The Namib Desert, stretching over 1900 km (1200 mi) along the coast of Namibia in southwest Africa, receives less than
two inches of rain a year, as a result of the effects of the cold Benguela Current. This region has sparse vegetation and few
Anthony Bannister/Oxford Scientific Films
Desert, term applied to regions of the earth that are characterized by less than 254 mm (10 in) of annual rainfall, an
evaporation rate that exceeds precipitation, and, in most cases, a high average temperature. Because of a lack of moisture
in the soil and low humidity in the atmosphere, most of the sunlight penetrates to the ground. Daytime temperatures can
reach 55° C (131° F) in the shade. At night the desert floor radiates heat back to the atmosphere, and the temperature can
drop to near freezing.
Deserts are caused by a combination of climate patterns and geological features.
II WIND SYSTEMS
World Desert Regions
Most deserts arise due to atmospheric wind conditions. Warm air masses created two belts of desert, one along the Tropic
of Cancer and the other along the Tropic of Capricorn. Other deserts result from the effects of ocean currents on
landmasses, where cool air masses carry fog and mist, but little rain, along coastal regions.
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Most desert regions have been formed by movements of air masses over the planet. As the earth turns on its axis, it
produces gigantic air swirls. Hot air rising over the equator flows northward and southward; the currents cool in the upper
regions and descend as high-pressure areas in two subtropical zones. North and south of these zones are two more areas
of ascending air and low pressure. Still farther north and south are the two polar regions of descending air. As air rises, it
cools and loses its moisture. As it descends, it warms and picks up moisture, drying out the land.
The downward movements of warm air masses over the earth have produced two belts of deserts, one along the tropic of
Cancer, in the northern hemisphere, and the other along the tropic of Capricorn, in the southern hemisphere. Among the
northern deserts are the Gobi in China, the deserts of southwestern North America, the Sahara in North Africa, and the
Arabian and Iranian deserts in the Middle East. Along the southern belt lie Patagonia in Argentina, the Kalahari Desert of
southern Africa, and the Great Victoria and Great Sandy deserts of Australia.
Other desert areas result from the influence of ocean currents on landmasses. As cold waters move from the Arctic and
Antarctic regions toward the equator and come into contact with the edges of continents, they are augmented by upwellings
of cold water from the ocean depths. Air currents cool as they move across cold water; they carry fog and mist but little rain.
Such currents flow across the coastal regions of southern California, Baja California, southwest Africa, and Chile; although
often shrouded in mist, these coasts are deserts.
III LAND FORMATION
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Mountain ranges influence the development of deserts by creating rain shadows. As moisture-laden winds flow upward over
the windward slopes, they cool and lose their moisture in the form of rain and snow. Dry air descending over the leeward
slopes evaporates moisture from the soil. The Great Basin, a desert of North America, results from the rain shadow
produced by the Sierra Nevada.
Other desert areas in the interiors of some continents have formed because the prevailing winds are far removed from large
bodies of water and have lost much of their moisture by the time they reach those regions. Such deserts are the Gobi and
Turkistan of Eurasia.
The largest desert in Asia, the Gobi straddles the border between Mongolia and China. It contains a series of basins that are
divided by low, flat-topped ranges and isolated hills, as pictured here.
Oxford Scientific Films /Photolibrary
The desert landscape is stark, shaped by wind and, paradoxically, water. When rains do come to the desert, the soil,
unprotected by vegetation, easily erodes. Canyons called arroyos form where water rushes down from the hills. From the
eroded angular peaks of more resistant rocks, alluvial fans lead away to deposit large slopes of debris, called bajadas, at
the base. These slopes level off to form low basins called playas. During the infrequent rains, the basins fill with water. The
rainwater then evaporates, leaving behind on the surface a layer of glistening salt dissolved from the ground. Such salt lakes
are a common feature of some deserts. In the Great Salt Lake of Utah, a remnant of an inland sea fed by some inflow of
fresh water, evaporation is never complete, but it is sufficient to concentrate salt in the lake water.
