A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than 400 millimetres (16 in).[1] A common definition distinguishes between true deserts, which receive less than 250 millimetres (10 in) of average annual precipitation, and semideserts or steppes, which receive between 250 millimetres (10 in) and 400 to 500 millimetres (16 to 20 in).[1][2]
Deserts can also be described as areas where more water is lost by evapotranspiration than falls as precipitation.[1] 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.[3][4]
Deserts are part of a wide classification of regions that, on an average annual basis, have a moisture deficit (i.e. they lose more moisture than they receive).[2] 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.[5] Other regions of the world have cold deserts, including areas of the Himalayas[6] and other high altitude areas in other parts of the world.[7] Polar deserts cover much of the ice free areas of the arctic and Antarctic.[8][9] An alternative definition describes deserts as parts of earth that don't have a sufficient vegetation cover to support human population.[10]