Deserts are generally defined as geographic regions that receive an annual average precipitation of 10 inches or less. Many factors combine to produce such a low level of precipitation but three conditions tend to predominate.
Most of the deserts on this planet are aligned along the Tropics of Cancer and Capricorn where persistent high pressure zones cause the air to sink; sinking air compresses, dries out and heats up, creating an atmosphere of low humidity and sparse cloud cover. The dry air, abundant sunshine and radiant nocturnal cooling lead to wide swings in temperature throughout the 24-hour cycle; hot, sunny days and chilly, clear nights characterize these desert regions for most of the year. These conditions are hostile to most species of vegetation, minimizing plant transpiration and further reducing the natural processes that humidify the atmosphere.
Similar climatological factors often lead to desert formation on the lee side of mountain ranges. Prevailing winds carry moisture-laden air into the mountains where it is forced to rise and condense, dropping most of its rain and snow on the windward side of the divide; moving beyond the summit of the range, the air descends, heats up and dries out. The deserts of eastern Oregon, Nevada and southeastern California, lying east of the Sierra Nevada, are classic examples of mountain-induced desert formation.
Finally, a couple of the driest deserts on Earth, the Atacama of Chile and the Namib of West Africa, lie along western coastlines bordered by cold ocean currents. Prevailing winds reach these deserts from the east, having crossed the girth of the Continent and mountain ranges along the way; what little moisture remains is lost as the air descends toward the shore. Occasional disruptions in this flow may allow westerlies to sweep in from the ocean but their moisture precipitates as the air crosses the cold waters of the sea; only a shroud of sea fog reaches these coastal deserts, offering droplets of water to the parched landscape.
