Desert

In geography, a desert is a landscape form or region that receives little precipitation, less than 250 mm per year.. Deserts have a reputation for supporting very little life. Compared to wetter regions this may be true, although upon closer examination, deserts often harbor a wealth of life that usually remains hidden (especially during the daylight) to preserve moisture. Approximately one-third of Earth's land surface is desert. (See a map of the world's non-polar deserts, http://pubs.usgs.gov/gip/deserts/what/world.html)

Types of desert

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Most classifications rely on some combination of the number of days of rainfall, the total amount of annual rainfall, temperature, humidity, or other factors. In 1953, Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received. In this now widely accepted system, extremely arid lands have at least 12 consecutive months without rainfall, arid lands have less than 250 millimeters of annual rainfall, and semiarid lands have a mean annual precipitation of between 250 and 500 millimeters. Arid and extremely arid land are deserts, and semiarid grasslands generally are referred to as steppes.

Related Topics:
Rain - Humidity - Peveril Meigs - Steppes

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However, lack of rainfall alone can't provide an accurate description of what a desert is. For example, Phoenix, Arizona receives less than 250 millimeters, (10 inches), of precipitation per year, and is immediately recognized as being located in the desert. The North Slope of Alaska's Brooks Range also receives less than 250 millimeters of precipitation per year, but is not generally recognized as a desert region.

Related Topics:
Phoenix, Arizona - North Slope - Brooks Range

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The difference lies in something termed "potential evapotranspiration."

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The water budget of an area can be calcuated using the formula P-PE+/-S, whereby P is precipitation, PE is potential evapotranspiration rates and S is amount of surface storage of water.

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Evapotranspiration is the combination of water loss through atmospheric evaporation, coupled with the evaporative loss of water through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. Tucson, Arizona receives about 300 millimeters, (12 inches), of rain per year, however about 2500 millimeters, (100 inches), of water could evaporate over the course of a year. In other words, about 8 times more water could evaporate from the region than actually falls. Rates of evapotranspiration in other regions such as Alaska are much lower, so while these regions receive minimal precipitation, they should be designated as specifically different from the simple definition of a desert: a place where evaporation exceeds precipitation.

Related Topics:
Evaporation - Tucson, Arizona

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That said, there are different forms of deserts. Cold deserts can be covered in snow; such locations don't receive much precipitation, and what does fall remains frozen as snow pack; these are more commonly referred to as tundra if a short season of above-freezing temperatures is experienced, or as an ice cap if the temperature remains below freezing year-round, rendering the land almost completely lifeless.

Related Topics:
Snow - Tundra - Ice cap

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Most non-polar deserts are hot because they have little water. Water tends to have a cooling, or at least a moderating, effect in environments where it is plentiful. In some parts of the world deserts are created by a rain shadow effect in which air masses lose much of their moisture as they move over a mountain range; other areas are arid by virtue of being very far from the nearest available sources of moisture (this is true in some middle-latitude landmass interior locations, particularly in Asia).

Related Topics:
Water - Rain shadow - Mountain range - Asia

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Deserts are also classified by their geographical location and dominant weather pattern as trade wind, midlatitude, rain shadow, coastal, monsoon, or polar deserts. Former desert areas presently in nonarid environments are paleodeserts, and extraterrestrial deserts exist on other planets.

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Trade wind deserts

The trade winds in two belts on the equatorial sides of the horse latitudes heat up as they move toward the Equator. These dry winds dissipate cloud cover, allowing more sunlight to heat the land. Most of the major deserts of the world lie in areas crossed by the trade winds. The world's second largest desert (second to Antarctica), the Sahara of North Africa, which has experienced temperatures as high as 57°C, is a trade wind desert.

Related Topics:
Trade wind - Horse latitudes - Antarctica - Sahara - North Africa

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Midlatitude deserts

Midlatitude deserts occur between 30° and 50° N. and S., poleward of the subtropical highpressure zones. These deserts are in interior watersheds far from oceans and have a wide range of annual temperatures. The Sonoran Desert of southwestern North America is a typical midlatitude desert. The Tengger Desert of China is another example.

