The desert climate or arid climate, is a climate in 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.
There are two variations of a desert climate: a hot desert climate, and a cold desert climate, To delineate 'hot desert climates' from 'cold desert climates', there are three widely used isotherms: most commonly a mean annual temperature of 18 °C, or sometimes a mean temperature of 0 or −3 °C 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', and a location with the appropriate temperature below the given isotherm is classified as 'cold arid',
Most desert/arid climates receive between 25 and 200 mm of rainfall annually, although some of the most consistently hot areas of Central Australia, 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.