Desert

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.

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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 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.

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