An arid and empty region of the Earth, characterized by little or no vegetation, and meagre and intermittent rainfall, high evaporation rates, and low humidity and cloud cover. Low-latitude deserts such as the Sahara are hot and dry, caused by high pressure air masses which prevent precipitation. Mid-latitude deserts such as the Gobi are cold and dry, and are related to mountain barriers which seal off moist maritime winds. Polar deserts of the Arctic and Antarctic are permanently covered by snow and ice. Approximately one third of the Earth's land surface is desert.
For the food course, usually served after dinner, see dessert. For the act of abandoning or withdrawing support from an entity, see desertion.In geography, a desert is a landscape form or region that receives very little precipitation. Generally deserts are defined as areas that receive an average annual precipitation of less than 250 mm (10 inches). The terminology used to define deserts is complex. Deserts are however part of a wider classification of regions that, on an average annual basis, have a moisture deficit (i.e. Because desert is a vague term, the use of 'dryland', and its subdivisions of hyper arid, arid, semiarid and dry-subhumid, is to be preferred, and is approved by the United Nations.
Deserts cover at least one-fifth of the Earth's land surface. Deserts are very arid (dry) and can have high temperatures in excess of 50°C. Even though the desert is very hot in the day, it is extremely cold at night. Deserts have quite rough terrain, many sand dunes and present an overwhelmingly hostile environment. Humans that travel unprepared into deserts have a slim chance of survival due to the relative dearth of water.
Deserts have a reputation for supporting very little life, but in reality deserts often have high biodiversity, including animals that remain hidden during daylight hours to control body temperature or to limit moisture needs.
Desert landscapes have certain distinctive features. Deserts are often composed of sand and rocky surfaces. Sand dunes called ergs and stony surfaces called Reg or hamada surfaces compose a minority of desert surfaces. Eolian (wind-driven) processes are major factors in shaping desert landscapes. Cold deserts (also known as polar deserts) have similar features but the main form of precipation is snow rather than rain. The largest cold desert is Antarctica (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 dry, they are ideal places for human artifacts and fossils to be preserved.
Deserts usually have an extreme temperature range. Most deserts have a low temperature at night. This is because the air is very dry (contains little moisture) and therefore holds little heat so as soon as the sun sets, the desert cools quickly.
In the Köppen climate classification system, deserts are classed as (BW).
Etymology
The English, French (désert), Italian (deserto), all come from the Latin deserta.
Types of desert
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.
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 a 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. 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.
That said, there are different forms of deserts. Cold deserts can be covered in snow;
Most non-polar deserts are hot because they have little water. 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;
Deserts are also classified by their geographical location and dominant weather pattern as trade wind, mid-latitude, rain shadow, coastal, monsoon, or polar deserts. Former desert areas presently in non-arid environments are paleodeserts.
Montane deserts
Montane deserts are arid places with a very high altitude; Deserts are normally cold.
Rain Shadow Deserts
Rain shadow deserts form when tall mountain ranges block clouds from reaching areas in the direction the wind is going. Moisture almost never reaches the downwind side of the mountain, therefore causing a desert. The air then warms and expands and blows across the desert. The warm air takes all the small amounts of moisture in the desert away.
Desert features
Sand covers only about 20 percent of Earth's deserts. In general, there are 6 forms of deserts:
Mountain and basin deserts; Hamada deserts, which comprise of plateaux landforms;Nearly all desert surfaces are plains where eolian deflation—removal of fine-grained material by the wind—has exposed loose gravels consisting predominantly of pebbles but with occasional cobbles.
The remaining surfaces of arid lands are composed of exposed bedrock outcrops, desert soils, and fluvial deposits including alluvial fans, playas, desert lakes, and oases.
There are several different types of dunes.
Oases are vegetated areas moistened by springs, wells, or by irrigation. Oases are often the only places in deserts that support crops and permanent habitation.
Vegetation
Most desert plants are drought- or salt-tolerant, such as xerophytes. Other desert plants have long tap roots that penetrate to the water table if present.
Deserts typically have a plant cover that is sparse but enormously diverse. The Sonoran Desert of the American Southwest has the most complex desert vegetation on Earth. The giant saguaro cacti provide nests for desert birds and serve as "trees" of the desert. They dot the Sonoran and reinforce the general impression of deserts as cactus-rich land.
Although cacti are often thought of as characteristic desert plants, other types of plants have adapted well to the arid environment. Cold deserts have grasses and shrubs as dominant vegetation.
Water
Rain does fall occasionally in deserts, and desert storms are often violent.
Though little rain falls in deserts, deserts receive runoff from ephemeral, or short-lived, streams fed considerable quantities of sediment for a day or two. Although most deserts are in basins with closed, or interior drainage, a few deserts are crossed by 'exotic' rivers that derive their water from outside the desert. Such rivers infiltrate soils and evaporate large amounts of water on their journeys through the deserts, but their volumes are such that they maintain their continuity. The Nile River, the Colorado River, and the Yellow River are exotic rivers that flow through deserts to deliver their sediments to the sea. Deserts may also have underground springs, rivers, or reservoirs that lay close to the surface, or deep underground. Plants that have not completely adapted to sporadic rainfalls in a desert environment may tap into underground water sources that do not exceed the reach of their root systems.
Lakes form where rainfall or meltwater in interior drainage basins is sufficient. Desert lakes are generally shallow, temporary, and salty. There are more than a hundred playas in North American deserts.
When the occasional precipitation does occur, it erodes the desert rocks quickly and powerfully. Winds are the other factor that erodes deserts - they are constant yet slow.
The flat terrains of hardpans and playas make them excellent race tracks and natural runways for airplanes and spacecraft.
Mineral resources
Some mineral deposits are formed, improved, or preserved by geologic processes that occur in arid lands as a consequence of climate.
Evaporation in arid lands enriches mineral accumulation in their lakes.
Significant evaporite resources occur in the Great Basin Desert of the United States, mineral deposits made forever famous by the "20-mule teams" that once hauled borax-laden wagons from Death Valley to the railroad. Borates are mined from evaporite deposits at Searles Lake, California, and other desert locations.
The Atacama Desert of South America is unique among the deserts of the world in its great abundance of saline minerals.
Valuable minerals located in arid lands include copper in the United States, Chile, Peru, and Iran;
The Green River Formation of Colorado, Wyoming, and Utah contains alluvial fan deposits and playa evaporites created in a huge lake whose level fluctuated for millions of years.
Some of the more productive petroleum areas on Earth are found in arid and semiarid regions of Africa and the Mideast, although the oil fields were originally formed in shallow marine environments.
Other oil reservoirs, however, are presumed to be eolian in origin and are presently found in humid environments.
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