Desert

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Not to be confused with Dessert. Desert_sentence_0

This article is about dry terrain. Desert_sentence_1

For arid climate, see Desert climate. Desert_sentence_2

For the act of abandoning or withdrawing support, see Desertion. Desert_sentence_3

For other uses, see Desert (disambiguation). Desert_sentence_4

A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. Desert_sentence_5

The lack of vegetation exposes the unprotected surface of the ground to the processes of denudation. Desert_sentence_6

About one-third of the land surface of the world is arid or semi-arid. Desert_sentence_7

This includes much of the polar regions, where little precipitation occurs, and which are sometimes called polar deserts or "cold deserts". Desert_sentence_8

Deserts can be classified by the amount of precipitation that falls, by the temperature that prevails, by the causes of desertification or by their geographical location. Desert_sentence_9

Deserts are formed by weathering processes as large variations in temperature between day and night put strains on the rocks, which consequently break in pieces. Desert_sentence_10

Although rain seldom occurs in deserts, there are occasional downpours that can result in flash floods. Desert_sentence_11

Rain falling on hot rocks can cause them to shatter, and the resulting fragments and rubble strewn over the desert floor are further eroded by the wind. Desert_sentence_12

This picks up particles of sand and dust and wafts them aloft in sand or dust storms. Desert_sentence_13

Wind-blown sand grains striking any solid object in their path can abrade the surface. Desert_sentence_14

Rocks are smoothed down, and the wind sorts sand into uniform deposits. Desert_sentence_15

The grains end up as level sheets of sand or are piled high in billowing sand dunes. Desert_sentence_16

Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of a mosaic of smooth stones. Desert_sentence_17

These areas are known as desert pavements, and little further erosion takes place. Desert_sentence_18

Other desert features include rock outcrops, exposed bedrock and clays once deposited by flowing water. Desert_sentence_19

Temporary lakes may form and salt pans may be left when waters evaporate. Desert_sentence_20

There may be underground sources of water, in the form of springs and seepages from aquifers. Desert_sentence_21

Where these are found, oases can occur. Desert_sentence_22

Plants and animals living in the desert need special adaptations to survive in the harsh environment. Desert_sentence_23

Plants tend to be tough and wiry with small or no leaves, water-resistant cuticles, and often spines to deter herbivory. Desert_sentence_24

Some annual plants germinate, bloom and die in the course of a few weeks after rainfall, while other long-lived plants survive for years and have deep root systems able to tap underground moisture. Desert_sentence_25

Animals need to keep cool and find enough food and water to survive. Desert_sentence_26

Many are nocturnal, and stay in the shade or underground during the heat of the day. Desert_sentence_27

They tend to be efficient at conserving water, extracting most of their needs from their food and concentrating their urine. Desert_sentence_28

Some animals remain in a state of dormancy for long periods, ready to become active again during the rare rainfall. Desert_sentence_29

They then reproduce rapidly while conditions are favorable before returning to dormancy. Desert_sentence_30

People have struggled to live in deserts and the surrounding semi-arid lands for millennia. Desert_sentence_31

Nomads have moved their flocks and herds to wherever grazing is available, and oases have provided opportunities for a more settled way of life. Desert_sentence_32

The cultivation of semi-arid regions encourages erosion of soil and is one of the causes of increased desertification. Desert_sentence_33

Desert farming is possible with the aid of irrigation, and the Imperial Valley in California provides an example of how previously barren land can be made productive by the import of water from an outside source. Desert_sentence_34

Many trade routes have been forged across deserts, especially across the Sahara Desert, and traditionally were used by caravans of camels carrying salt, gold, ivory and other goods. Desert_sentence_35

Large numbers of slaves were also taken northwards across the Sahara. Desert_sentence_36

Some mineral extraction also takes place in deserts, and the uninterrupted sunlight gives potential for the capture of large quantities of solar energy. Desert_sentence_37

Etymology Desert_section_0

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". Desert_sentence_38

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. Desert_sentence_39

In English before 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). Desert_sentence_40

Phrases such as "desert island" and "Great American Desert", or Shakespeare's "deserts of Bohemia" (The Winter's Tale) in previous centuries did not necessarily imply sand or aridity; their focus was the sparse population. Desert_sentence_41

Physical geography Desert_section_1

A desert is a region of land that is very dry because it receives low amounts of precipitation (usually in the form of rain, but it may be snow, mist or fog), often has little coverage by plants, and in which streams dry up unless they are supplied by water from outside the area. Desert_sentence_42

Deserts generally receive less than 250 mm (10 in) of precipitation each year. Desert_sentence_43

The potential evapotranspiration may be large but (in the absence of available water) the actual evapotranspiration may be close to zero. Desert_sentence_44

Semi-deserts are regions which receive between 250 and 500 mm (10 and 20 in) and when clad in grass, these are known as steppes. Desert_sentence_45

Classification Desert_section_2

Deserts have been defined and classified in a number of ways, generally combining total precipitation, number of days on which this falls, temperature, and humidity, and sometimes additional factors. Desert_sentence_46

For example, Phoenix, Arizona, receives less than 250 mm (9.8 in) of precipitation per year, and is immediately recognized as being located in a desert because of its aridity-adapted plants. Desert_sentence_47

The North Slope of Alaska's Brooks Range also receives less than 250 mm (9.8 in) of precipitation per year and is often classified as a cold desert. Desert_sentence_48

Other regions of the world have cold deserts, including areas of the Himalayas and other high-altitude areas in other parts of the world. Desert_sentence_49

Polar deserts cover much of the ice-free areas of the Arctic and Antarctic. Desert_sentence_50

A non-technical definition is that deserts are those parts of the Earth's surface that have insufficient vegetation cover to support a human population. Desert_sentence_51

Potential evapotranspiration supplements the measurement of precipitation in providing a scientific measurement-based definition of a desert. Desert_sentence_52

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 the amount of surface storage of water. Desert_sentence_53

Evapotranspiration is the combination of water loss through atmospheric evaporation and through the life processes of plants. Desert_sentence_54

Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. Desert_sentence_55

As an example, Tucson, Arizona receives about 300 mm (12 in) of rain per year, however about 2,500 mm (98 in) of water could evaporate over the course of a year. Desert_sentence_56

In other words, about eight times more water could evaporate from the region than actually falls as rain. Desert_sentence_57

Rates of evapotranspiration in cold regions such as Alaska are much lower because of the lack of heat to aid in the evaporation process. Desert_sentence_58

Deserts are sometimes classified as "hot" or "cold", "semiarid" or "coastal". Desert_sentence_59

The characteristics of hot deserts include high temperatures in summer; greater evaporation than precipitation, usually exacerbated by high temperatures, strong winds and lack of cloud cover; considerable variation in the occurrence of precipitation, its intensity and distribution; and low humidity. Desert_sentence_60

Winter temperatures vary considerably between different deserts and are often related to the location of the desert on the continental landmass and the latitude. Desert_sentence_61

