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.