   #copyright

Desert

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather;
Geology and geophysics

   Erg Chebbi, Morocco
   Enlarge
   Erg Chebbi, Morocco

   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).

Etymology

   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

   The Thar Desert near Jaisalmer, India
   Enlarge
   The Thar Desert near Jaisalmer, India
   High desert in Eastern Oregon, United States
   Enlarge
   High desert in Eastern Oregon, United States
   The Agasthiyamalai hills, cuts off Tirunelveli(India) from the
   monsoons, creating a rainshadow region
   Enlarge
   The Agasthiyamalai hills, cuts off Tirunelveli(India) from the
   monsoons, creating a rainshadow region

   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

   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.

Desert features

   Satellite view of Al-Dahna desert in Saudi Arabia. Different
   depositional features can be clearly seen.
   Enlarge
   Satellite view of Al-Dahna desert in Saudi Arabia. Different
   depositional features can be clearly seen.

   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.

Vegetation

   Flora of Baja California Desert, Cataviña region, Mexico
   Enlarge
   Flora of Baja California Desert, Cataviña region, Mexico

   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.

Water

   The shifting sands simulator at Questacon, Canberra
   Enlarge
   The shifting sands simulator at Questacon, Canberra

   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.

Trivia

     * 2006 is the International Year of Deserts and Desertification.

   Retrieved from " http://en.wikipedia.org/wiki/Desert"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