Daily variations in temperature can be as great as 22 °C (40 °F) or more, with heat loss by radiation at night being increased by the clear skies. Desert_sentence_62

Cold deserts, sometimes known as temperate deserts, occur at higher latitudes than hot deserts, and the aridity is caused by the dryness of the air. Desert_sentence_63

Some cold deserts are far from the ocean and others are separated by mountain ranges from the sea, and in both cases, there is insufficient moisture in the air to cause much precipitation. Desert_sentence_64

The largest of these deserts are found in Central Asia. Desert_sentence_65

Others occur on the eastern side of the Rocky Mountains, the eastern side of the southern Andes and in southern Australia. Desert_sentence_66

Polar deserts are a particular class of cold desert. Desert_sentence_67

The air is very cold and carries little moisture so little precipitation occurs and what does fall, usually snow, is carried along in the often strong wind and may form blizzards, drifts and dunes similar to those caused by dust and sand in other desert regions. Desert_sentence_68

In Antarctica, for example, the annual precipitation is about 50 mm (2 in) on the central plateau and some ten times that amount on some major peninsulas. Desert_sentence_69

Based on precipitation alone, hyperarid deserts receive less than 25 mm (1 in) of rainfall a year; they have no annual seasonal cycle of precipitation and experience twelve-month periods with no rainfall at all. Desert_sentence_70

Arid deserts receive between 25 and 200 mm (1 and 8 in) in a year and semiarid deserts between 200 and 500 mm (8 and 20 in). Desert_sentence_71

However, such factors as the temperature, humidity, rate of evaporation and evapotranspiration, and the moisture storage capacity of the ground have a marked effect on the degree of aridity and the plant and animal life that can be sustained. Desert_sentence_72

Rain falling in the cold season may be more effective at promoting plant growth, and defining the boundaries of deserts and the semiarid regions that surround them on the grounds of precipitation alone is problematic. Desert_sentence_73

A semi-arid desert or a steppe is a version of the arid desert with much more rainfall, vegetation and higher humidity. Desert_sentence_74

These regions feature a semi-arid climate and are less extreme than regular deserts. Desert_sentence_75

Like arid deserts, temperatures can vary greatly in semi deserts. Desert_sentence_76

They share some characteristics of a true desert and are usually located at the edge of deserts and continental dry areas. Desert_sentence_77

They usually receive precipitation from 250 mm (10 in) to 500 mm (20 in) but this can vary due to evapotranspiration and soil nutrition. Desert_sentence_78

Semi deserts can be found in the Tabernas Desert (and some of the Spanish Plateau), The Sahel, The Eurasian Steppe, most of Central Asia, the Western US, most of Northern Mexico, portions of South America (especially in Argentina) and the Australian Outback. Desert_sentence_79

They usually feature BSh (hot steppe) or BSk (temperate steppe) in the Köppen climate classification. Desert_sentence_80

Coastal deserts are mostly found on the western edges of continental land masses in regions where cold currents approach the land or cold water upwellings rise from the ocean depths. Desert_sentence_81

The cool winds crossing this water pick up little moisture and the coastal regions have low temperatures and very low rainfall, the main precipitation being in the form of fog and dew. Desert_sentence_82

The range of temperatures on a daily and annual scale is relatively low, being 11 °C (20 °F) and 5 °C (9 °F) respectively in the Atacama Desert. Desert_sentence_83

Deserts of this type are often long and narrow and bounded to the east by mountain ranges. Desert_sentence_84

They occur in Namibia, Chile, southern California and Baja California. Desert_sentence_85

Other coastal deserts influenced by cold currents are found in Western Australia, the Arabian Peninsula and Horn of Africa, and the western fringes of the Sahara. Desert_sentence_86

In 1961, Peveril Meigs divided desert regions on Earth into three categories according to the amount of precipitation they received. Desert_sentence_87

In this now widely accepted system, extremely arid lands have at least twelve consecutive months without precipitation, arid lands have less than 250 mm (10 in) of annual precipitation, and semiarid lands have a mean annual precipitation of between 250 and 500 mm (10–20 in). Desert_sentence_88

Both extremely arid and arid lands are considered to be deserts while semiarid lands are generally referred to as steppes when they are grasslands. Desert_sentence_89

Deserts are also classified, according to their geographical location and dominant weather pattern, as trade wind, mid-latitude, rain shadow, coastal, monsoon, or polar deserts. Desert_sentence_90

Trade wind deserts occur either side of the horse latitudes at 30° to 35° North and South. Desert_sentence_91

These belts are associated with the subtropical anticyclone and the large-scale descent of dry air moving from high-altitudes toward the poles. Desert_sentence_92

The Sahara Desert is of this type. Desert_sentence_93

Mid-latitude deserts occur between 30° and 50° North and South. Desert_sentence_94

They are mostly in areas remote from the sea where most of the moisture has already precipitated from the prevailing winds. Desert_sentence_95

They include the Tengger and Sonoran Deserts. Desert_sentence_96

Monsoon deserts are similar. Desert_sentence_97

They occur in regions where large temperature differences occur between sea and land. Desert_sentence_98

Moist warm air rises over the land, deposits its water content and circulates back to sea. Desert_sentence_99

Further inland, areas receive very little precipitation. Desert_sentence_100

The Thar Desert near the India/Pakistan border is of this type. Desert_sentence_101

In some parts of the world, deserts are created by a rain shadow effect. Desert_sentence_102

Orographic lift occurs as air masses rise to pass over high ground. Desert_sentence_103

In the process they cool and lose much of their moisture by precipitation on the windward slope of the mountain range. Desert_sentence_104

When they descend on the leeward side, they warm and their capacity to hold moisture increases so an area with relatively little precipitation occurs. Desert_sentence_105

The Taklamakan Desert is an example, lying in the rain shadow of the Himalayas and receiving less than 38 mm (1.5 in) precipitation annually. Desert_sentence_106

Other areas are arid by virtue of being a very long way from the nearest available sources of moisture. Desert_sentence_107

Montane deserts are arid places with a very high altitude; the most prominent example is found north of the Himalayas, in the Kunlun Mountains and the Tibetan Plateau. Desert_sentence_108

Many locations within this category have elevations exceeding 3,000 m (9,800 ft) and the thermal regime can be hemiboreal. Desert_sentence_109

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 and are often in the lee of mountain ranges. Desert_sentence_110

Montane deserts are normally cold, or may be scorchingly hot by day and very cold by night as is true of the northeastern slopes of Mount Kilimanjaro. Desert_sentence_111

Polar deserts such as McMurdo Dry Valleys remain ice-free because of the dry katabatic winds that flow downhill from the surrounding mountains. Desert_sentence_112

Former desert areas presently in non-arid environments, such as the Sandhills in Nebraska, are known as paleodeserts. Desert_sentence_113

In the Köppen climate classification system, deserts are classed as BWh (hot desert) or BWk (temperate desert). Desert_sentence_114

In the Thornthwaite climate classification system, deserts would be classified as arid megathermal climates. Desert_sentence_115

