Thedesert climate orarid climate (in theKöppen climate classificationBWh andBWk) is adry climate sub-type in which there is a severe excess ofevaporation overprecipitation. 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 thePolar climate.[1]
There are two variations of adesert climate according to theKöppen 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 18 °C (64.4 °F) is used as an isotherm so that a location with aBW type climate with the appropriate temperature above this isotherm is classified as "hot arid subtype" (BWh), and a location with the appropriate temperature below the isotherm is classified as "cold arid subtype" (BWk).
Most desert/arid climates receive between 25 and 200 mm (1 and 8 in) of rainfall annually,[2][3] although some of the most consistently hot areas ofCentral Australia, theSahel andGuajira Peninsula can be, due to extremepotential evapotranspiration, classed as arid with the annual rainfall as high as 430 millimetres or 17 inches.
Although no part of Earth is known for certain to be rainless, in theAtacama Desert of northern Chile, the average annual rainfall over 17 years was only 5 millimetres (0.20 in). Some locations in theSahara Desert such asKufra,Libya, record an even drier 0.86 mm (0.034 in) of rainfall annually. The official weather station inDeath Valley, United States reports 60 mm (2.4 in) annually, but in 40 months between 1931 and 1934 a total of just 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 summer half of the year (April through September in theNorthern 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 aBW (desert climate), while 50–100% of the threshold results in asemi-arid climate.[1]
Hot desert climates (BWh) are typically found under thesubtropical ridge in the lower middle latitudes or thesubtropics, often between 20° and 33° north and south latitudes. In these locations, stable descending air and high pressure aloft clear clouds and create hot, arid conditions with intense sunshine. Hot desert climates are found across vast areas ofNorth Africa,West Asia, northwestern parts of theIndian subcontinent, southwestern Africa, interior Australia, theSouthwestern United States, northernMexico, sections of southeasternSpain, the coast ofPeru andChile and parts of the Braziliansertão. This makes hot deserts present in every continent except Antarctica.
At the time of high sun (summer), scorching, desiccating heat prevails. Hot-month average temperatures are normally between 29 and 35 °C (84 and 95 °F), and midday readings of 43–46 °C (109–115 °F) are common. The world's absolute heat records, over 50 °C (122 °F), are generally in the hot deserts, where the heat potential can be the highest on the planet. This includes the record of 56.7 °C (134.1 °F) inDeath Valley, which is currently considered thehighest temperature recorded on Earth.[4] Some deserts in thetropics 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 30 °C (86 °F), up to nearly 35 °C (95 °F) inDallol, Ethiopia. This last feature is seen in sections of Africa andArabia. 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 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 (BWk) usually feature hot (or warm in a few instances), dry summers, though summers are not typically as hot as hot desert climates. Unlike hot desert climates, cold desert climates tend to feature cold, dry winters. Snow tends to be rare in regions with this climate. TheGobi Desert in northern China and Mongolia is one example of a cold desert. Though hot in the summer, it shares the freezing winters of the rest ofInner Asia. Summers in South America'sAtacama Desert are mild, with only slight temperature variations between seasons. Cold desert climates are typically found at higher altitudes than hot desert climates and are usually drier than hot desert climates.
The Atacama Desert in ChileThe Gobi Desert in Mongolia
Cold desert climates are typically located in temperate zones in the 30s and 40s latitudes, usually in the leewardrain shadow of high mountains, restricting precipitation from the westerly winds. An example of this is thePatagonian Desert in Argentina, bounded by theAndes ranges to its west. In the case of Central Asia, mountains restrict precipitation from theeastern monsoon. TheKyzyl Kum,Taklamakan andKatpana Desert deserts ofCentral Asia are other significant examples ofBWk climates. TheLadakh region and the city ofLeh in theGreat Himalayas in India also have a cold desert climate. In North America, the cold desert climate occurs in the drier parts of theGreat Basin Desert, theBighorn Basin inWyoming, and theColumbia Plateau in easternWashington and easternOregon. TheHautes Plaines, located in the northeastern section ofMorocco and inAlgeria, is another prominent example of a cold desert climate. InEurope, this climate only occurs in some inland parts of southeastern Spain, such as inLorca.[7][6]
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 theMcMurdo Dry Valleys of Antarctica. These areas are generally classified as havingpolar climates because they have average summer temperatures below 10 °C (50 °F) even if they have some characteristics of extreme non-polar deserts.[8]
^"Karachi Climate Normals 1991–2020".World Meteorological Organization Climatological Standard Normals (1991–2020). National Oceanic and Atmospheric Administration.Archived from the original on 10 September 2024. Retrieved17 September 2023.
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