Winds literally sandblast rocks into unusual shapes and also build up dunes. In sandy deserts such as the Sahara and parts
of the North American desert, sand dunes (see Sand Dune) are typical features. Wind-built mounds of sand can reach
heights of more than 200 m (more than 650 ft) in the Sahara, Arabian, and Iranian deserts. In deserts where prevailing
winds are strong and sand is relatively scarce, as in the coastal deserts of Peru, dunes may take on regular crescent shapes
that move continuously across the desert floor. Dunes may be longitudinal ridges resulting from winds blowing only in one
direction, or they may be star shaped in regions where the wind blows from all directions.
IV PLANT ADAPTATIONS
All but the most arid desert lands support life that is frequently abundant and well adapted to the scarcity of water and the
Desert plants have evolved ways of conserving and efficiently using the water available to them. Some flowering desert
plants are ephemeral; they live for a few days at most. Their seeds lie dormant in the soil, sometimes for years, until a
soaking rain enables them to germinate and quickly bloom. Woody desert plants either have long root systems that reach
deep water sources or have spreading shallow roots that are able to take up surface moisture quickly from heavy dews and
occasional rains. Desert plants usually have small leaves. This conserves water by reducing surface area from which
transpiration can take place. Other plants drop their leaves during the dry period. The process of photosynthesis—by which
sunlight is converted to energy and usually conducted primarily in leaves—is taken over in the desert by the stems. A
number of desert plants are succulents, storing water in leaves, stems, and roots. Thorns, which are modified leaves, serve
to guard the water from animal invaders. These plants may take in and store carbon dioxide only at night; during the day
their stomata, or pores, are closed to prevent evaporation. Desert plants growing on saline soils may concentrate salt in their
sap and then secrete the salt through their leaves.
V ANIMAL ADAPTATIONS
The camel is adapted to an arid environment, at home among the dunes of Asian and North African deserts. Pictured here
are Arabian camels, which have one hump on their backs. The animals are used as beasts of burden by people from
northwestern India to the Arabian peninsula and North Africa.
ORF Enterprise Ges.m.b.H
Among the desert animals, the few amphibian species are capable of long-term dormancy during dry periods. When the
rains come, they mature rapidly, mate, and lay eggs. Many birds and rodents reproduce only during or following periods of
winter rain that stimulate the growth of vegetation. Some desert rodents, such as the North American kangaroo rat and the
African gerbil, feed on dry seeds; their metabolic processes are extremely efficient at conserving and recycling water, and
their urine is highly concentrated. A number of desert mammals, such as the camel, are able to withstand considerable
dehydration. Most desert mammals and reptiles are nocturnal, remaining in cool underground burrows or in the shade by
day. Some desert reptiles, such as the horned toad, can control their metabolic heat production by varying their rate of
heartbeat and the rate of body metabolism. Some mammals, among them the desert oryx, vary their body temperatures,
storing heat by day and releasing it at night.
To move across the shifting sands of its desert habitat, the sidewinder throws its body sideways in “S”-shaped loops so that
it moves forward at an oblique angle.
BBC Worldwide Americas, Inc.
VI HUMAN INFLUENCES
Because so little water moves through the desert soil to carry nutrients away, desert soils are naturally fertile. Crops are
grown on desert lands with water provided by irrigation from rivers or wells. Such transformations of deserts are not without
problems. Evaporation of the irrigation water results in the accumulation of salt on the surface soil, eventually rendering it
useless for further crop production. By tapping reservoirs of fossil water deep beneath the desert, humans are, in effect,
mining water. Once this water is gone, it is irreplaceable. Burning and overgrazing of semiarid lands on the periphery of
deserts can irreversibly damage the plants that concentrate moisture and hold the soil together, thus enabling deserts to
encroach on arable land. This encroachment, a serious world problem, is called desertification. A 1984 report of a
desertification study made for the United Nations stated that 35 percent of the earth’s land surface was at least threatened
by such processes.
See also Ecology; Environment.
Robert Leo Smith