Giddens, age 12, of Shreveport, La., for her question:
WHAT MAKES A DESERT?
A desert is a term that is applied to regions of the earth that are characterized by less than 10 inches of annual rainfall, an evaporation rate that exceeds precipitation and, in most cases, a high average temperature. Deserts are caused by a combination of climate patterns and geological features.
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.
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 Near East.
Along the southern belt lie Patagonia in Argentina, the Kalihari 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 land masses. 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 upweilings 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.
Mountain ranges can also influence the development of deserts by creating rain shadows.
As moisture laden winds flow upward over the windward slopes of some mountains, they cool and lose their moisture. Dry air descending over the leeward slopes evaporates moisture from the soil. The Great Basin, a cool desert in 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 these regions. Such deserts are the Gobi and Turkestan of Eurasia.
In sandy deserts such as the Sahara and parts of the North American desert, sand dunes are typical features. Wind built mounds of sand may reach heights of more than 650 feet.
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.
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.