Where are Deserts? Draw a line around the world, starting mid-center between Joshua Tree and Death Valley, and you will touch or come close to many of the world's great deserts: Mojave, Great Basin, Sahara, Arabian, Iranian, Gobi. Most deserts occur between the latitudes of 15 and 40 degrees on either side of the equator. They are found around the world on every continent, covering approximately 20 percent of Earth's land area. Sand dunes cover only about 10 percent of this area. Some deserts are very mountainous. Most are hot, with warm daytime temperatures much of the year, but others are cold, getting over half their moisture from snow.
Deserts can be divided into four types: subtropical, coastal, interior, and rain shadow, depending on the conditions creating them. Subtropical deserts lie along the Tropic of Cancer (23N latitude) and the Tropic of Capricorn (23S latitude). Near the equator hot, moist air rises. It cools, dropping heavy rains on tropical areas. The resulting cooler, drier air then descends, creating zones of high atmospheric pressure as it moves away from the equator. The descending air hinders cloud formation and precipitation. It also warms up, absorbing any available moisture. The Sahara, the world's largest hot desert, is a subtropical desert about the size of the United States. Coastal deserts are also in areas of high pressure. Damp, chilly fog forms when air, chilled by water contact as it blows toward shore, meets warm air over land.
Although humidity is high, atmospheric disturbances that can cause rainfall are not present. Two coastal deserts, the Atacama of Chile and the Namib in southern Africa, are among the driest places in the world. Interior deserts, like the Gobi, exist because they are too far from moisture-laden, ocean winds. By the time these winds reach the center of a large landmass, the air is very dry. Rain shadow deserts are created when mountain ranges lie parallel to moist, coastal areas. Prevailing winds moving inland cool as air is forced to rise over the mountains. Carried moisture falls on slopes facing the winds. When the winds move over the crest and down the far side, they are very dry. Descending air also makes it hard for additional clouds and precipitation to form. Without another source of moisture, rain shadow deserts are formed on the far side of these mountain ranges. The Mojave Desert, which includes most of Joshua Tree National Park, is a rain shadow desert.