Weathering processes Desert_section_3

Main article: Weathering Desert_sentence_116

Deserts usually have a large diurnal and seasonal temperature range, with high daytime temperatures falling sharply at night. Desert_sentence_117

The diurnal range may be as much as 20 to 30 °C (36 to 54 °F) and the rock surface experiences even greater temperature differentials. Desert_sentence_118

During the day the sky is usually clear and most of the sun's radiation reaches the ground, but as soon as the sun sets, the desert cools quickly by radiating heat into space. Desert_sentence_119

In hot deserts, the temperature during daytime can exceed 45 °C (113 °F) in summer and plunge below freezing point at night during winter. Desert_sentence_120

Such large temperature variations have a destructive effect on the exposed rocky surfaces. Desert_sentence_121

The repeated fluctuations put a strain on exposed rock and the flanks of mountains crack and shatter. Desert_sentence_122

Fragmented strata slide down into the valleys where they continue to break into pieces due to the relentless sun by day and chill by night. Desert_sentence_123

Successive strata are exposed to further weathering. Desert_sentence_124

The relief of the internal pressure that has built up in rocks that have been underground for aeons can cause them to shatter. Desert_sentence_125

Exfoliation also occurs when the outer surfaces of rocks split off in flat flakes. Desert_sentence_126

This is believed to be caused by the stresses put on the rock by repeated thermal expansions and contractions which induces fracturing parallel to the original surface. Desert_sentence_127

Chemical weathering processes probably play a more important role in deserts than was previously thought. Desert_sentence_128

The necessary moisture may be present in the form of dew or mist. Desert_sentence_129

Ground water may be drawn to the surface by evaporation and the formation of salt crystals may dislodge rock particles as sand or disintegrate rocks by exfoliation. Desert_sentence_130

Shallow caves are sometimes formed at the base of cliffs by this means. Desert_sentence_131

As the desert mountains decay, large areas of shattered rock and rubble occur. Desert_sentence_132

The process continues and the end products are either dust or sand. Desert_sentence_133

Dust is formed from solidified clay or volcanic deposits whereas sand results from the fragmentation of harder granites, limestone and sandstone. Desert_sentence_134

There is a certain critical size (about 0.5 mm) below which further temperature-induced weathering of rocks does not occur and this provides a minimum size for sand grains. Desert_sentence_135

As the mountains are eroded, more and more sand is created. Desert_sentence_136

At high wind speeds, sand grains are picked up off the surface and blown along, a process known as saltation. Desert_sentence_137

The whirling airborne grains act as a sand blasting mechanism which grinds away solid objects in its path as the kinetic energy of the wind is transferred to the ground. Desert_sentence_138

The sand eventually ends up deposited in level areas known as sand-fields or sand-seas, or piled up in dunes. Desert_sentence_139

Dust storms and sandstorms Desert_section_4

Main article: Dust storm Desert_sentence_140

Sand and dust storms are natural events that occur in arid regions where the land is not protected by a covering of vegetation. Desert_sentence_141

Dust storms usually start in desert margins rather than the deserts themselves where the finer materials have already been blown away. Desert_sentence_142

As a steady wind begins to blow, fine particles lying on the exposed ground begin to vibrate. Desert_sentence_143

At greater wind speeds, some particles are lifted into the air stream. Desert_sentence_144

When they land, they strike other particles which may be jerked into the air in their turn, starting a chain reaction. Desert_sentence_145

Once ejected, these particles move in one of three possible ways, depending on their size, shape and density; suspension, saltation or creep. Desert_sentence_146

Suspension is only possible for particles less than 0.1 mm (0.004 in) in diameter. Desert_sentence_147

In a dust storm, these fine particles are lifted up and wafted aloft to heights of up to 6 km (3.7 mi). Desert_sentence_148

They reduce visibility and can remain in the atmosphere for days on end, conveyed by the trade winds for distances of up to 6,000 km (3,700 mi). Desert_sentence_149

Denser clouds of dust can be formed in stronger winds, moving across the land with a billowing leading edge. Desert_sentence_150

The sunlight can be obliterated and it may become as dark as night at ground level. Desert_sentence_151

In a study of a dust storm in China in 2001, it was estimated that 6.5 million tons of dust were involved, covering an area of 134,000,000 km (52,000,000 sq mi). Desert_sentence_152

The mean particle size was 1.44 μm. Desert_sentence_153

A much smaller scale, short-lived phenomenon can occur in calm conditions when hot air near the ground rises quickly through a small pocket of cooler, low-pressure air above forming a whirling column of particles, a dust devil. Desert_sentence_154

Sandstorms occur with much less frequency than dust storms. Desert_sentence_155

They are often preceded by severe dust storms and occur when the wind velocity increases to a point where it can lift heavier particles. Desert_sentence_156

These grains of sand, up to about 0.5 mm (0.020 in) in diameter are jerked into the air but soon fall back to earth, ejecting other particles in the process. Desert_sentence_157

Their weight prevents them from being airborne for long and most only travel a distance of a few meters (yards). Desert_sentence_158

The sand streams along above the surface of the ground like a fluid, often rising to heights of about 30 cm (12 in). Desert_sentence_159

In a really severe steady blow, 2 m (6 ft 7 in) is about as high as the sand stream can rise as the largest sand grains do not become airborne at all. Desert_sentence_160

They are transported by creep, being rolled along the desert floor or performing short jumps. Desert_sentence_161

During a sandstorm, the wind-blown sand particles become electrically charged. Desert_sentence_162

Such electric fields, which range in size up to 80 kV/m, can produce sparks and cause interference with telecommunications equipment. Desert_sentence_163

They are also unpleasant for humans and can cause headaches and nausea. Desert_sentence_164

The electric fields are caused by the collision between airborne particles and by the impacts of saltating sand grains landing on the ground. Desert_sentence_165

The mechanism is little understood but the particles usually have a negative charge when their diameter is under 250 μm and a positive one when they are over 500 μm. Desert_sentence_166

Major deserts Desert_section_5

See also: List of deserts by area Desert_sentence_167

Deserts take up about one third of the Earth's land surface. Desert_sentence_168

Bottomlands may be salt-covered flats. Desert_sentence_169

Eolian processes are major factors in shaping desert landscapes. Desert_sentence_170

Polar deserts (also seen as "cold deserts") have similar features, except the main form of precipitation is snow rather than rain. Desert_sentence_171

Antarctica is the world's largest cold desert (composed of about 98% thick continental ice sheet and 2% barren rock). Desert_sentence_172

Some of the barren rock is to be found in the so-called Dry Valleys of Antarctica that almost never get snow, which can have ice-encrusted saline lakes that suggest evaporation far greater than the rare snowfall due to the strong katabatic winds that even evaporate ice. Desert_sentence_173

Deserts, both hot and cold, play a part in moderating the Earth's temperature. Desert_sentence_174

This is because they reflect more of the incoming light and their albedo is higher than that of forests or the sea. Desert_sentence_175

