation.

Global climate goes through several different variations that

can cause belts of aridity to shift back and forth with time.

The Sahel region of Africa has experienced some of the more

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