Not to be confused with dessert In Western culture dessert is a course that typically comes at the end of a meal, usually consisting of sweet food. The word comes from the French language as dessert and this from Old French desservir, "to clear the table" and "to serve." Common desserts include cakes, cookies, pastries, ice cream, pie, and candies. Fruit may.

Largest deserts Hills in the Judean desert Judea or Judæa is the name given to the mountainous southern part of the historic Land of Israel (Hebrew: ארץ ישראל‎ Eretz Yisrael), an area now divided between Israel and the West Bank (itself partly under Palestinian Authority administration and Israeli military rule)

A desert is a landscape Landscape comprises the visible features of an area of land, including the physical elements of landforms, water bodies such as rivers, lakes and the sea, living elements of land cover including indigenous vegetation, human elements including land uses, buildings and structures, and transitory elements such as lighting and weather conditions or region Region is most commonly a geographical term that is used in various ways among the different branches of geography. In general, a region may be seen as a collection of smaller units or as one part of a larger whole (as in "the New England region of the United States"). Regions can be defined by physical characteristics, human that receives an extremely low amount of precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that is pulled down by gravity and deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel. It occurs when the atmosphere, a large gaseous solution, becomes saturated with water vapour and the water, less than enough to support growth of most plants. Deserts are defined as areas with an average annual precipitation of less than 250 millimetres (10 in) per year,^[1][2] or as areas where more water is lost by evapotranspiration Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies. Transpiration accounts for the movement of water within a plant and the subsequent than falls as precipitation.^[3] In the Köppen climate classification 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, deserts are classed as BWh (hot desert) or BWk (temperate desert). In the Thornthwaite climate classification system, deserts would be classified as arid megathermal Megathermal climates are sometimes split into two temperature-based subsets — equatorial and tropical — with "equatorial" denoting little or no variation in temperature throughout the year and "tropical" denoting significant seasonal variation, even though no month has an average temperature of below 18°C. In addition, climates.^[4][5]

Contents 1 Geography 2 Etymology 3 Classification 4 Definition 5 Desert features 5.1 Fauna and flora 5.2 Water 6 Formation of hot deserts 7 Mineral resources 8 Solar energy resources 9 Human life in deserts 10 See also 11 References 12 External links

Geography

A satellite image of the Sahara The Sahara is the world's largest hot desert. At over 9,400,000 square kilometres (3,630,000 sq mi), it covers most of Northern Africa, making it almost as large as Europe or the United States. The only larger desert in the world is Antarctica. The Sahara Desert stretches from the Red Sea, including parts of the Mediterranean coasts, to the; the world's largest hot desert and third largest desert after Antarctica Antarctica (pronounced /ænˈtɑrktɪkə/ ) is Earth's southernmost continent, underlying the South Pole. It is situated in the Antarctic region of the southern hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Ocean. At 14.0 million km2 (5.4 million sq mi), it is the fifth-largest continent in area after and the Arctic The Arctic is the region around the Earth's North Pole, opposite the Antarctic region around the South Pole. The Arctic includes the Arctic Ocean (which overlies the North Pole) and parts of Canada, Greenland (a territory of Denmark), Russia, the United States (Alaska), Iceland, Norway, Sweden and Finland. The snow surface at Dome C Station in Antarctica is representative of the majority of the continent's surface.