Features Desert_section_6

Many people think of deserts as consisting of extensive areas of billowing sand dunes because that is the way they are often depicted on TV and in films, but deserts do not always look like this. Desert_sentence_176

Across the world, around 20% of desert is sand, varying from only 2% in North America to 30% in Australia and over 45% in Central Asia. Desert_sentence_177

Where sand does occur, it is usually in large quantities in the form of sand sheets or extensive areas of dunes. Desert_sentence_178

A sand sheet is a near-level, firm expanse of partially consolidated particles in a layer that varies from a few centimeters to a few meters thick. Desert_sentence_179

The structure of the sheet consists of thin horizontal layers of coarse silt and very fine to medium grain sand, separated by layers of coarse sand and pea-gravel which are a single grain thick. Desert_sentence_180

These larger particles anchor the other particles in place and may also be packed together on the surface so as to form a miniature desert pavement. Desert_sentence_181

Small ripples form on the sand sheet when the wind exceeds 24 km/h (15 mph). Desert_sentence_182

They form perpendicular to the wind direction and gradually move across the surface as the wind continues to blow. Desert_sentence_183

The distance between their crests corresponds to the average length of jumps made by particles during saltation. Desert_sentence_184

The ripples are ephemeral and a change in wind direction causes them to reorganise. Desert_sentence_185

Sand dunes are accumulations of windblown sand piled up in mounds or ridges. Desert_sentence_186

They form downwind of copious sources of dry, loose sand and occur when topographic and climatic conditions cause airborne particles to settle. Desert_sentence_187

As the wind blows, saltation and creep take place on the windward side of the dune and individual grains of sand move uphill. Desert_sentence_188

When they reach the crest, they cascade down the far side. Desert_sentence_189

The upwind slope typically has a gradient of 10° to 20° while the lee slope is around 32°, the angle at which loose dry sand will slip. Desert_sentence_190

As this wind-induced movement of sand grains takes place, the dune moves slowly across the surface of the ground. Desert_sentence_191

Dunes are sometimes solitary, but they are more often grouped together in dune fields. Desert_sentence_192

When these are extensive, they are known as sand seas or ergs. Desert_sentence_193

The shape of the dune depends on the characteristics of the prevailing wind. Desert_sentence_194

Barchan dunes are produced by strong winds blowing across a level surface and are crescent-shaped with the concave side away from the wind. Desert_sentence_195

When there are two directions from which winds regularly blow, a series of long, linear dunes known as seif dunes may form. Desert_sentence_196

These also occur parallel to a strong wind that blows in one general direction. Desert_sentence_197

Transverse dunes run at a right angle to the prevailing wind direction. Desert_sentence_198

Star dunes are formed by variable winds, and have several ridges and slip faces radiating from a central point. Desert_sentence_199

They tend to grow vertically; they can reach a height of 500 m (1,600 ft), making them the tallest type of dune. Desert_sentence_200

Rounded mounds of sand without a slip face are the rare dome dunes, found on the upwind edges of sand seas. Desert_sentence_201

A large part of the surface area of the world's deserts consists of flat, stone-covered plains dominated by wind erosion. Desert_sentence_202

In "eolian deflation", the wind continually removes fine-grained material, which becomes wind-blown sand. Desert_sentence_203

This exposes coarser-grained material, mainly pebbles with some larger stones or cobbles, leaving a desert pavement, an area of land overlaid by closely packed smooth stones forming a tessellated mosaic. Desert_sentence_204

Different theories exist as to how exactly the pavement is formed. Desert_sentence_205

It may be that after the sand and dust is blown away by the wind the stones jiggle themselves into place; alternatively, stones previously below ground may in some way work themselves to the surface. Desert_sentence_206

Very little further erosion takes place after the formation of a pavement, and the ground becomes stable. Desert_sentence_207

Evaporation brings moisture to the surface by capillary action and calcium salts may be precipitated, binding particles together to form a desert conglomerate. Desert_sentence_208

In time, bacteria that live on the surface of the stones accumulate a film of minerals and clay particles, forming a shiny brown coating known as desert varnish. Desert_sentence_209

Other non-sandy deserts consist of exposed outcrops of bedrock, dry soils or aridisols, and a variety of landforms affected by flowing water, such as alluvial fans, sinks or playas, temporary or permanent lakes, and oases. Desert_sentence_210

A hamada is a type of desert landscape consisting of a high rocky plateau where the sand has been removed by aeolian processes. Desert_sentence_211

Other landforms include plains largely covered by gravels and angular boulders, from which the finer particles have been stripped by the wind. Desert_sentence_212

These are called "reg" in the western Sahara, "serir" in the eastern Sahara, "gibber plains" in Australia and "saï" in central Asia. Desert_sentence_213

The Tassili Plateau in Algeria is an impressive jumble of eroded sandstone outcrops, canyons, blocks, pinnacles, fissures, slabs and ravines. Desert_sentence_214

In some places the wind has carved holes or arches, and in others, it has created mushroom-like pillars narrower at the base than the top. Desert_sentence_215

In the Colorado Plateau it is water that has been the eroding force. Desert_sentence_216

Here the Colorado River has cut its way over the millennia through the high desert floor creating a canyon that is over a mile (6,000 feet or 1,800 meters) deep in places, exposing strata that are over two billion years old. Desert_sentence_217

Water Desert_section_7

One of the driest places on Earth is the Atacama Desert. Desert_sentence_218

It is virtually devoid of life because it is blocked from receiving precipitation by the Andes mountains to the east and the Chilean Coast Range to the west. Desert_sentence_219

The cold Humboldt Current and the anticyclone of the Pacific are essential to keep the dry climate of the Atacama. Desert_sentence_220

The average precipitation in the Chilean region of Antofagasta is just 1 mm (0.039 in) per year. Desert_sentence_221

Some weather stations in the Atacama have never received rain. Desert_sentence_222

Evidence suggests that the Atacama may not have had any significant rainfall from 1570 to 1971. Desert_sentence_223

It is so arid that mountains that reach as high as 6,885 m (22,589 ft) 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 m (14,400 ft) and is continuous above 5,600 m (18,400 ft). Desert_sentence_224

Nevertheless, there is some plant life in the Atacama, in the form of specialist plants that obtain moisture from dew and the fogs that blow in from the Pacific. Desert_sentence_225

When rain falls in deserts, as it occasionally does, it is often with great violence. Desert_sentence_226

The desert surface is evidence of this with dry stream channels known as arroyos or wadis meandering across its surface. Desert_sentence_227

These can experience flash floods, becoming raging torrents with surprising rapidity after a storm that may be many kilometers away. Desert_sentence_228

Most deserts are in basins with no drainage to the sea but some are crossed by exotic rivers sourced in mountain ranges or other high rainfall areas beyond their borders. Desert_sentence_229

The River Nile, the Colorado River and the Yellow River do this, losing much of their water through evaporation as they pass through the desert and raising groundwater levels nearby. Desert_sentence_230