Related Topics:
Watershed - Sonoran Desert - North America - Tengger Desert - China

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Rain Shadow Deserts

Rain shadow deserts are formed because tall mountain ranges prevent moisture-rich clouds from reaching areas on the lee, or protected side, of the range. As air rises over the mountain, water is precipitated and the air loses its moisture content. A desert is formed in the leeside "shadow" of the range. Examples are the Judean Desert in Israel and Palestine, the Great Basin of the western United States, and the Antelope Valley portion of the Mojave Desert, also called the High Desert, in southern California.

Related Topics:
Judean Desert - Israel - Palestine - Great Basin - Antelope Valley - Mojave Desert - High Desert - California

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Coastal deserts

Coastal deserts generally are found on the western edges of continents near the Tropics of Cancer and Capricorn. They are affected by cold ocean currents that parallel the coast. Because local wind systems dominate the trade winds, these deserts are less stable than other deserts. Winter fogs, produced by upwelling cold currents, frequently blanket coastal deserts and block solar radiation. Coastal deserts are relatively complex because they are at the juncture of terrestrial, oceanic, and atmospheric systems. A coastal desert, the Atacama Desert of South America, is Earth's driest desert. In the Atacama, measurable rainfall—1 millimeter or more of rain—may occur as infrequently as once every 5–20 years. Other coastal deserts include the Sechura desert of South America, the Baja California desert of North America, and the Atlantic coastal desert and Namib desert of Africa.

Related Topics:
Tropics - Ocean current - Fog - Atacama Desert - South America - Sechura desert - Baja California desert - North America - Atlantic coastal desert - Namib desert - Africa

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Crescent-shaped dunes are common in coastal deserts such as the Namib Desert, with prevailing onshore winds.

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Monsoon deserts

"Monsoon," derived from an Arabic word for "season," refers to a wind system with pronounced seasonal reversal. Monsoons develop in response to temperature variations between continents and oceans. The southeast trade winds of the Indian Ocean, for example, provide heavy summer rains in India as they move onshore. As the monsoon crosses India, it loses moisture on the eastern slopes of the Aravalli Range. The Thar Desert of India and the Cholistan Desert of Pakistan are parts of a monsoon desert region west of the range.

Related Topics:
Monsoon - Indian Ocean - India - Aravalli Range - Thar Desert - Cholistan Desert - Pakistan

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Montane deserts

Montane deserts are arid places with a very high altitude; the most prominent example is found north of the Himalaya, in parts of the Kunlun Mountains and the Tibetan Plateau. Many locations within this category have elevations exceeding 3,000 meters (9,843 feet) and the thermal regime can be hemiboreal or even boreal. These places owe their profound aridity (the average annual precipitation is often less than 40mm) to being very far from the nearest available sources of moisture.

Related Topics:
Himalaya - Kunlun Mountains - Tibetan Plateau - Hemiboreal - Boreal

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Polar deserts

Polar deserts are areas with annual precipitation less than 250 millimeters and a mean temperature during the warmest month of less than 10° C. Polar deserts on Earth cover nearly 5 million square kilometers and are mostly bedrock or gravel plains. Sand dunes are not prominent features in these deserts, but snow dunes occur commonly in areas where precipitation is locally more abundant. Temperature changes in polar deserts frequently cross the freezing point of water. This "freeze-thaw" alternation forms patterned textures on the ground, as much as 5 meters in diameter.

Related Topics:
Bedrock - Gravel

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The Dry Valleys of Antarctica have been ice-free for thousands of years.

Related Topics:
Dry Valley - Antarctica

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Extraterrestrial deserts

Mars is the only other planet on which we have identified wind-shaped (eolian) features. Although its surface atmospheric pressure is only about one-hundredth that of Earth, global circulation patterns on Mars have formed a circumpolar sand sea of more than five million square kilometers, an area greater than the Empty Quarter of Saudi Arabia, the largest sand sea on our planet. Martian sand seas consist predominantly of crescent-shaped dunes on plains near the perennial ice cap of the north polar area. Smaller dune fields occupy the floors of many large craters in the polar regions.

Related Topics:
Mars - Empty Quarter - Saudi Arabia

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Defining desert by lack of precipitation alone, rather than also requiring eolian features, classifies essentially all known extraterrestrial bodies as such. The only known body where there is considered to be a strong possibility of precipitation is Titan, the moon of Saturn; it does not have liquid water, however, instead potentially possessing lakes of liquid methane and other hydrocarbons.

Related Topics:
Titan - Moon - Saturn - Methane - Hydrocarbon

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