2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather; Geology and geophysics
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. 'True deserts' where vegetation cover is exceedingly sparse, correspond to the 'hyperarid regions of the earth, where rainfall is exceedingly rare and infrequent. Deserts are however part of a wider classification of regions that, on an average annual basis, have a moisture deficit (i.e. they can potentially lose more than is received). These areas are collectively called ' drylands' and extent over almost half of the earth's land surface. 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. Exposures of rocky terrain are typical, and reflect minimal soil development and sparseness of vegetation. Bottom lands may be salt-covered flats. 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. Also, cloudless skies increase the release of heat at night.
In the Köppen climate classification system, deserts are classed as (BW).
The English, French (désert), Italian (deserto), all come from the Latin deserta. This name is derived from the old Egyptian language, from the word deshert, meaning the 'red land' that bordered the black land (kemet) in the nile valley from the east and the west.
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.
The difference lies in something termed "potential evapotranspiration." The water budget of an area can be calculated using the formula P-PE+/-S, wherein P is precipitation, PE is potential evapotranspiration rates and S is amount of surface storage of water. 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.
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.
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).
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 are arid places with a very high altitude; the most prominent example is found north of the Himalaya especially in Laddakh region of Jammu and Kashmir (India), 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. 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. 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. As the air moves over the mountains, it cools and condenses, causing precipitation on the upwind side. Moisture almost never reaches the downwind side of the mountain, therefore causing a desert. When that air reaches the downwind side, the air is dry, because it has already lost all of its moisture. 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.
Sand covers only about 20 percent of Earth's deserts. Most of the sand is in sand sheets and sand seas—vast regions of undulating dunes resembling ocean waves "frozen" in an instant of time. In general, there are 6 forms of deserts:
- Mountain and basin deserts;
- Hamada deserts, which comprise of plateaux landforms;
- Regs which consist of rock pavements;
- Ergs which are formed by sand seas;
- Intermontane Basins; and
- Badlands which are located at the margins of arid lands comprising of clay-rich soil.
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. Bedrock outcrops commonly occur as small mountains surrounded by extensive erosional plains.
There are several different types of dunes. Barchan dunes are produced by strong winds blowing across a level surface and are crescent-shaped. Longitudinal or seif dunes are dunes that are parallel to a strong wind that blows in one general direction. Transverse dunes run at a right angle to the constant wind direction. Star dunes are star-shaped and have several ridges that spread out around a point.
Oases are vegetated areas moistened by springs, wells, or by irrigation. Many are artificial. Oases are often the only places in deserts that support crops and permanent habitation.
Most desert plants are drought- or salt-tolerant, such as xerophytes. Some store water in their leaves, roots, and stems. Other desert plants have long tap roots that penetrate to the water table if present. The stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect the ground from erosion. Even small fungi and microscopic plant organisms found on the soil surface (so-called cryptobiotic soil) can be a vital link in preventing erosion and providing support for other living organisms.
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. Saguaro grow slowly but may live 200 years. When 9 years old, they are about 15 centimeters high. After about 75 years, the cacti develop their first branches. When fully grown, saguaro are 15 meters tall and weigh as much as 10 tons. 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. They include the pea and sunflower families. Cold deserts have grasses and shrubs as dominant vegetation.
Rain does fall occasionally in deserts, and desert storms are often violent. A record 44 millimeters of rain once fell within 3 hours in the Sahara. Large Saharan storms may deliver up to 1 millimeter per minute. Normally dry stream channels, called arroyos or wadis, can quickly fill after heavy rains, and flash floods make these channels dangerous.
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. Because these lakes are shallow and have a low bottom gradient, wind stress may cause the lake waters to move over many square kilometers. When small lakes dry up, they leave a salt crust or hardpan. The flat area of clay, silt, or sand encrusted with salt that forms is known as a playa. There are more than a hundred playas in North American deserts. Most are relics of large lakes that existed during the last ice age about 12,000 years ago. Lake Bonneville was a 52,000-square-kilometer lake almost 300 meters deep in Utah, Nevada, and Idaho during the Ice Age. Today the remnants of Lake Bonneville include Utah's Great Salt Lake, Utah Lake, and Sevier Lake. Because playas are arid land forms from a wetter past, they contain useful clues to climatic change.
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. Ground-vehicle speed records are commonly established on Bonneville Speedway, a race track on the Great Salt Lake hardpan. Space shuttles land on Rogers Lake Playa at Edwards Air Force Base in California.
- 2006 is the International Year of Deserts and Desertification.