There may also be underground sources of water in deserts in the form of springs, aquifers, underground rivers or lakes. Desert_sentence_231

Where these lie close to the surface, wells can be dug and oases may form where plant and animal life can flourish. Desert_sentence_232

The Nubian Sandstone Aquifer System under the Sahara Desert is the largest known accumulation of fossil water. Desert_sentence_233

The Great Man-Made River is a scheme launched by Libya's Muammar Gadaffi to tap this aquifer and supply water to coastal cities. Desert_sentence_234

Kharga Oasis in Egypt is 150 km (93 mi) long and is the largest oasis in the Libyan Desert. Desert_sentence_235

A lake occupied this depression in ancient times and thick deposits of sandy-clay resulted. Desert_sentence_236

Wells are dug to extract water from the porous sandstone that lies underneath. Desert_sentence_237

Seepages may occur in the walls of canyons and pools may survive in deep shade near the dried up watercourse below. Desert_sentence_238

Lakes may form in basins where there is sufficient precipitation or meltwater from glaciers above. Desert_sentence_239

They are usually shallow and saline, and wind blowing over their surface can cause stress, moving the water over nearby low-lying areas. Desert_sentence_240

When the lakes dry up, they leave a crust or hardpan behind. Desert_sentence_241

This area of deposited clay, silt or sand is known as a playa. Desert_sentence_242

The deserts of North America have more than one hundred playas, many of them relics of Lake Bonneville which covered parts of Utah, Nevada and Idaho during the last ice age when the climate was colder and wetter. Desert_sentence_243

These include the Great Salt Lake, Utah Lake, Sevier Lake and many dry lake beds. Desert_sentence_244

The smooth flat surfaces of playas have been used for attempted vehicle speed records at Black Rock Desert and Bonneville Speedway and the United States Air Force uses Rogers Dry Lake in the Mojave Desert as runways for aircraft and the space shuttle. Desert_sentence_245

Ecology and biogeography Desert_section_8

Deserts and semi-deserts are home to ecosystems with low or very low biomass and primary productivity in arid or semi-arid climates. Desert_sentence_246

They are mostly found in subtropical high-pressure belts and major continental rain shadows. Desert_sentence_247

Primary productivity depends on low densities of small photoautotrophs that sustain a sparse trophic network. Desert_sentence_248

Plant growth is limited by rainfall, temperature extremes and desiccating winds. Desert_sentence_249

Deserts have strong temporal variability in the availability of resources due to the total amount of annual rainfall and the size of individual rainfall events. Desert_sentence_250

Resources are often ephemeral or episodic, and this triggers sporadic animal movements and ‘pulse and reserve’ or ‘boom-bust’ ecosystem dynamics. Desert_sentence_251

Erosion and sedimentation are high due to the sparse vegetation cover and the activities of large mammals and people. Desert_sentence_252

Plants and animals in deserts are mostly adapted to extreme and prolonged water deficits, but their reproductive phenology often responds to short episodes of surplus. Desert_sentence_253

Competitive interactions are weak. Desert_sentence_254

Flora Desert_section_9

Plants face severe challenges in arid environments. Desert_sentence_255

Problems they need to solve include how to obtain enough water, how to avoid being eaten and how to reproduce. Desert_sentence_256

Photosynthesis is the key to plant growth. Desert_sentence_257

It can only take place during the day as energy from the sun is required, but during the day, many deserts become very hot. Desert_sentence_258

Opening stomata to allow in the carbon dioxide necessary for the process causes evapotranspiration, and conservation of water is a top priority for desert vegetation. Desert_sentence_259

Some plants have resolved this problem by adopting crassulacean acid metabolism, allowing them to open their stomata during the night to allow CO2 to enter, and close them during the day, or by using C4 carbon fixation. Desert_sentence_260

Many desert plants have reduced the size of their leaves or abandoned them altogether. Desert_sentence_261

Cacti are desert specialists, and in most species, the leaves have been dispensed with and the chlorophyll displaced into the trunks, the cellular structure of which has been modified to allow them to store water. Desert_sentence_262

When rain falls, the water is rapidly absorbed by the shallow roots and retained to allow them to survive until the next downpour, which may be months or years away. Desert_sentence_263

The giant saguaro cacti of the Sonoran Desert form "forests", providing shade for other plants and nesting places for desert birds. Desert_sentence_264

Saguaro grows slowly but may live for up to two hundred years. Desert_sentence_265

The surface of the trunk is folded like a concertina, allowing it to expand, and a large specimen can hold eight tons of water after a good downpour. Desert_sentence_266

Cacti are present in both North and South America with a post-Gondwana origin. Desert_sentence_267

Other xerophytic plants have developed similar strategies by a process known as convergent evolution. Desert_sentence_268

They limit water loss by reducing the size and number of stomata, by having waxy coatings and hairy or tiny leaves. Desert_sentence_269

Some are deciduous, shedding their leaves in the driest season, and others curl their leaves up to reduce transpiration. Desert_sentence_270

Others store water in succulent leaves or stems or in fleshy tubers. Desert_sentence_271

Desert plants maximize water uptake by having shallow roots that spread widely, or by developing long taproots that reach down to deep rock strata for ground water. Desert_sentence_272

The saltbush in Australia has succulent leaves and secretes salt crystals, enabling it to live in saline areas. Desert_sentence_273

In common with cacti, many have developed spines to ward off browsing animals. Desert_sentence_274

Some desert plants produce seed which lies dormant in the soil until sparked into growth by rainfall. Desert_sentence_275

With annuals, such plants grow with great rapidity and may flower and set seed within weeks, aiming to complete their development before the last vestige of water dries up. Desert_sentence_276

For perennial plants, reproduction is more likely to be successful if the seed germinates in a shaded position, but not so close to the parent plant as to be in competition with it. Desert_sentence_277

Some seed will not germinate until it has been blown about on the desert floor to scarify the seed coat. Desert_sentence_278

The seed of the mesquite tree, which grows in deserts in the Americas, is hard and fails to sprout even when planted carefully. Desert_sentence_279

When it has passed through the gut of a pronghorn it germinates readily, and the little pile of moist dung provides an excellent start to life well away from the parent tree. Desert_sentence_280

The stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect the ground from erosion. Desert_sentence_281

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. Desert_sentence_282

Cold deserts often have high concentrations of salt in the soil. Desert_sentence_283

Grasses and low shrubs are the dominant vegetation here and the ground may be covered with lichens. Desert_sentence_284

Most shrubs have spiny leaves and shed them in the coldest part of the year. Desert_sentence_285

Fauna Desert_section_10

Main article: Xerocole Desert_sentence_286

Animals adapted to live in deserts are called xerocoles. Desert_sentence_287

There is no evidence that body temperature of mammals and birds is adaptive to the different climates, either of great heat or cold. Desert_sentence_288

In fact, with a very few exceptions, their basal metabolic rate is determined by body size, irrespective of the climate in which they live. Desert_sentence_289

