Scientists measure a region's aridity by comparing the amount of precipitation (rain, sleet, or snow) to the rate of evaporation. Evaporation always exceeds precipitation. Deserts can be classified as extremely arid (less than 1 inch [2.5 centimeters] of rain per year); arid (up to 10 inches [25 centimeters]); and semiarid (as much as 20 inches [50 centimeters] of rain per year, but are so hot that moisture evaporates rapidly). Most true deserts receive less than 4 inches (10 centimeters).
Except for those at the North and South Poles, which are special cases, deserts are generally classified as hot or cold. Daytime average temperatures in hot deserts are warm during all seasons of the year, usually above 65°F (18°C). Nighttime temperatures are chilly and sometimes go below freezing. Typical hot deserts include the Sahara and the Namib Desert of Namibia. Cold deserts have hot summers and cold winters. For at least one month during the year, the mean temperature is below 45°F (7°C). Cold deserts include the Turkestan in Kazakhstan and Uzbekistan, the Gobi (GOH-bee) in China and Mongolia, and the Great Salt Lake Desert in Utah. These deserts usually get some precipitation in the form of snow.
Deserts can be further characterized by their appearance and plant life. They may be flat, mountainous, broken by gorges and ravines, or covered by a sea of sand. Plants may range from nearly invisible fungi to towering cacti and trees.
Source: "Desert." U·X·L Encyclopedia of Biomes, Vol. 1. U·X·L, Detroit: 2000.