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    Desert Experiments & Lesson Plans

    Desert Background Information

    Definition

    A desert is a landscape or region that receives almost no precipitation. Deserts are defined as areas with an average annual precipitation of less than 250 millimetres (10 in) per year, or as areas where more water is lost by evapotranspiration than falls as precipitation.

    Basics

    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 extend 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-fourth of the Earth's land surface. The desert is very arid (dry) and can have high temperatures in excess of 50°C (122 F). Even though the desert is often very hot in the day, it can be extremely cold by night. Deserts have quite rough terrain, and many sand dunes present an overwhelmingly hostile environment. Humans who travel into deserts unprepared have a slim chance of survival due to the relative dearth of water.

    The 10 largest deserts
    Rank Desert Area (km²) Area (mi²)
    1 Antarctic Desert (Antarctica) 13,829,430 5,339,573
    2 Arctic 13,700,000+ 5,300,000+
    3 Sahara (Africa) 9,100,000+ 3,320,000+
    4 Arabian Desert (Middle East) 2,330,000 900,000
    5 Gobi Desert (Asia) 1,300,000 500,000
    6 Kalahari Desert (Africa) 900,000 360,000
    7 Patagonian Desert (South America) 670,000 260,000
    8 Great Victoria Desert (Australia) 647,000 250,000
    9 Syrian Desert (Middle East) 520,000 200,000
    10 Great Basin Desert (North America) 492,000 190,000

    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. Bottomlands 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 precipitation 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 in general or specifically BWh (hot desert) or BWk (temperate desert). In the Thornthwaite climate classification system, deserts would be classified as arid megathermal climates.

    What is a desert?

    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 lands are deserts, and semiarid grasslands are generally 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).

    Desert features

    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 six forms of deserts:

    • Mountain and basin deserts
    • Hamada deserts, which comprise of plateau landforms
    • Regs, which consist of rock pavements
    • Ergs, which are formed by sand seas
    • Intermontane Basins
    • 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

    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 up to 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

    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 landforms 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.

    Mineral resources

    Some mineral deposits are formed, improved, or preserved by geologic processes that occur in arid lands as a consequence of climate. Ground water leaches ore minerals and redeposits them in zones near the water table. This leaching process concentrates these minerals as ore that can be mined.

    Evaporation in arid lands enriches mineral accumulation in their lakes. Playas may be sources of mineral deposits formed by evaporation. Water evaporating in closed basins precipitates minerals such as gypsum, salts (including sodium nitrate and sodium chloride), and borates. The minerals formed in these evaporite deposits depend on the composition and temperature of the saline waters at the time of deposition.

    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. Boron, from borax and borate evaporites, is an essential ingredient in the manufacture of glass, enamel, agricultural chemicals, water softeners, and pharmaceuticals. Borates are mined from evaporite deposits at Searles Lake, California, and other desert locations. The total value of chemicals that have been produced from Searles Lake substantially exceeds US$1 billion.

    The Atacama Desert of South America is unique among the deserts of the world in its great abundance of saline minerals. Sodium nitrate has been mined for explosives and fertilizer in the Atacama since the middle of the 19th century. Nearly 3 million tonnes were mined during World War I.

    Valuable minerals located in arid lands include copper in the United States, Chile, Peru, and Iran; iron and lead-zinc ore in Australia; chromite in Turkey; and gold, silver, and uranium deposits in Australia and the United States. Nonmetallic mineral resources and rocks such as beryllium, mica, lithium, clays, pumice, and scoria also occur in arid regions. Sodium carbonate, sulfate, borate, nitrate, lithium, bromine, iodine, calcium, and strontium compounds come from sediments and near-surface brines formed by evaporation of inland bodies of water, often during geologically recent times.

    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. Economically significant deposits of trona, a major source of sodium compounds, and thick layers of oil shale were created in the arid environment.

    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. Recent climate change has placed these reservoirs in an arid environment. It's noteworthy that Ghawar, the world's largest and most productive oilfield is mostly under the Empty Quarter and Al-Dahna deserts. Currently the verification of SALINITY is unavailable on this page.