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. Desert_sentence_290

One well-studied example is the specializations of mammalian kidneys shown by desert-inhabiting species. Desert_sentence_291

Many examples of convergent evolution have been identified in desert organisms, including between cacti and Euphorbia, kangaroo rats and jerboas, Phrynosoma and Moloch lizards. Desert_sentence_292

Deserts present a very challenging environment for animals. Desert_sentence_293

Not only do they require food and water but they also need to keep their body temperature at a tolerable level. Desert_sentence_294

In many ways, birds are the ablest to do this of the higher animals. Desert_sentence_295

They can move to areas of greater food availability as the desert blooms after local rainfall and can fly to faraway waterholes. Desert_sentence_296

In hot deserts, gliding birds can remove themselves from the over-heated desert floor by using thermals to soar in the cooler air at great heights. Desert_sentence_297

In order to conserve energy, other desert birds run rather than fly. Desert_sentence_298

The cream-colored courser flits gracefully across the ground on its long legs, stopping periodically to snatch up insects. Desert_sentence_299

Like other desert birds, it is well-camouflaged by its coloring and can merge into the landscape when stationary. Desert_sentence_300

The sandgrouse is an expert at this and nests on the open desert floor dozens of kilometers (miles) away from the waterhole it needs to visit daily. Desert_sentence_301

Some small diurnal birds are found in very restricted localities where their plumage matches the color of the underlying surface. Desert_sentence_302

The desert lark takes frequent dust baths which ensures that it matches its environment. Desert_sentence_303

Water and carbon dioxide are metabolic end products of oxidation of fats, proteins, and carbohydrates. Desert_sentence_304

Oxidising a gram of carbohydrate produces 0.60 grams of water; a gram of protein produces 0.41 grams of water; and a gram of fat produces 1.07 grams of water, making it possible for xerocoles to live with little or no access to drinking water. Desert_sentence_305

The kangaroo rat for example makes use of this water of metabolism and conserves water both by having a low basal metabolic rate and by remaining underground during the heat of the day, reducing loss of water through its skin and respiratory system when at rest. Desert_sentence_306

Herbivorous mammals obtain moisture from the plants they eat. Desert_sentence_307

Species such as the addax antelope, dik-dik, Grant's gazelle and oryx are so efficient at doing this that they apparently never need to drink. Desert_sentence_308

The camel is a superb example of a mammal adapted to desert life. Desert_sentence_309

It minimizes its water loss by producing concentrated urine and dry dung, and is able to lose 40% of its body weight through water loss without dying of dehydration. Desert_sentence_310

Carnivores can obtain much of their water needs from the body fluids of their prey. Desert_sentence_311

Many other hot desert animals are nocturnal, seeking out shade during the day or dwelling underground in burrows. Desert_sentence_312

At depths of more than 50 cm (20 in), these remain at between 30 to 32 °C (86 to 90 °F) regardless of the external temperature. Desert_sentence_313

Jerboas, desert rats, kangaroo rats and other small rodents emerge from their burrows at night and so do the foxes, coyotes, jackals and snakes that prey on them. Desert_sentence_314

Kangaroos keep cool by increasing their respiration rate, panting, sweating and moistening the skin of their forelegs with saliva. Desert_sentence_315

Mammals living in cold deserts have developed greater insulation through warmer body fur and insulating layers of fat beneath the skin. Desert_sentence_316

The arctic weasel has a metabolic rate that is two or three times as high as would be expected for an animal of its size. Desert_sentence_317

Birds have avoided the problem of losing heat through their feet by not attempting to maintain them at the same temperature as the rest of their bodies, a form of adaptive insulation. Desert_sentence_318

The emperor penguin has dense plumage, a downy under layer, an air insulation layer next to the skin and various thermoregulatory strategies to maintain its body temperature in one of the harshest environments on Earth. Desert_sentence_319

Being ectotherms, reptiles are unable to live in cold deserts but are well-suited to hot ones. Desert_sentence_320

In the heat of the day in the Sahara, the temperature can rise to 50 °C (122 °F). Desert_sentence_321

Reptiles cannot survive at this temperature and lizards will be prostrated by heat at 45 °C (113 °F). Desert_sentence_322

They have few adaptations to desert life and are unable to cool themselves by sweating so they shelter during the heat of the day. Desert_sentence_323

In the first part of the night, as the ground radiates the heat absorbed during the day, they emerge and search for prey. Desert_sentence_324

Lizards and snakes are the most numerous in arid regions and certain snakes have developed a novel method of locomotion that enables them to move sidewards and navigate high sand-dunes. Desert_sentence_325

These include the horned viper of Africa and the sidewinder of North America, evolutionarily distinct but with similar behavioural patterns because of convergent evolution. Desert_sentence_326

Many desert reptiles are ambush predators and often bury themselves in the sand, waiting for prey to come within range. Desert_sentence_327

Amphibians might seem unlikely desert-dwellers, because of their need to keep their skins moist and their dependence on water for reproductive purposes. Desert_sentence_328

In fact, the few species that are found in this habitat have made some remarkable adaptations. Desert_sentence_329

Most of them are fossorial, spending the hot dry months aestivating in deep burrows. Desert_sentence_330

While there they shed their skins a number of times and retain the remnants around them as a waterproof cocoon to retain moisture. Desert_sentence_331

In the Sonoran Desert, Couch's spadefoot toad spends most of the year dormant in its burrow. Desert_sentence_332

Heavy rain is the trigger for emergence and the first male to find a suitable pool calls to attract others. Desert_sentence_333

Eggs are laid and the tadpoles grow rapidly as they must reach metamorphosis before the water evaporates. Desert_sentence_334

As the desert dries out, the adult toads rebury themselves. Desert_sentence_335

The juveniles stay on the surface for a while, feeding and growing, but soon dig themselves burrows. Desert_sentence_336

Few make it to adulthood. Desert_sentence_337

The water holding frog in Australia has a similar life cycle and may aestivate for as long as five years if no rain falls. Desert_sentence_338

The Desert rain frog of Namibia is nocturnal and survives because of the damp sea fogs that roll in from the Atlantic. Desert_sentence_339

Invertebrates, particularly arthropods, have successfully made their homes in the desert. Desert_sentence_340

Flies, beetles, ants, termites, locusts, millipedes, scorpions and spiders have hard cuticles which are impervious to water and many of them lay their eggs underground and their young develop away from the temperature extremes at the surface. Desert_sentence_341

The Saharan silver ant (Cataglyphis bombycina) uses a heat shock protein in a novel way and forages in the open during brief forays in the heat of the day. Desert_sentence_342

The long-legged darkling beetle in Namibia stands on its front legs and raises its carapace to catch the morning mist as condensate, funnelling the water into its mouth. Desert_sentence_343

Some arthropods make use of the ephemeral pools that form after rain and complete their life cycle in a matter of days. Desert_sentence_344

The desert shrimp does this, appearing "miraculously" in new-formed puddles as the dormant eggs hatch. Desert_sentence_345

