Deserts Are An environmental extreme, characterized by low rainfall that is highly variable intra-annu-ally and interannually, with a coverage of about 27.7 Mkm.2, equivalent to 35 percent of the earth's surface and, thus, representing one of the largest terrestrial biomes. They occur mainly within latitudes between 5-35 degrees north of the equator and are characterized by very high aridity, very little vegetation cover and large surfaces of exposed bare soil, and high adaptations of plants and animals for survival during long droughts. According to bio-ecological definitions, the world's deserts represent all ecoregions of the world that harbor desert vegetation, identified by the xerophilous life-forms and the general desert-adapted physiognomy of the dominant plants.

Desert climate is characterized by precipitation of less than 9.84 in. (250 mm.) with very high variability, high diurnal variations of temperature, and strong solar radiation. Desert air is very dry; incoming solar and outgoing terrestrial radiation are intense, with large daily temperature fluctuations; and potential evaporation is high. Extreme desert systems already experience wide fluctuations in rainfall and are adapted to coping with sequences of extreme conditions.

Deserts can be hot or cold. Among the hot deserts are ones with two rain seasons (Sonora desert, Karoo); with one rainy season (Northern Sahara, Mohave Desert, Middle-Asian deserts); deserts with summer rain (Southern Sahara, inner Namib, Atacama); deserts with few rains at any season (Central Australia); coastal deserts without rains, but with fog (North Chilean coastal deserts, outer Namib); and deserts without any rain or vegetation (Central Sahara). The Sahara in north Africa and the Namib desert in southwest Africa are classified as the hottest deserts in the world, with average monthly temperatures above 86 degrees F (30 degrees C) during the warmest months and extremes above 122 degrees F (50 degrees C). The diurnal temperature range often is large; winter nights in the Namib Desert sometimes are as cold as minus 14 degrees F (10 degrees C) or lower.

Aridity is the most prominent indicator, commonly measured by the Aridity Index, an estimator for the ratio between mean annual precipitation and mean annual potential evapotranspiration, which is less than -40 for arid deserts and -20 for semi-deserts. Aridity is highest in the Saharan and Chilean-Peruvian deserts, followed by the Arabian, East African, Gobi, Australian, and South African deserts, and lower in the Thar and North American deserts. This high aridity, as well as typical pulse-type variations in desert environments, are caused by global atmospheric and oceanic phenomena, such as the position of the jet streams, the movement of polar-front boundaries, the intensity of the summer monsoon, El NiƱo Southern Oscillation events, and even longer-term ocean cycles, such as the Pacific Decadal Oscillation.

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