When people think of deserts, they usually think of regions that are sandy, hot, and dry. However, there are other types of deserts as well. One of the world's major ecosystems, a desert is any large, barren region that supports very little life. Deserts cover at least 20 percent of the Earth's land surface.

Where deserts are found

Most of the Earth's deserts are located along the latitudes known as the Tropic of Cancer and the Tropic of Capricorn. These hot, dry deserts lie mainly on the west coasts of continents, though they also extend across North Africa and far into Asia and Australia. Tropical deserts include the Sahara of northern Africa, the Thar Desert of the Indian subcontinent, the Victoria of Australia, and the Kalahari of southwestern Africa.

The deserts located on the inside of continents extend towards the Earth's poles and are often called middle-latitude deserts. The Great Basin Desert in western North America is an example of an interior desert.

Ice cap and tundra regions around the poles, such as the barren lands of Antarctica and Greenland, are sometimes called cold deserts or high-latitude deserts.

Climate

One of the most characteristic features of most of the desert regions is that they get less than about 10 inches (25 centimeters) of rain in a year. Some deserts in coastal regions can experience fog and high humidity, while the interior desert regions tend to be very dry. When rain will occur is usually uncertain. For example, at Iquique in northern Chile, records show that during one period of almost four years the area received no rain at all. When precipitation does occur, it may be very heavy for short periods.

Deserts have extreme climates, and the temperatures range widely. Hot deserts, as their name indicates, experience very high temperatures by day, especially in summer. The highest recorded temperature occurred in Libya, where it reached 136.4° F (58° C).

The winters in deserts located far from the equator are quite cold. In the month of July, Luktchin, a region in central Asia, records an average temperature of 90° F (32° C), while the January average is 13° F (-10.6° C). The Gobi in Central Asia has average temperatures below freezing for as many as six months each year. During the hottest months, the air temperature may average 105° to 110° F (41° to 43° C). In addition to temperature changes from summer to winter, there are also temperature changes from day to night, sometimes as great as 60 to 70 Fahrenheit degrees (33 to 39 Celsius degrees).

Land forms

Deserts occur on all the continents. They differ greatly in the appearance and nature of their surface features, which range from mountains to plateaus to plains.

Very often deserts are associated with huge sand dunes. However, they are not as common or widespread as is believed. Even in deserts with much sand, such as the Arabian, dunes occupy only about 30 percent of the total area. If sand accumulations on plains pile up and look like a sea of sand, they are called ergs.

The more common type of desert consists of rugged mountains separated by basins called bolsons. These mountains receive most of their rain in downpours. As the water rushes down the slopes, it cuts deep gullies and carries rock fragments, gravel, and sand to the bolson. As the water slows down or evaporates, these materials get deposited, producing rugged forms such as the terrain in Death Valley, California. They are called badlands.

Sometimes when floodwaters collect in basins with no outlets, temporary shallow lakes called playa lakes are formed in desert regions. Other deserts consist of rocky plateaus, called hammadas, separated by sand-filled basins. Sometimes where streams or wind wear away the weaker rocks, strong rock formations called mesas or cuestas are formed in deserts. Gullies, known as wadis in Arabia and arroyos in the American southwest, are sometimes cut deep into the hammadas by rushing water.

Plants and animals

Desert plants and animals have special traits to help them survive in the harsh climate. Some plants have deep roots in order to reach water underground. Others may spread their roots over a large area close to the surface of the soil to absorb the small amount of rain that falls. Desert plants often have a waxy coating to prevent water loss. Some even shed their leaves during the dry season. Many plants grow only when rain falls, passing most of the time as seeds or bulbs. Common desert plants are agaves, cacti, date palms, and yuccas.

Typical desert animals are insects, reptiles, birds, rodents, and a few large mammals. The smaller animals hide under stones and vegetation during the day when it is hot and emerge to feed at night. The camel can survive for many days on fat stored in its hump.

Human habitation

Despite the hardships involved, people do live in deserts. The Tuareg of the Sahara, the San of the Kalahari, the Bedouins of the Middle East and North Africa, and the Aboriginals of Australia are some such people. However, the total population of all desert regions is estimated at only about 85 million, or less than 8 people to each square mile. The only densely settled spots are irrigated places such as the lower Nile River valley.

Water and drainage

People can live and grow crops in a desert only at places where they can get water. Such places are called oases. Land is usually fertile in oases and cultivated intensively. In North African and Asian oases, the chief food crops are dates, figs, cotton, sugarcane, wheat, barley, rice, and beans. In the United States, irrigated lands are mainly used for citrus fruits, dates, winter vegetables, and cotton.

Usually groundwater lies at greater depths in deserts than in humid lands. In the wadis, or gullies, ordinary wells can usually tap a supply of good water. Oasis settlements, therefore, are most often found where wadis are numerous. Ergs, into which many wadis drain, may also have a water supply.

Deep wells are also bored where the rock structures hold water under pressure. The most generous supply of water for irrigation, however, is obtained from streams that rise in rainy regions outside deserts. The largest and best-known rivers that supply water to desert areas are the Nile in Egypt, the Tigris and the Euphrates in Iraq, the Indus in Pakistan, and the Colorado in the United States.

Desert development

As humans have learned to irrigate the lands, improve methods of transportation, and take advantage of valuable resources such as petroleum, they have found it easier to live in the deserts. Human activity has also had a harmful effect on the land, however. In the 20th century and into the 21st a process known as desertification became a problem for many people living in areas near deserts. As the populations continue to increase, more of those lands are used for farming and grazing to support the people. Because of poor farming methods as well as droughts, that land has slowly become more desertlike. It is a problem in many parts of the tropics and means that not enough food can be grown. Scientists are trying to find ways of stopping this process.