Others, such as brine shrimps, fairy shrimps and tadpole shrimps, are cryptobiotic and can lose up to 92% of their bodyweight, rehydrating as soon as it rains and their temporary pools reappear. Desert_sentence_346

Human relations Desert_section_11

Humans have long made use of deserts as places to live, and more recently have started to exploit them for minerals and energy capture. Desert_sentence_347

Deserts play a significant role in human culture with an extensive literature. Desert_sentence_348

History Desert_section_12

People have been living in deserts for millennia. Desert_sentence_349

Many, such as the Bushmen in the Kalahari, the Aborigines in Australia and various tribes of North American Indians, were originally hunter-gatherers. Desert_sentence_350

They developed skills in the manufacture and use of weapons, animal tracking, finding water, foraging for edible plants and using the things they found in their natural environment to supply their everyday needs. Desert_sentence_351

Their self-sufficient skills and knowledge were passed down through the generations by word of mouth. Desert_sentence_352

Other cultures developed a nomadic way of life as herders of sheep, goats, cattle, camels, yaks, llamas or reindeer. Desert_sentence_353

They travelled over large areas with their herds, moving to new pastures as seasonal and erratic rainfall encouraged new plant growth. Desert_sentence_354

They took with them their tents made of cloth or skins draped over poles and their diet included milk, blood and sometimes meat. Desert_sentence_355

The desert nomads were also traders. Desert_sentence_356

The Sahara is a very large expanse of land stretching from the Atlantic rim to Egypt. Desert_sentence_357

Trade routes were developed linking the Sahel in the south with the fertile Mediterranean region to the north and large numbers of camels were used to carry valuable goods across the desert interior. Desert_sentence_358

The Tuareg were traders and the goods transported traditionally included slaves, ivory and gold going northwards and salt going southwards. Desert_sentence_359

Berbers with knowledge of the region were employed to guide the caravans between the various oases and wells. Desert_sentence_360

Several million slaves may have been taken northwards across the Sahara between the 8th and 18th centuries. Desert_sentence_361

Traditional means of overland transport declined with the advent of motor vehicles, shipping and air freight, but caravans still travel along routes between Agadez and Bilma and between Timbuktu and Taoudenni carrying salt from the interior to desert-edge communities. Desert_sentence_362

Round the rims of deserts, where more precipitation occurred and conditions were more suitable, some groups took to cultivating crops. Desert_sentence_363

This may have happened when drought caused the death of herd animals, forcing herdsmen to turn to cultivation. Desert_sentence_364

With few inputs, they were at the mercy of the weather and may have lived at bare subsistence level. Desert_sentence_365

The land they cultivated reduced the area available to nomadic herders, causing disputes over land. Desert_sentence_366

The semi-arid fringes of the desert have fragile soils which are at risk of erosion when exposed, as happened in the American Dust Bowl in the 1930s. Desert_sentence_367

The grasses that held the soil in place were ploughed under, and a series of dry years caused crop failures, while enormous dust storms blew the topsoil away. Desert_sentence_368

Half a million Americans were forced to leave their land in this catastrophe. Desert_sentence_369

Similar damage is being done today to the semi-arid areas that rim deserts and about twelve million hectares of land are being turned to desert each year. Desert_sentence_370

Desertification is caused by such factors as drought, climatic shifts, tillage for agriculture, overgrazing and deforestation. Desert_sentence_371

Vegetation plays a major role in determining the composition of the soil. Desert_sentence_372

In many environments, the rate of erosion and run off increases dramatically with reduced vegetation cover. Desert_sentence_373

Natural resource extraction Desert_section_13

Deserts contain substantial mineral resources, sometimes over their entire surface, giving them their characteristic colors. Desert_sentence_374

For example, the red of many sand deserts comes from laterite minerals. Desert_sentence_375

Geological processes in a desert climate can concentrate minerals into valuable deposits. Desert_sentence_376

Leaching by ground water can extract ore minerals and redeposit them, according to the water table, in concentrated form. Desert_sentence_377

Similarly, evaporation tends to concentrate minerals in desert lakes, creating dry lake beds or playas rich in minerals. Desert_sentence_378

Evaporation can concentrate minerals as a variety of evaporite deposits, including gypsum, sodium nitrate, sodium chloride and borates. Desert_sentence_379

Evaporites are found in the USA's Great Basin Desert, historically exploited by the "20-mule teams" pulling carts of borax from Death Valley to the nearest railway. Desert_sentence_380

A desert especially rich in mineral salts is the Atacama Desert, Chile, where sodium nitrate has been mined for explosives and fertilizer since around 1850. Desert_sentence_381

Other desert minerals are copper from Chile, Peru, and Iran, and iron and uranium in Australia. Desert_sentence_382

Many other metals, salts and commercially valuable types of rock such as pumice are extracted from deserts around the world. Desert_sentence_383

Oil and gas form on the bottom of shallow seas when micro-organisms decompose under anoxic conditions and later become covered with sediment. Desert_sentence_384

Many deserts were at one time the sites of shallow seas and others have had underlying hydrocarbon deposits transported to them by the movement of tectonic plates. Desert_sentence_385

Some major oilfields such as Ghawar are found under the sands of Saudi Arabia. Desert_sentence_386

Geologists believe that other oil deposits were formed by aeolian processes in ancient deserts as may be the case with some of the major American oil fields. Desert_sentence_387

Farming Desert_section_14

Main article: Desert farming Desert_sentence_388

Traditional desert farming systems have long been established in North Africa, irrigation being the key to success in an area where water stress is a limiting factor to growth. Desert_sentence_389

Techniques that can be used include drip irrigation, the use of organic residues or animal manures as fertilisers and other traditional agricultural management practices. Desert_sentence_390

Once fertility has been built up, further crop production preserves the soil from destruction by wind and other forms of erosion. Desert_sentence_391

It has been found that plant growth-promoting bacteria play a role in increasing the resistance of plants to stress conditions and these rhizobacterial suspensions could be inoculated into the soil in the vicinity of the plants. Desert_sentence_392

A study of these microbes found that desert farming hampers desertification by establishing islands of fertility allowing farmers to achieve increased yields despite the adverse environmental conditions. Desert_sentence_393

A field trial in the Sonoran Desert which exposed the roots of different species of tree to rhizobacteria and the nitrogen fixing bacterium Azospirillum brasilense with the aim of restoring degraded lands was only partially successful. Desert_sentence_394

The Judean Desert was farmed in the 7th century BC during the Iron Age to supply food for desert forts. Desert_sentence_395

Native Americans in the south western United States became agriculturalists around 600 AD when seeds and technologies became available from Mexico. Desert_sentence_396

They used terracing techniques and grew gardens beside seeps, in moist areas at the foot of dunes, near streams providing flood irrigation and in areas irrigated by extensive specially built canals. Desert_sentence_397

The Hohokam tribe constructed over 500 miles (800 km) of large canals and maintained them for centuries, an impressive feat of engineering. Desert_sentence_398