Deserts are part of a wide classification of regions that, on an average annual basis, have a moisture deficit (i.e. they can potentially lose more than is received). Deserts are located where vegetation cover is sparse to almost nonexistent.^[1][6] Deserts take up about one third (33%) of the Earth's land surface.^[1] Hot deserts usually have a large diurnal Diurnal temperature variation is a meteorological term that relates to the variation in temperature that occurs from the highs of the day to the cool of nights and seasonal temperature range, with high daytime temperatures, and low nighttime temperatures (due to extremely low humidity Humidity is the amount of water vapor in the air. Relative humidity is defined as the ratio of the partial pressure of water vapor in a parcel of air to the saturated vapor pressure of water vapor at a prescribed temperature. Humidity may also be expressed as specific humidity. Relative humidity is an important metric used in forecasting weather). In hot deserts the temperature in the daytime can reach 45 °C/113 °F or higher in the summer, and dip to 0 °C/32 °F or lower in the winter. Water acts to trap infrared Infrared light is electromagnetic radiation with a wavelength between 0.7 and 300 micrometres, which equates to a frequency range between approximately 1 and 430 THz radiation from both the sun and the ground, and dry desert air is incapable of blocking sunlight Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon during the day or trapping heat The term thermal insulation can refer to materials used to reduce the rate of heat transfer, or the methods and processes used to reduce heat transfer. Heat energy can be transferred by conduction, convection, radiation or by actual movement of material from one location to another. For the purposes of this discussion only the first three during the night. Thus, during daylight most of the sun The Sun is the star at the center of the Solar System. It has a diameter of about 1,392,000 kilometers , about 109 times that of Earth, and its mass (about 2 × 1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen, while the rest is's heat reaches the ground, and as soon as the sun sets the desert cools quickly by radiating its heat into space. Urban areas in deserts lack large (more than 14 °C/25 °F) daily temperature variations, partially due to the urban heat island An urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night than during the day, and is most apparent when winds effect.

Many deserts are formed by rain shadows The condition exists because as warm moist air rises through "orographic lifting" to the top of a mountain range or large mountain, where due to decreasing atmospheric pressure with increasing altitude, it has expanded and adiabatically cooled to the point that the air reaches its adiabatic dew point . At the adiabatic dew point,; mountains blocking the path of precipitation to the desert (on the lee side of the mountain). Deserts are often composed of sand Sand is a naturally occurring granular material composed of finely divided rock and mineral particles and rocky In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids surfaces. Sand dunes In physical geography, a dune is a hill of sand built by aeolian processes. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind. The valley or trough between dunes is called ergs An erg is a large, relatively flat area of desert covered with wind-swept sand with little or no vegetative cover. The term takes its name from the Arabic word erg (عرق), meaning "dune field". Strictly speaking, an erg is defined as a desert area that contains more than 125 square kilometers of aeolian or wind-blown sand and where and stony surfaces called hamada A hamada is a type of desert landscape consisting of largely barren, hard, rocky plateaus, with very little sand. A hamada may sometimes also be called a reg (pronounced "rej"), though this more properly refers to a stony plain rather than a highland surfaces compose a minority of desert surfaces. Exposures of rocky In geology, rock is a naturally occurring solid aggregate of minerals and/or mineraloids terrain are typical, and reflect minimal soil development and sparseness of vegetation Vegetation is the plant life or the plant ground cover of a region, not the particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader than the term flora which refers exclusively to species composition. Perhaps the closest synonym is plant community, but vegetation can, and. The soil is rocky because of the low chemical weathering.

Bottomlands may be salt Salt is a mineral that is composed primarily of sodium chloride. It is essential for animal life in small quantities, but is harmful to animals and plants in excess. Salt flavor is one of the basic tastes, making salt one of the oldest, most ubiquitous food seasonings. Salting is an important method of food preservation-covered flats. Eolian processes are major factors in shaping desert landscapes. Polar deserts (also seen as "cold deserts") have similar features, except the main form of precipitation is snow Snow is a type of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. Snowflakes come in a variety of rather than rain Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface. On Earth, it is the condensation of atmospheric water vapor into drops of water heavy. Antarctica Antarctica (pronounced /ænˈtɑrktɪkə/ ) is Earth's southernmost continent, underlying the South Pole. It is situated in the Antarctic region of the southern hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Ocean. At 14.0 million km2 (5.4 million sq mi), it is the fifth-largest continent in area after is the world's largest cold desert (composed of about 98 percent thick continental A continent is one of several large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents – they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia ice sheet An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier. The only current ice sheets are in Antarctica and Greenland; during the last glacial period at Last Glacial Maximum (LGM) the Laurentide ice sheet covered much of Canada and North America, the Weichselian and 2 percent barren rock). Some of the barren rock is to be found in the so-called Dry Valleys The McMurdo Dry Valleys are a row of valleys in Antarctica located within Victoria Land west of McMurdo Sound. The region includes many interesting geological features including Lake Vida and the Onyx River, Antarctica's longest river. It is also one of the world's most extreme deserts of Antarctica that almost never get snow, which can have ice-encrusted saline lakes A salt lake or saline lake is a landlocked body of water which has a concentration of salts and other minerals significantly higher than most lakes (often defined as at least three grams of salt per liter). In some cases, salt lakes have a higher concentration of salt than sea water, but such lakes would also be termed hypersaline lakes that suggest evaporation far greater than the rare snowfall due to the strong katabatic winds A katabatic wind, from the Greek word katabatikos meaning "going downhill", is the technical name for a drainage wind, a wind that carries high density air from a higher elevation down a slope under the force of gravity. Such winds are sometimes also called fall winds that evaporate even ice.

