Deserts are characterized by very low rainfall; indeed, so-called true deserts receive less than 100 mm of rainfall per year. Because of the scarcity of water, vegetation is not abundant, and there is reduced chemical weathering of rock and thin soils.
Most of the world's deserts are located 30 degrees north or 30 degrees south of the equator. Air rising from the equator cools and releases moisture over the tropical latitudes. Some of the dried air moves north and some south, and it then drops downward about 30 degrees to either side of the equator. Moisture there evaporates from the surface, creating desert conditions. Most of the annual rainfall in these deserts occurs during the summer.
Hot deserts have richer and more diverse vegetation than the drier cold deserts. Cacti, for example, store large quantities of water in their expandable stems, and photosynthesis occurs primarily within stems rather than leaves.
Cold deserts are located in middle to high latitudes, 30 to 50 degrees, and they are usually in the middle of continents, where seasonal temperature changes are great. There is a relatively low diversity of plants and animals; rain usually falls in the winter, and plant growth is concentrated in the spring.
Some deserts lie in the wind shadow behind mountains. Moisture precipitates from air rising and cooling above a mountain; once over
the mountain, the now dry air descends, drying out the surface as it flows across the land.
Most of the processes that take place in the desert—weathering and erosion, for example—are the same as those elsewhere, but they are altered somewhat because of the lack of water. Today about one-third of the earth's land is arid or semiarid, with half of it so dry that it is uninhabitable. Characteristic of the desert floor is what is called desert pavement, which is developed as wind—a significant feature of desert areas—moves over the surface, picking up dust- to sand-size particles. Left behind is a lag deposit of coarser material, the desert pavement. The constant wind and supply of sand builds up dunes. Desert varnish, a dark coating on rocks large and small, forms from the slow precipitation of manganese and iron compounds, as well as minute clay particles. Large areas of dunes are called sand seas, or ergs; they are found in large deserts, usually taking up no more than 10 percent of the area.
Wind also builds deposits of loess, consisting of finely disintegrated rock debris that is very common, for example, in the north of China, where it is carried by the prevailing winds from the Gobi Desert in Mongolia. In northern China the loess covers extensive areas and reaches a thickness of more than 60 m. Easily eroded and carried away by streams, it is the reason that the Huang Ho (Yellow River) got its name. Although the weak loess forms vertical cliffs, it is easily excavated and provides a great number of people with shelter. Wind carries this material to cities such as Beijing, where the air is smoglike, and to adjacent countries such as Japan and Korea, and then out to sea. The wind-blown dust usually accumulates in semiarid regions along the edge of deserts.
Deserts expand and contract with climatic fluctuations. Their margins, where they are in transition to wetter ecosystems, are heavily exploited by humans. Grazing and trampling by cattle, the collection of firewood, over-cultivation, and salinization caused by irrigation stress the environment, causing the deserts to expand, sometimes at a rapid pace. This process is called desertification. Today about 20 percent of the terrestrial surface is arid, and about an additional 15 percent is threatened.
See also: Communities; Ecology; Hydrologic Cycle; Topsoil Formation
Cooke, Ronald U., Andrew Warren, and Andrew Goudie. 1993. Desert Geomorphology. England: UCL Press; Gleick, Peter. 2001. "Making Every Drop Count." Scientific American 284 (2): 40-45; Griffin, Dale W., et al. 2002. "The Global Transport of Dust." American Scientist 90: 228-235; Laing, David. 1991. The Earth System: An Introduction to Earth Science. Dubuque, IA: Wm. C. Brown; Montgomery, Carla. 1996. Fundamentals of Geology, 3rd ed. New York: McGraw Hill Professional Publishing.
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