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[[File:BW climate.png|thumb|right|upright=1.6|Regions with desert climates
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The desert climate or arid climate (in the Köppen climate classification BWh and BWk) is a dry climate sub-type in which there is a severe excess of evaporation over precipitation. The typically bald, rocky, or sandy surfaces in desert climates are dry and hold little moisture, quickly evaporating the already 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 the Polar climate.
There are two variations of a desert climate according to the Köppen climate classification: a hot desert climate ( BWh), and a cold desert climate ( BWk). To delineate "hot desert climates" from "cold desert climates", a mean annual temperature of is used as an isotherm so that a location with a BW type climate with the appropriate temperature above this isotherm is classified as "hot arid subtype" ( BW h ), and a location with the appropriate temperature below the isotherm is classified as "cold arid subtype" (BWk).
Most desert/arid climates receive between 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 the annual rainfall as high as .
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 summer 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 there. If the area's annual precipitation is less than half the threshold (50%), it is classified as a BW (desert climate), while 50–100% of the threshold results in a semi-arid climate.
At the time of high sun (summer), scorching, desiccating heat prevails. Hot-month average temperatures are normally between , and midday readings of are common. The world's absolute heat records, over , are generally in the hot deserts, where the heat potential can be the highest on the planet. This includes the record of in Death Valley, which is currently considered the highest temperature recorded on Earth. Some deserts in the tropics consistently experience very high temperatures all year long, even during wintertime. These locations feature some of the highest annual average temperatures recorded on Earth, exceeding , up to nearly in Dallol, Ethiopia. This last feature is seen in sections of Africa and Arabia. During colder periods of the year, night-time temperatures can drop to freezing or below due to the exceptional radiation loss under the clear skies. However, temperatures rarely drop far below freezing under the hot subtype. Hot desert climates can be found in the deserts of North Africa such as the wide Sahara Desert, the Libyan Desert or the Nubian Desert; deserts of the Horn of Africa such as the Danakil Desert or the Grand Bara; deserts of Southern Africa such as the Namib Desert or the Kalahari Desert; deserts of West Asia such as the Arabian Desert, or the Syrian Desert; deserts of South Asia such as Dasht-e Lut and Dasht-e Kavir of Iran or the Thar Desert of India and Pakistan; deserts of the United States and Mexico such as the Mojave Desert, the Sonoran Desert or the Chihuahuan Desert; deserts of Australia such as the Simpson Desert or the Great Victoria Desert and many other regions. In Europe, the hot desert climate can only be found on southeastern coast of Spain as well as small inland parts of southeastern, especially parts of the Tabernas Desert.
Hot deserts are lands of extremes: most of them are among the hottest, the driest, and the sunniest places on Earth because of nearly constant high pressure; the almost permanent removal of low-pressure systems, dynamic fronts, and atmospheric disturbances; sinking air motion; dry atmosphere near the surface and aloft; the exacerbated exposure to the sun where solar angles are always high makes this desert inhospitable to most species.
Cold desert climates are typically located in temperate zones in the 30s and 40s latitudes, usually in the leeward rain shadow of high mountains, restricting precipitation from the westerly winds. An example of this is the Patagonian Desert in Argentina, bounded by the Andes to its west. In the case of Central Asia, mountains restrict precipitation from the eastern monsoon. The Kyzyl Kum, Taklamakan and Katpana Desert deserts of Central Asia are other significant examples of BWk climates. The Ladakh region and the city of Leh in the Great Himalayas in India also have a cold desert climate. In North America, the cold desert climate occurs in the drier parts of the Great Basin Desert and the Bighorn Basin in Big Horn and Washakie County in Wyoming. The Hautes Plaines, located in the northeastern section of Morocco and in Algeria, is another prominent example of a cold desert climate. In Europe, this climate only occurs in some inland parts of southeastern Spain, such as in Lorca.
Polar climate desert areas in the Arctic and Antarctic regions receive very little precipitation during the year owing to the cold, dry air freezing most precipitation. Polar desert climates have desert-like features that occur in cold desert climates, including intermittent streams, hypersaline lakes, and extremely barren terrain in unglaciated areas such as the McMurdo Dry Valleys of Antarctica. These areas are generally classified as having because they have average summer temperatures below even if they have some characteristics of extreme non-polar deserts.
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