The largest hot desert is the Sahara in northern Africa, covering 9 million square kilometres and 12 countries.

Deserts sometimes contain valuable mineral deposits that were formed in the arid environment or that were exposed by erosion. Due to extreme and consistent dryness, some deserts are ideal places for natural preservation of artifacts and fossils.

The 10 largest deserts

Rank	Desert	Area (km²)	Area (mi²)
1	Antarctic Desert Antarctica (pronounced /ænˈtɑrktɪkə/ ) is Earth's southernmost continent, underlying the South Pole. It is situated in the Antarctic region of the southern hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Ocean. At 14.0 million km2 (5.4 million sq mi), it is the fifth-largest continent in area after (Antarctica Antarctica (pronounced /ænˈtɑrktɪkə/ ) is Earth's southernmost continent, underlying the South Pole. It is situated in the Antarctic region of the southern hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Ocean. At 14.0 million km2 (5.4 million sq mi), it is the fifth-largest continent in area after)	13,829,430	5,339,573
2	Arctic The Arctic is the region around the Earth's North Pole, opposite the Antarctic region around the South Pole. The Arctic includes the Arctic Ocean (which overlies the North Pole) and parts of Canada, Greenland (a territory of Denmark), Russia, the United States (Alaska), Iceland, Norway, Sweden and Finland	13,700,000+	5,300,000+
3	Sahara The Sahara is the world's largest hot desert. At over 9,400,000 square kilometres (3,630,000 sq mi), it covers most of Northern Africa, making it almost as large as Europe or the United States. The only larger desert in the world is Antarctica. The Sahara Desert stretches from the Red Sea, including parts of the Mediterranean coasts, to the (Africa Africa is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people (as of 2009, see table) in 61 territories, it accounts for about 14.72% of the world's human population)	9,100,000+	3,320,000+
4	Arabian Desert The Arabian Desert is a vast desert wilderness stretching from Yemen to the Persian Gulf and Oman to Jordan and Iraq. It occupies most of the Arabian Peninsula with an area of 222 070 square kilometers. At its center is the Rub'al-Khali, one of the largest continuous bodies of sand in the world. Gazelles, oryx, sand cats, and spiny-tailed lizards (Middle East The Middle East is a region that encompasses southwestern Asia and Egypt. In some contexts, the term has recently been expanded in usage to sometimes include Afghanistan and Pakistan, the Caucasus and Central Asia, and North Africa. It's often used as a synonym for Near East, in opposition to Far East. The corresponding adjective is Middle-Eastern)	2,330,000	900,000
5	Gobi Desert The Gobi (Mongolian: Говь, Govi, "semidesert"; Chinese: 戈壁 Gēbì (Shāmò)) is a large desert region in Asia. It covers parts of northern and northwestern China, and of southern Mongolia. The desert basins of the Gobi are bounded by the Altai Mountains and the grasslands and steppes of Mongolia on the north, by the Hexi Corridor (Asia Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.6% of the Earth's total surface area and with approximately 4 billion people, it hosts 60% of the world's current human population. During the 20th century Asia's population nearly quadrupled)	1,300,000	500,000
6	Kalahari Desert The Kalahari Desert is a large arid to semi-arid sandy area in Southern Africa extending 900,000 square kilometres (350,000 sq mi), covering much of Botswana and parts of Namibia and South Africa, as semi-desert, with huge tracts of excellent grazing after good rains. The Kalahari Desert is the southern part of Africa, and the geography is a (Africa Africa is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area. With a billion people (as of 2009, see table) in 61 territories, it accounts for about 14.72% of the world's human population)	900,000	360,000
7	Patagonian Desert The Patagonian Desert, also known as the Patagonia Desert or the Patagonian Steppe, is the largest desert in America and is the 7th largest desert in the world by area, occupying 260,000 square miles . It is located primarily in Argentina with small parts in Chile and is bounded by the Andes, to its west, and the Atlantic Ocean to its east, in the (South America South America is the southern continent of America, situated in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. It is bordered on the west by the Pacific Ocean and on the north and east by the Atlantic Ocean; North America and the Caribbean Sea lie to the northwest)	670,000	260,000
8	Great Victoria Desert The Great Victoria Desert is a barren, arid, and sparsely populated desert ecoregion in southern Australia. It falls inside the states of South Australia and Western Australia and consists of many small sandhills, grasslands and salt lakes. It is over 700 kilometres wide (from west to east) and covers an area of 424,400 square kilometres (163,900 (Australia For at least 40,000 years before European settlement in the late 18th century, Australia was inhabited by indigenous Australians, who belonged to one or more of the roughly 250 language groups. After sporadic visits by fishermen from the immediate north and discovery by Dutch explorers in 1606, Australia's eastern half was claimed by the British)	647,000	250,000
9	Syrian Desert (Middle East)	520,000	200,000
10	Great Basin Desert (North America)	492,000	190,000

