Introduction to Desert
Desert, an arid region with little or no vegetation. The word comes from a Latin word meaning abandoned or forsaken, signifying lack of human habitation. Although deserts seem to be uninhabited, some kinds of plants and animals have adapted to the harsh conditions. People, too, live in or near deserts—traditionally as oasis dwellers or nomads. In the 20th century, permanent settlement in some deserts has increased, largely due to improved means of obtaining water and to such inventions as air conditioning.
The term desert usually refers to areas that, in addition to being extremely dry, have high daytime temperatures, particularly in the summer. Many scientists also consider polar regions to be deserts, since they are arid and sustain little or no vegetation.
For information on polar areas, the discussion in this article is concerned with deserts in the usual sense.
Deserts occupy between 15 and 20 per cent of the earth's land area. They usually receive less than 10 inches (250 mm) of precipitation yearly. Precipitation, usually rain, tends to come in a few heavy storms of short duration. Deserts are often bordered by semiarid areas.
Kinds of Deserts
Though most scientists who study deserts agree that there are several types of deserts, they do not agree on how to classify these types. Some scientists classify deserts according to the amount of annual precipitation received. Others classify them according to the types of vegetation and soil present. Still others classify deserts by average summer and winter temperatures. In a general sense, deserts can be categorized by where they are located.
Where Deserts Are Found
The world's deserts are generally found in four areas: in the subtropics; in continental interior areas at middle latitudes; on the leeward side of mountain ranges; and along coastal areas. Most of the earth's greatest deserts are found in the subtropics. The subtropical deserts center on the tropic of Cancer and the tropic of Capricorn. The aridity of these areas is due primarily to the general circulation of air around the earth. Subtropical areas have consistently high atmospheric pressure because of descending air currents and are swept by hot, dry winds. Such conditions keep moisture-bearing winds from entering the region and prevent the formation of clouds.
The absence of cloud cover is largely responsible for the high temperatures that occur in subtropical deserts. Without the protection of clouds, the land receives nearly the full heating effect of the sun's rays. Although temperatures are relatively high the year round, in summer the heat is extreme. Summer daytime temperatures often exceed 100° F. (38° C.) and sometimes rise to as much as 120° F. (49° C.). The earth's highest recorded temperature, 136.4° F. (58° C.), occurred in the Libyan Desert of northern Africa. At night, however, with no clouds to provide insulation, daytime heat dissipates quickly and temperatures drop abruptly. Nighttime temperatures average around 70° F. (21° C.) in summer and can fall below freezing during winter.
The world's largest desert, the Sahara, is a subtropical desert. It occupies about 3,500,000 square miles (9,000,000 km2) in Africa.
Middle-latitude continental interior deserts include the vast Gobi and Taklimakan deserts of Central Asia. The scant precipitation in middle-latitude deserts is due mainly to their inland location, far from the oceans The air that enters such areas is usually extremely dry; whatever moisture it may have contained was released before reaching the interior. These deserts, too, are cloudless for most of the year. Summer daytime temperatures can rise as high as those of subtropical deserts; winter temperatures, however, are considerably lower. Average daily temperatures below freezing are common during winter.
Deserts found on the leeward sides of mountains include the deserts of the southwestern United States. Mountain ranges act as barriers to moisture-carrying winds. Clouds release precipitation on the windward slopes, leaving the dry air to pass down the leeward sides. This phenomenon is known as the “rain shadow effect.”
The presence of cold coastal water can contribute to the creation of a desert along the adjoining shore. This occurs because cold ocean currents tend to stabilize the air over the coast and inhibit cloud formation. Large coastal deserts include the Atacama Desert in Chile and the Namib Desert in Namibia.
Desert Landforms
Deserts display a great variety of landscape features. Sand dunes, the landforms commonly thought of as typical of deserts, occupy only a relatively small part of the world's deserts. Some deserts, however, have great expanses of dunes, known as sand seas, or ergs. Bare rock surfaces, called yardangs, are found in many deserts; they occur where wind and water have removed the surface particles and left the hard bedrock exposed. Commonly, flat areas are covered by what is known as a desert pavement, a closely compacted layer of rock and gravel. In some places, especially at the foot of a mountain, there are large deposits of gravel and other coarse debris, which are sometimes quite deep.
In many deserts flash floods form temporary rivers that carve steep-sided valleys called dry washes, arroyos, or wadis. Badlands and deep gullies are also common in arid lands.
The water carried by flash floods sometimes accumulates in shallow depressions in the desert floor, called playas. Playa lakes usually disappear quickly and leave behind sand, silt, and dissolved minerals, which may form salt flats. Though most rivers and lakes are intermittent, there can be permanent water features—such as the Great Salt Lake, in Utah, and the Dead Sea in Israel and Jordan.
Vegetation and Wildlife
Desert plants and animals have developed specialized means of survival in the harsh climate. Some plants have extensive root systems that reach deep into the earth to collect groundwater or that spread out widely near the surface to absorb surface moisture. Many desert plants have small waxy or leathery leaves that limit moisture loss. Others, such as cacti, retain water in their roots and stems. Certain plants appear only after drenching rains, go through an entire life cycle, and leave seeds that will germinate only after the next heavy rain.
Many animals have developed forms of behavior suited to desert life, such as burrowing underground to escape the heat and being active only at night when it is cool. Desert animals, in general, are especially well adapted for conserving water and regulating body temperature. The animals most commonly found in desert and semidesert areas are small mammals, particularly rodents; reptiles, including many species of snakes and lizards; and birds. Spiders, scorpions, and insects, notably ants and beetles, are widespread.
Humans and the Desert
In some deserts and steppes traditional ways of life have changed, especially since the mid-20th century. Many nomadic groups have adopted settled ways of life or have been restricted to fixed areas by their country's government. Some oasis settlements have grown into modern cities.
Many of the changes occurring in the world's deserts have been caused by the development of desert resources. Especially important has been the extraction of oil and natural gas, which has brought more people to desert areas. Also contributing to changes in the deserts are technological advances that have brought security from drought to many areas and have made possible the development of large settlements. Reservoirs, canals, pipelines, desalinization plants, and deep wells are some of the means used to provide a steady supply of water. Careful water and land management along with the development of drought-resistant crops have helped make farming successful in some arid lands.
Human activities have often damaged the desert environment. Also, in semiarid areas along the fringes of deserts, human activities (especially those related to large-scale agricultural development) have contributed to desertification—reduction in the soil's ability to capture and store water.
