The world's hot and cold desert regions.
Terkezi Oasis in the Sahara Desert.
A desert is:
1. An ecosystem with less than 250 mm (10 in) of precipitation per year.
2. A wide, open, comparatively barren tract of land with few forms of life and little rainfall. See also desertification.
Deserts cover about one third of Earth's land area. There are two types.
These typically lie between latitudes 20° and 30° north and south, though they also exist farther from the equator in the centers of continental landmasses. They can be described as areas where water precipitation from the atmosphere is greatly exceeded by surface evaporation and plant transpiration.
The worlds's major deserts, including the Australian, Kalahari, and Sahara, are found in the horse latitudes, where the permanent high pressure causes drought throughout the year. Deserts occur in various west coast areas where the influence of cool currents offshore makes the land even drier, with less than 100 mm (4 in) rainfall in places; for example the Atacama Desert in Peru (with the Humboldt Current offshore) and the Namib Desert in South Africa (with the Benguela Current offshore). There are deserts in the middle of the largest continents where no onshore winds can reach to bring any rainfall; for example, the Gobi Desert in Asia, and smaller examples in Nevada and New Mexico.
Groundwater exists but is normally far below the surface; here and there it is accessible as springs or wells. Irrigation has enabled reclamation of some desert land. Landscapes generally result from the surface's extreme vulnerability to erosion. Features include arroyos, buttes, dunes, mesas, and wadis. Human influence may assist peripheral areas to become susceptible to erosion; and thus temporarily advance the desert's boundaries.
Plants may survive by being able to store water, like the cactus; by having tiny leaves to reduce evaporation loss, like the paloverde; or by having extensive root systems to capture maximum moisture, like the mesquite. Animals may be nomadic, or spend the daylight hours underground. Best adapted of all is the camel.
In cold deserts, water is unavailable during most of the year as it is trapped in the form of ice. Cold deserts include the Antarctic polar icecaps, the barren wastes of Greenland, and much of the tundra. (See also glacier.) Eskimos, Lapps, and Samoyeds are among the ethnic groups inhabiting such areas in the northern hemisphere. Their animal neighbors include seals and the polar bear.