Desert - geography.

Desert - geography. 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 d...