Etymology

The Thar Desert near Jaisalmer, India.

English desert and its Romance cognates (including Italian and Portuguese deserto, French désert and Spanish desierto) all come from the ecclesiastical Latin dēsertum (originally "an abandoned place"), a participle of dēserere, "to abandon." (See desertion.) The correlation between aridity and sparse population is complex and dynamic, varying by culture, era, and technologies; thus the use of the word desert can cause confusion. In English prior to the 20th century, desert was often used in the sense of "unpopulated area", without specific reference to aridity; but today the word is most often used in its climate-science sense (an area of low precipitation)—and a desert may be quite heavily populated, with millions of inhabitants. Phrases such as "desert island" and "Great American Desert" in previous centuries did not necessarily imply sand or aridity; their focus was the sparse population. However, the connotation of a hot, parched, and sandy place often influences today's popular interpretation of those phrases.

Tadrart Acacus desert in western Libya, part of the Sahara.

Classification

In 1961, 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 mm (10 in) of annual rainfall, and semiarid lands have a mean annual precipitation of between 250 and 500 mm (10–20 in). Arid and extremely arid lands are deserts, and semiarid areas are generally referred to as steppes.^[1]

Definition

Measurement of rainfall alone cannot provide an accurate definition of what a desert is because being arid also depends on evaporation, which depends in part on temperature. For example, Phoenix, Arizona receives less than 250 millimeters (10 in) of precipitation per year, and is immediately recognized as being located in a desert due to its arid adapted plants. The North Slope of Alaska's Brooks Range also receives less than 250 millimeters (10 in) of precipitation per year and is often classified as a cold desert.^[7] Other regions of the world have cold deserts, including areas of the Himalayas^[8] and other high altitude areas in other parts of the world.^[9] Polar deserts cover much of the ice free areas of the arctic and antarctic.^[10][11]

Potential evapotranspiration supplements the measurement of rainfall in providing a scientific measurement-based definition of a desert. 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 and through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. As an example, Tucson, Arizona receives about 300 millimeters (12 in) of rain per year, however about 2500 millimeters (100 in) of water could evaporate over the course of a year.^{[citation needed]} In other words, about 8 times more water could evaporate from the region than actually falls. Rates of evapotranspiration in cold regions such as Alaska are much lower because of the lack of heat to aid in the evaporation process.

There are different forms of deserts. Cold deserts can be covered in snow or ice; frozen water unavailable to plant life. 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 in the day and chilly at night (for the latitude) because of the lack of the moderating effect of 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; other areas are arid by virtue of being very far from the nearest available sources of moisture.