They grew maize, beans, squash and peppers. Desert_sentence_399

A modern example of desert farming is the Imperial Valley in California, which has high temperatures and average rainfall of just 3 in (76 mm) per year. Desert_sentence_400

The economy is heavily based on agriculture and the land is irrigated through a network of canals and pipelines sourced entirely from the Colorado River via the All-American Canal. Desert_sentence_401

The soil is deep and fertile, being part of the river's flood plains, and what would otherwise have been desert has been transformed into one of the most productive farming regions in California. Desert_sentence_402

Other water from the river is piped to urban communities but all this has been at the expense of the river, which below the extraction sites no longer has any above-ground flow during most of the year. Desert_sentence_403

Another problem of growing crops in this way is the build-up of salinity in the soil caused by the evaporation of river water. Desert_sentence_404

The greening of the desert remains an aspiration and was at one time viewed as a future means for increasing food production for the world's growing population. Desert_sentence_405

This prospect has proved false as it disregarded the environmental damage caused elsewhere by the diversion of water for desert project irrigation. Desert_sentence_406

Solar energy capture Desert_section_15

Deserts are increasingly seen as sources for solar energy, partly due to low amounts of cloud cover. Desert_sentence_407

Many solar power plants have been built in the Mojave Desert such as the Solar Energy Generating Systems and Ivanpah Solar Power Facility. Desert_sentence_408

Large swaths of this desert are covered in mirrors. Desert_sentence_409

The potential for generating solar energy from the Sahara Desert is huge, the highest found on the globe. Desert_sentence_410

Professor David Faiman of Ben-Gurion University has stated that the technology now exists to supply all of the world's electricity needs from 10% of the Sahara Desert. Desert_sentence_411

Desertec Industrial Initiative was a consortium seeking $560 billion to invest in North African solar and wind installations over the next forty years to supply electricity to Europe via cable lines running under the Mediterranean Sea. Desert_sentence_412

European interest in the Sahara Desert stems from its two aspects: the almost continual daytime sunshine and plenty of unused land. Desert_sentence_413

The Sahara receives more sunshine per acre than any part of Europe. Desert_sentence_414

The Sahara Desert also has the empty space totalling hundreds of square miles required to house fields of mirrors for solar plants. Desert_sentence_415

The Negev Desert, Israel, and the surrounding area, including the Arava Valley, receive plenty of sunshine and are generally not arable. Desert_sentence_416

This has resulted in the construction of many solar plants. Desert_sentence_417

David Faiman has proposed that "giant" solar plants in the Negev could supply all of Israel's needs for electricity. Desert_sentence_418

Warfare Desert_section_16

Main article: Desert warfare Desert_sentence_419

The Arabs were probably the first organized force to conduct successful battles in the desert. Desert_sentence_420

By knowing back routes and the locations of oases and by utilizing camels, Muslim Arab forces were able to successfully overcome both Roman and Persian forces in the period 600 to 700 AD during the expansion of the Islamic caliphate. Desert_sentence_421

Many centuries later, both world wars saw fighting in the desert. Desert_sentence_422

In the First World War, the Ottoman Turks were engaged with the British regular army in a campaign that spanned the Arabian peninsula. Desert_sentence_423

The Turks were defeated by the British, who had the backing of irregular Arab forces that were seeking to revolt against the Turks in the Hejaz, made famous in T.E. Desert_sentence_424 Lawrence's book Seven Pillars of Wisdom. Desert_sentence_425

In the Second World War, the Western Desert Campaign began in Italian Libya. Desert_sentence_426

Warfare in the desert offered great scope for tacticians to use the large open spaces without the distractions of casualties among civilian populations. Desert_sentence_427

Tanks and armoured vehicles were able to travel large distances unimpeded and land mines were laid in large numbers. Desert_sentence_428

However, the size and harshness of the terrain meant that all supplies needed to be brought in from great distances. Desert_sentence_429

The victors in a battle would advance and their supply chain would necessarily become longer, while the defeated army could retreat, regroup and resupply. Desert_sentence_430

For these reasons, the front line moved back and forth through hundreds of kilometers as each side lost and regained momentum. Desert_sentence_431

Its most easterly point was at El Alamein in Egypt, where the Allies decisively defeated the Axis forces in 1942. Desert_sentence_432

In culture Desert_section_17

The desert is generally thought of as a barren and empty landscape. Desert_sentence_433

It has been portrayed by writers, film-makers, philosophers, artists and critics as a place of extremes, a metaphor for anything from death, war or religion to the primitive past or the desolate future. Desert_sentence_434

There is an extensive literature on the subject of deserts. Desert_sentence_435

An early historical account is that of Marco Polo (c. 1254–1324), who travelled through Central Asia to China, crossing a number of deserts in his twenty four year trek. Desert_sentence_436

Some accounts give vivid descriptions of desert conditions, though often accounts of journeys across deserts are interwoven with reflection, as is the case in Charles Montagu Doughty's major work, Travels in Arabia Deserta (1888). Desert_sentence_437

Antoine de Saint-Exupéry described both his flying and the desert in Wind, Sand and Stars and Gertrude Bell travelled extensively in the Arabian desert in the early part of the 20th century, becoming an expert on the subject, writing books and advising the British government on dealing with the Arabs. Desert_sentence_438

Another woman explorer was Freya Stark who travelled alone in the Middle East, visiting Turkey, Arabia, Yemen, Syria, Persia and Afghanistan, writing over twenty books on her experiences. Desert_sentence_439

The German naturalist Uwe George spent several years living in deserts, recording his experiences and research in his book, In the Deserts of this Earth. Desert_sentence_440

The American poet Robert Frost expressed his bleak thoughts in his poem, Desert Places, which ends with the stanza "They cannot scare me with their empty spaces / Between stars – on stars where no human race is. Desert_sentence_441

/ I have it in me so much nearer home / To scare myself with my own desert places." Desert_sentence_442

Deserts on other planets Desert_section_18

Main article: Desert planets Desert_sentence_443

Mars is the only other planet in the Solar System besides Earth on which deserts have been identified. Desert_sentence_444

Despite its low surface atmospheric pressure (only 1/100 of that of the Earth), the patterns of atmospheric circulation on Mars have formed a sea of circumpolar sand more than 5 million km (1.9 million sq mi) in the area, larger than most deserts on Earth. Desert_sentence_445

The Martian deserts principally consist of dunes in the form of half-moons in flat areas near the permanent polar ice caps in the north of the planet. Desert_sentence_446

The smaller dune fields occupy the bottom of many of the craters situated in the Martian polar regions. Desert_sentence_447

Examination of the surface of rocks by laser beamed from the Mars Exploration Rover have shown a surface film that resembles the desert varnish found on Earth although it might just be surface dust. Desert_sentence_448

The surface of Titan, a moon of Saturn, also has a desert-like surface with dune seas. Desert_sentence_449

See also Desert_section_19

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Desert.