I | INTRODUCTION |
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 |
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 |
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 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 daytime heat.
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 |
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.
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.
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