    Other oil reservoirs, however, are presumed to be eolian in origin and are presently found in humid environments. The Rotliegendes, a hydrocarbon reservoir in the North Sea, is associated with extensive evaporite deposits. Many of the major U.S. hydrocarbon resources may come from eolian sands. Ancient alluvial fan sequences may also be hydrocarbon reservoirs.

    Solar energy resources: Deserts are increasingly seen as sources for solar energy. The Negev Desert and the surrounding area, including the Arava Valley, are the sunniest parts of Israel and little of this land is arable, which is why it has become the center of the Israeli solar industry. David Faiman, a world expert on solar energy, feels the energy needs of a country like Israel could be met by building solar energy plants in the Negev. Faiman also feels the technology now exists to supply all of the world's electricity needs with 10 per cent of the Sahara. Solel has nine fields of solar collectors in the Mojave Desert of California. It recently signed a contract to build the Mojave Solar Park, which will be the world's largest solar generating plant.

    Human life in deserts

    A desert is a hostile, potentially deadly environment for unprepared humans. In hot deserts, high temperatures cause rapid loss of water due to sweating, and the absence of water sources with which to replenish it can result in dehydration and death within a few days. In addition, unprotected humans are also at risk from heatstroke.

    Humans may also have to adapt to sandstorms in some deserts, not just in their adverse effects on respiratory systems and eyes, but also in their potentially harmful effects on equipment such as filters, vehicles and communication equipment. Sandstorms can last for hours, sometimes even days. This makes surviving in the desert quite difficult for humans.

    Despite this, some cultures have made hot deserts their home for thousands of years, including the Bedouin, Tuareg and Pueblo people. Modern technology, including advanced irrigation systems, desalinization and air conditioning have made deserts much more hospitable. In the United States and Israel for example, desert farming has found extensive use.

    In cold deserts, hypothermia and frostbite are the chief hazards, as well as dehydration in the absence of a source of heat to melt ice for drinking. Falling through pack-ice or surface ice layers into freezing water is a particular danger requiring emergency action to prevent rapid hypothermia. Starvation is also a hazard; in low temperatures the body requires much more food energy to maintain body heat and to move. As with hot deserts, some people such as the Inuit have adapted to the harsh conditions of cold deserts.

    Most traditional human life in deserts is nomadic. It depends in hot deserts on finding water, and on following infrequent rains to obtain grazing for livestock. In cold deserts, it depends on finding good hunting and fishing grounds, on sheltering from blizzards and winter extremes, and on storing enough food for winter. Permanent settlement in both kinds of deserts requires permanent water and food sources and adequate shelter, or the technology and energy sources to provide it.

    Many deserts are flat and featureless, lacking landmarks, or composed of repeating landforms such as sand dunes or the jumbled ice-fields of glaciers. Advanced skills or devices are required to navigate through such landscapes and inexperienced travellers may perish when supplies run out after becoming lost. In addition sandstorms or blizzards may cause disorientation in severely reduced visibility.

    The danger represented by wild animals in deserts has been featured in explorers' accounts but does not cause higher rates of death than in other environments such as rain forests or savanna woodland, and generally does not by itself affect human distribution. Defence against polar bears may be advisable in some areas of the Arctic, as may precautions against venomous snakes and scorpions in choosing sites at which to camp in some hot deserts.

    Topics of Interest

    The flat terrains of desert hardpans and playas make them excellent racetracks and natural runways for airplanes and spacecraft. Ground-vehicle speed records are commonly established on Bonneville Speedway, a racetrack on the Great Salt Lake hardpan. Space shuttles land on Rogers Lake Playa at Edwards Air Force Base in California.

    The Köppen climate classification is one of the most widely used climate classification systems. It was first published by German climatologist Wladimir Köppen in 1884, with several later modifications by Köppen himself notably in 1918 and 1936. Later, German climatologist Rudolf Geiger collaborated with Köppen on changes to the classification system, which is thus sometimes referred to as the Köppen–Geiger climate classification system.

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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