The desert climate or arid climate (in the Köppen climate classification BWh and BWk), is a climate which there is an excess of evaporation over precipitation. The typically bald, rocky, or sandy surfaces in desert climates hold little moisture and evaporate the little rainfall they receive. Covering 14.2% of earth's land area, hot deserts are the second most common type of climate on earth after polar climate.
Although no part of Earth is known for certain to be absolutely rainless, in the Atacama Desert in northern Chile, the average annual rainfall over a period of 17 years was only 5 millimetres (0.20 in). Some locations in the Sahara Desert such as Kufra, Libya record only 0.86 mm (0.034 in) of rainfall annually. The official weather station in Death Valley, United States reports 60 mm (2.4 in) annually, but in a 40-month period between 1931 and 1934 a total of 16 mm (0.63 in) of rainfall was measured.
To determine whether a location has an arid climate, the precipitation threshold is determined. The precipitation threshold (in millimetres) involves first multiplying the average annual temperature in °C by 20, then adding 280 if 70% or more of the total precipitation is in the high-sun half of the year (April through September in the Northern Hemisphere, or October through March in the Southern), or 140 if 30–70% of the total precipitation is received during the applicable period, or 0 if less than 30% of the total precipitation is so received. If the area's annual precipitation is less than half the threshold, it is classified as a BW (desert climate).
There are two variations of a desert climate: a hot desert climate (BWh), and a cold desert climate (BWk). To delineate "hot desert climates" from "cold desert climates", there are three widely used isotherms: most commonly a mean annual temperature of 18 °C (64.4 °F), or sometimes a mean temperature of 0 or −3 °C (32.0 or 26.6 °F) in the coldest month, so that a location with a BW type climate with the appropriate temperature above whichever isotherm is being used is classified as "hot arid" (BWh), and a location with the appropriate temperature below the given isotherm is classified as "cold arid" (BWk).
Most desert/arid climates receive between 25 and 200 mm (1 and 8 in) of rainfall annually, although some of the most consistently hot areas of the Sahel and Guajira Peninsula can be due to extreme potential evapotranspiration classed as arid with annual rainfall as high as 430 millimetres or 17 inches.