Deserts are classified by their geographical location and dominant weather pattern as _trade wind_, _midlatitude_, _rain shadow_, _coastal_, _monsoon_, or _polar_ deserts. Former desert areas presently in nonarid environments are _paleodeserts_, and _extraterrestrial_ deserts exist on other planets. Trade wind deserts The trade winds in two belts on the equatorial sides of the Horse Latitudes heat up as they move toward the Equator. These dry winds dissipate cloud cover, allowing more sunlight to heat the land. Most of the major deserts of the world lie in areas crossed by the trade winds. The world’s largest desert, the Sahara of North Africa, which has experienced temperatures as high as 57° C, is a trade wind desert. Midlatitude deserts Midlatitude deserts occur between 30° and 50° N. and S., poleward of the subtropical high-pressure zones. These deserts are in interior drainage basins far from oceans and have a wide range of annual temperatures. The Sonoran Desert of southwestern North America is a typical midlatitude desert. Rain shadow deserts Rain shadow deserts are formed because tall mountain ranges prevent moisture-rich clouds from reaching areas on the lee, or protected side, of the range. As air rises over the mountain, water is precipitated and the air loses its moisture content. A desert is formed in the leeside “shadow” of the range. [Illustration: The Sahara of Africa is the world’s largest desert. It contains complex linear dunes that are separated by almost 6 kilometers (Skylab photograph).] [Illustration: A rare rain in the Tengger, a midlatitude desert of China, exposes ripples and a small blowout on the left. Winds will shortly cover or remove these features.] [Illustration: This Landsat image shows the Turpan Depression in the rain shadow desert of the Tian Shan of China. A sand sea is in the lower center on the right, but desert pavement, gray in color, dominates this desert. The few oases in the desert and the vegetation in the mountains at the top are in red. A blanket of snow separates the vegetation in the Tian Shan from the rain shadow desert.] Coastal deserts Coastal deserts generally are found on the western edges of continents near the Tropics of Cancer and Capricorn. They are affected by cold ocean currents that parallel the coast. Because local wind systems dominate the trade winds, these deserts are less stable than other deserts. Winter fogs, produced by upwelling cold currents, frequently blanket coastal deserts and block solar radiation. Coastal deserts are relatively complex because they are at the juncture of terrestrial, oceanic, and atmospheric systems. A coastal desert, the Atacama of South America, is the Earth’s driest desert. In the Atacama, measurable rainfall—1 millimeter or more of rain—may occur as infrequently as once every 5-20 years.