A large, dry, barren region, usually having sandy or rocky soil and little or no vegetation. Water lost to evaporation and transpiration in a desert exceeds the amount of precipitation; most deserts average less than 25 cm (9.75 inches) of precipitation each year, concentrated in short local bursts. Deserts cover about one fifth of the Earth's surface, with the principal warm deserts located mainly along the Tropic of Cancer and the Tropic of Capricorn, where warm, rising equatorial air masses that have already lost most of their moisture descend over the subtropical regions. Cool deserts are located at higher elevations in the temperate regions, often on the lee side of a barrier mountain range where the prevailing winds drop their moisture before crossing the range.
A Closer Look A desert is defined not by temperature but by the sparse amount of water found in a region. An area with an annual rainfall of fewer than 25 centimeters (9.75 inches) generally qualifies as a desert. In spite of the dryness, however, some animals and plants have adapted to desert life and thrive in these harsh environments. While different animals live in different types of deserts, the dominant animals of warm deserts are reptiles, including snakes and lizards, small mammals, such as ground squirrels and mice, and arthropods, such as scorpions and beetles. These animals are usually nocturnal, spending the day resting in the shade of plants or burrowed in the ground, and emerging in the evenings to hunt or eat. Warm-desert plants are mainly ground-hugging shrubs, small wooded trees, and cacti. Plant and animal life is scarcer in the cool desert, where the precipitation falls mainly as snow. Plants are generally scattered mosses and grasses that are able to survive the cold by remaining low to the ground, avoiding the wind, and animal life can include both large and small mammals, such as deer and jackrabbits, as well as a variety of raptors and other birds.