A desert is a landscape or region of land that is very dry because of low rainfall amounts (precipitation), often has little coverage by plants, and in which streams dry up unless they are supplied by water from outside areas. Deserts can also be described as areas where more water is lost by evapotranspiration than falls as precipitation. Desert plants must have special adaptations to survive with this little water. Deserts generally receive less than 25 cm of rain (precipitation) each year. Semideserts or steppes are regions which receive between 250 millimetres (10 in) and 400 to 500 millimetres (16 to 20 in) . In the Köppen climate classification system, deserts are classed as BWh (hot desert) or BWk (temperate desert). In the Thornthwaite climate classification system, deserts would be classified as arid megathermal climates.
Deserts are part of a wide classification of regions that, on an average annual basis, have a moisture deficit (they lose more moisture than they receive). Measurement of rainfall alone cannot provide an accurate definition of what a desert is because being arid also depends on evaporation, which depends in part on temperature. For example, Phoenix, Arizona receives less than 250 millimeters (10 in) of precipitation per year, and is immediately recognized as being located in a desert due to its arid adapted plants. The North Slope of Alaska's Brooks Range also receives less than 250 millimeters (10 in) of precipitation per year and is often classified as a cold desert. Other regions of the world have cold deserts, including areas of the Himalayas and other high altitude areas in other parts of the world. Polar deserts cover much of the ice free areas of the arctic and Antarctic. An alternative definition describes deserts as parts of earth that don't have a sufficient vegetation cover to support human population .
Potential evapotranspiration supplements the measurement of rainfall in providing a scientific measurement-based definition of a desert. The water budget of an area can be calculated using the formula P − PE ± S, wherein P is precipitation, PE is potential evapotranspiration rates and S is amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric evaporation and through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. As an example, Tucson, Arizona receives about 300 millimeters (12 in) of rain per year, however about 2500 millimeters (100 in) of water could evaporate over the course of a year. In other words, about 8 times more water could evaporate from the region than actually falls. Rates of evapotranspiration in cold regions such as Alaska are much lower because of the lack of heat to aid in the evaporation process.