The Agasthiyamalai hills cut off Tirunelveli in India from the monsoons, creating a rainshadow region.

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 Ladakh region of Jammu and Kashmir, in parts of the Kunlun Mountains and the Tibetan Plateau. Many locations within this category have elevations exceeding 3,000 meters (10,000 ft) and the thermal regime can be hemiboreal. These places owe their profound aridity (the average annual precipitation is often less than 40 mm or 1.5 in) to being very far from the nearest available sources of moisture. Montane deserts are normally cold.

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 moisture condenses, causing precipitation on the windward side. When that air reaches the leeward side, it is dry because it has lost the majority of its moisture, resulting in a desert. The air then warms, expands, and blows across the desert. The warm, desiccated air takes with it any remaining moisture in the desert.

Desert features

Satellite view of Al-Dahna desert in Saudi Arabia showing different depositional 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 five forms of deserts:

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

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 occur as small mountains surrounded by extensive erosional plains.

Several different types of dunes exist. 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.

Fauna and flora

"Desert flower" redirects here. For other uses, see Desert flower (disambiguation). Prickly pear flower Organ Pipe cactus in Arizona Saguaro cactuses in the Sonoran Desert of Arizona. Flora of Baja California Desert, Cataviña region, Mexico

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. Some fauna includes the kangaroo rat, coyote, jack rabbit, and many lizards. These animals adapted to live in deserts are called xerocoles. Many desert animals (and plants) show especially clear evolutionary adaptations for water conservation or heat tolerance, and so are often studied in comparative physiology, ecophysiology, and evolutionary physiology. One well-studied example is the specializations of mammalian kidneys shown by desert-inhabiting species.^[12] Many examples of convergent evolution have been identified in desert organisms, including between cacti and Euphorbia, kangaroo rats and jerboas, Phrynosoma and Moloch lizards.

Some flora includes shrubs, Prickly Pears, Desert Holly, and the Brittlebush. 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 taproots that penetrate to the water table if present, or have adapted to the weather by having wide-spreading roots to absorb water from a greater area of the ground. Another adaptation is the development of small, spiny leaves which shed less moisture than deciduous leaves with greater surface areas. 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 giant saguaro cacti of the Sonoran Desert 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 (6 in) high. After about 75 years, the cacti develop their first branches. When fully grown, saguaro cacti are 15 meters (50 ft) 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

Atacama, the world's driest desert^[13]

Atacama is the driest place on Earth^{[13][14][15][16]} and is virtually sterile because it is blocked from moisture on both sides by the Andes mountains and by the Chilean Coast Range. The cold Humboldt Current and the anticyclone of the Pacific are essential to keep the dry climate of the Atacama. The average rainfall in the Chilean region of Antofagasta is just 1 mm per year. Some weather stations in the Atacama have never received rain. Evidence suggests that the Atacama may not have had any significant rainfall from 1570 to 1971. It is so arid that mountains that reach as high as 6,885 meters (22,590 feet) are completely free of glaciers and, in the southern part from 25°S to 27°S, may have been glacier-free throughout the Quaternary, though permafrost extends down to an altitude of 4,400 meters and is continuous above 5,600 meters.

Rain does fall occasionally in deserts, and desert storms are often violent. A record 44 millimeters (1.7 in) 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 lie 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.

While deserts are well-known for their lack of water, some groups have adapted ways to find water in this harsh environment. The Bedouin, for example, turn over half-buried stones just before dawn so dew forms on them.^[17]

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 or a sink. 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 (20,000 mi²) lake almost 300 meters (1000 ft) 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.

The flat terrains of hardpans and playas make them excellent racetracks and natural runways for airplanes and spacecraft. Ground-vehicle speed records have been established on the flat lakebeds of the Black Rock Desert in Nevada and Bonneville Speedway in Utah. Space shuttles and flight-test aircraft land on Rogers Lake Playa at Edwards Air Force Base in California.

