A desert is a landscape form or region that receives very little precipitation
Deserts are defined as areas that receive an average annual precipitation of less than 250
mm (10 in). In the Köppen climate classification system, deserts are classed as (BW).
A satellite image of the Sahara, the world's largest hot desert and second largest desert after Antarctica.
Deserts take up one-third of the Earth's land surface. They usually have a large diurnal and seasonal temperature range, with high daytime temperatures (in summer up to 45 °C or 113 °F), and low night-time temperatures (in winter down to 0 °C; 32 °F) due to extremely low humidity. Water acts to trap infrared radiation from $$$$ the sun and the ground, and dry desert air is incapable of blocking sunlight during the day or trapping heat during the night. Thus during daylight most of the sun's heat reaches the ground. As soon as the sun sets the desert cools quickly by radiating its heat into space. Urban areas in deserts lack large (more than 25 °F/14 °C) daily temperature ranges, partially due to the urban heat island effect.
Many deserts are formed by rain shadows, mountains blocking the path of precipitation to the desert. Deserts are often composed of sand and rocky surfaces. Sand dunes called ergs and stony surfaces called hamada surfaces compose a minority of desert surfaces. Exposures of rocky terrain are typical, and reflect minimal soil development and sparseness of vegetation.
The snow surface at Dome C Station in Antarctica is a representative of the majority of the continent's surface.
Bottomlands may be salt-covered flats. Eolian processes are major factors in shaping desert landscapes. Cold deserts (also known as polar deserts) have similar features but the main form of precipitation is snow rather than rain. Antarctica is the world's largest cold desert (composed of about 98 percent thick continental ice sheet and 2 percent barren rock). The largest hot desert is the Sahara.
Deserts sometimes contain valuable mineral deposits that were formed in the arid environment or that were exposed by erosion. Because deserts are so dry, they are ideal places for artifacts and fossils to be preserved.
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