Formation of hot deserts

There are four main, interlinked causes of hot deserts:^{[citation needed]}

• The formation of the subtropical high-pressure cell.
• The rain shadow effect in the belt of easterly trade winds.
• The effect of the cold currents off the west coast of the continents at these latitudes.
• The depositing sands of a desert along its border into the fertile land

It must be noted that also hot deserts (like cold deserts) may result in average temperature cooling^[18] because they reflect more of the incoming light (their albedo is higher than that of water or forests).

Mineral resources

A mining plant near Jodhpur, India

Deserts may contain great amounts of mineral resources over their entire surface. This occurrence in minerals also determines the color. For example, the red color of many sand deserts is a result of the occurrence of laterite.^[19]

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. Lake beds known as 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 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 Chile 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 metric tons 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; 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.

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.^[20] 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.^[21] Faiman also feels the technology now exists to supply all of the world's electricity needs with 10 per cent of the Sahara.^[21] Solel has nine fields of solar collectors in the Mojave Desert of California.^[22] It recently signed a contract to build the Mojave Solar Park, which will be the world's largest solar generating plant.^[23][24]

Human life in deserts

Mahktesh Gadol, an erosional basin in the Negev Desert of southern Israel Cooking in desert

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 Australia 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. Defense 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.

See also

Kolob Canyon, part of Zion National Park, Utah, United States is part of the larger desert the Colorado Plateau.

• Arid Lands Information Network
• Aridification
• Desert greening
• Desertification
• International Center for Agricultural Research in the Dry Areas
• List of deserts
• List of deserts by area (size)
• List of North American deserts
• Oasis
• Sediment precipitation

References

1. ^ ^{a b c d} What is a desert?
2. ^ According to What is a desert?, the 250 mm threshold definition is attributed to Peveril Meigs.
3. ^ "desert". Encyclopædia Britannica online. http://www.britannica.com/eb/article-70815/desert. Retrieved 2008-02-09.
4. ^ Fredlund, D.G.; Rahardjo, H. (1993) (pdf). Soil Mechanics for Unsaturated Soils. Wiley-Interscience. ISBN 978-0471850083. http://www.soilvision.com/subdomains/unsaturatedsoil.com/Docs/chapter1UST.pdf. Retrieved 2008-05-21.
5. ^ Glossary of Meteorology. Megathermal Climate. Retrieved on 2008-05-21.
6. ^ 1911 Encyclopedia Britannica
7. ^ Heinrich Walter; Siegmar-W. Breckle (2002). Walter's Vegetation of the earth: the ecological systems of the geo-biosphere. Springer. pp. 457–. ISBN 9783540433156. http://books.google.com/books?id=SdaCSwxK5bIC&pg=PA457. Retrieved 24 July 2010.
8. ^ S.S. Negi (1 March 2002). Cold Deserts of India. Indus Publishing. pp. 9–. ISBN 9788173871276. http://books.google.com/books?id=54RgJ6FgMl0C&pg=PA9. Retrieved 24 July 2010.
9. ^ Robert V. Rohli; Anthony J. Vega (2008). Climatology. Jones & Bartlett Learning. pp. 207–. ISBN 9780763738280. http://books.google.com/books?id=Zhbqbrg2XswC&pg=PA207. Retrieved 25 July 2010.
10. ^ David Neville Thomas; Gordon Elliott Fogg; P. Convey; C.H. Fritsen, J.-M. Gili (2008). The biology of polar regions. Oxford University Press. pp. 64–. ISBN 9780199298136. http://books.google.com/books?id=-ErSVcvhI4oC&pg=PA64. Retrieved 24 July 2010.
11. ^ W. Berry Lyons; C. Howard-Williams; Ian Hawes (1997). Ecosystem processes in Antarctic ice-free landscapes: proceedings of an International Workshop on Polar Desert Ecosystems : Christchurch, New Zealand, 1-4 July, 1996. Taylor & Francis. pp. 3–. ISBN 9789054109259. http://books.google.com/books?id=VKtCnLa5uaYC&pg=PA3. Retrieved 25 July 2010.
12. ^ Al-kahtani, M.A.; C. Zuleta, E. Caviedes-Vidal, and T. Garland, Jr. (2004). "Kidney mass and relative medullary thickness of rodents in relation to habitat, body size, and phylogeny". Physiological and Biochemical Zoology 77 (3): 346–365. doi:10.1086/420941. PMID 15286910. http://www.biology.ucr.edu/people/faculty/Garland/Al-kahtaniEA2004.pdf.
13. ^ ^{a b} Nationalgeographic.com
14. ^ Extremescience.com
15. ^ NASA.gov
16. ^ Boehm, Richard G.; Editors and writers of The World and Its People (2006). The World and Its People (2005 ed.). Columbus, Ohio: Glencoe. pp. 276. ISBN 0-07-860977-1.
17. ^ PBS.org
18. ^ Celsias.com
19. ^ Tsjaad by Dorrit van Dalen
20. ^ Head of Kibbutz Movement: We will not be discriminated against by the government, Ehud Zion Waldoks, Jerusalem Post, March 10, 2008.
21. ^ ^{a b} Giant solar plants in Negev could power Israel's future, John Lettice, The Register, January 25, 2008
22. ^ Israel21c.org
23. ^ Looking to the sun, Tom Parry, Canadian Broadcasting Corporation, August 15, 2007.
24. ^ Businessweek.com

External links

Wikimedia Commons has media related to: Deserts

• "The Desert Biome". University of California Museum of Paleontology. 1996. http://www.ucmp.berkeley.edu/glossary/gloss5/biome/deserts.html.
• "Global Deserts Outlook". United Nations Environment Programme (UNEP). 2006. http://www.unep.org/geo/gdoutlook/. , a report in the Global Environment Outlook (GEO) series.
• Map with biodiversity scenarios for desert areas, from the Global Deserts Outlook.
• Finding Water in the Desert - wikiHow page

Deserts Africa Bayuda Desert · Chalbi Desert · Danakil Desert · Eastern Desert · Ferio Desert · Kalahari Desert · Libyan Desert · Namib Desert · Nubian Desert · Nyiri Desert · Richtersveld · Sahara · Ténéré · White Desert Asia Ad-Dahna Desert · Arabian Desert · Aral Karakum · Aralkum · Badain Jaran Desert · Betpak-Dala · Cholistan Desert · Dasht-e Kavir · Dasht-e Lut · Dasht-e Margoh · Dasht-e Naomid · Gurbantünggüt Desert · Gobi Desert · Hami Desert · Indus Valley Desert · Judean Desert · Karakum Desert · Kharan Desert · Kumtag Desert · Kyzyl Kum · Lop Desert · Nefud · Negev · Ordos Desert · Qaidam · Rub' al Khali · Russian Arctic · Registan Desert · Saryesik-Atyrau Desert · Syrian Desert · Taklamakan Desert · Tengger Desert · Thal Desert · Thar · Tihamah · Ustyurt Plateau · Wahiba Sands · Liwa Desert Australia Gibson Desert · Great Sandy Desert · Great Victoria Desert · Little Sandy Desert · Nullarbor Plain · Painted Desert · Pedirka Desert · Simpson Desert · Strzelecki Desert · Sturt's Stony Desert · Tanami Desert · Tirari Desert Europe Accona Desert · Alpine · Bardenas Reales · Błędów Desert · Cabo de Gata · Deliblatska Peščara · Hálendi · Oleshky Sands · Oltenian Sahara · Ryn Desert · Tabernas Desert North America Alvord Desert · Amargosa Desert · Baja California Desert · Black Rock Desert · Carcross "Desert" · Channeled scablands · Chihuahuan Desert · Escalante Desert · Gran Desierto de Altar · Great Basin · Great Salt Lake Desert · Jornada del Muerto · Mojave Desert · Okanagan Desert · North American Arctic · Owyhee Desert · Painted Desert · Red Desert · Sonoran Desert Polar regions Antarctica · Arctic South America Atacama Desert · La Guajira Desert · Los Médanos de Coro · Monte Desert · Patagonian Desert · Sechura Desert · Tatacoa Desert Oceania Kaʻū Desert · Rangipo Desert See also List of deserts and List of deserts by area

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