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110-421: Prosperous Bay Plain forms part of the eastern arid area of Saint Helena, and covers about 2.25 km, comprising one of the largest areas of relatively level ground on the island. It was formed 8.5 million years ago by lava flows from Saint Helena's Southwest Volcano. The surface of the plain is covered by rocks, grit and dust, with what little soil there is containing high concentrations of mineral salts. Within

220-530: A mammal adapted to desert life. 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. Carnivores can obtain much of their water needs from the body fluids of their prey. Many other hot desert animals are nocturnal , seeking out shade during the day or dwelling underground in burrows. At depths of more than 50 cm (20 in), these remain at between 30 and 32 °C (86 and 90 °F) regardless of

330-455: A mosaic of smooth stones, often forming desert pavements , and little further erosion takes place. Other desert features include rock outcrops , exposed bedrock and clays once deposited by flowing water. Temporary lakes may form and salt pans may be left when waters evaporate. There may be underground sources of water, in the form of springs and seepages from aquifers . Where these are found, oases can occur. Plants and animals living in

440-456: A sand blasting mechanism which grinds away solid objects in its path as the kinetic energy of the wind is transferred to the ground. The sand eventually ends up deposited in level areas known as sand-fields or sand-seas, or piled up in dunes. 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. Across

550-428: A semi-arid climate and are less extreme than regular deserts. Like arid deserts, temperatures can vary greatly in semi deserts. They share some characteristics of a true desert and are usually located at the edge of deserts and continental dry areas. They usually receive precipitation from 250 to 500 mm (9.8 to 19.7 in) but this can vary due to evapotranspiration and soil nutrition. Semi-deserts can be found in

660-458: A tropical savanna climate or a humid subtropical climate . These climates tend to have hot, or sometimes extremely hot, summers and warm to cool winters, with some to minimal precipitation. Hot semi-arid climates are most commonly found around the fringes of subtropical deserts. Hot semi-arid climates are most commonly found in Africa , Australia , and South Asia . In Australia, a large portion of

770-438: A desert conglomerate . 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 . 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. A hamada

880-505: A destructive effect on the exposed rocky surfaces. The repeated fluctuations put a strain on exposed rock and the flanks of mountains crack and shatter. 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. Successive strata are exposed to further weathering. The relief of the internal pressure that has built up in rocks that have been underground for aeons can cause them to shatter. Exfoliation also occurs when

990-434: A location has a semi-arid climate, the precipitation threshold must first be determined. The method used to find the precipitation threshold (in millimeters): If the area's annual precipitation in millimeters is less than the threshold but more than half or 50% the threshold, it is classified as a BS (steppe, semi-desert, or semi-arid climate). Furthermore, to delineate hot semi-arid climates from cold semi-arid climates,

1100-415: 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. Herbivorous mammals obtain moisture from the plants they eat. 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. The camel is a superb example of

1210-409: A mean annual temperature of 18 °C (64.4 °F) is used as an isotherm. A location with a BS -type climate is classified as hot semi-arid ( BSh ) if its mean temperature is above this isotherm, and cold semi-arid ( BSk ) if not. Hot semi-arid climates (type "BSh") tend to be located from the high teens to mid-30s latitudes of the tropics and subtropics , typically in proximity to regions with

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1320-484: A number of ways, generally combining total precipitation, number of days on which this falls, temperature, and humidity, and sometimes additional factors. 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. The North Slope of Alaska's Brooks Range also receives less than 250 mm (9.8 in) of precipitation per year and

1430-423: A series of long, linear dunes known as seif dunes may form. These also occur parallel to a strong wind that blows in one general direction. Transverse dunes run at a right angle to the prevailing wind direction. Star dunes are formed by variable winds, and have several ridges and slip faces radiating from a central point. They tend to grow vertically; they can reach a height of 500 m (1,600 ft), making them

1540-461: A sparse trophic network . Plant growth is limited by rainfall , temperature extremes and desiccating winds. 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. Resources are often ephemeral or episodic, and this triggers sporadic animal movements and 'pulse and reserve' or 'boom-bust' ecosystem dynamics. Erosion and sedimentation are high due to

1650-439: A steady wind begins to blow, fine particles lying on the exposed ground begin to vibrate. At greater wind speeds, some particles are lifted into the air stream. When they land, they strike other particles which may be jerked into the air in their turn, starting a chain reaction . Once ejected, these particles move in one of three possible ways, depending on their size, shape and density; suspension , saltation or creep. Suspension

1760-571: A storm that may be many kilometers away. 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. 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. There may also be underground sources of water in deserts in

1870-485: A tolerable level. In many ways, birds are the ablest to do this of the higher animals. They can move to areas of greater food availability as the desert blooms after local rainfall and can fly to faraway waterholes. 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. In order to conserve energy, other desert birds run rather than fly. The cream-colored courser flits gracefully across

1980-632: A type of cold desert . While they do not lack water, having a persistent cover of snow and ice, this is merely due to marginal evaporation rates and low precipitation. The McMurdo dry valleys of Antarctica, which lack water (whether rain, ice, or snow) much like a non-polar desert and even have such desert features as hypersaline lakes and intermittent streams that resemble (except for being frozen at their surfaces) hot or cold deserts for extreme aridity and lack of precipitation of any kind. Extreme winds and not seasonal heat desiccate these nearly-lifeless terrains. The concept of "biological desert" redefines

2090-400: A year; they have no annual seasonal cycle of precipitation and experience twelve-month periods with no rainfall at all. 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). However, such factors as the temperature, humidity, rate of evaporation and evapotranspiration, and the moisture storage capacity of

2200-465: Is 150 km (93 mi) long and is the largest oasis in the Libyan Desert. A lake occupied this depression in ancient times and thick deposits of sandy-clay resulted. Wells are dug to extract water from the porous sandstone that lies underneath. Seepages may occur in the walls of canyons and pools may survive in deep shade near the dried up watercourse below. Lakes may form in basins where there

2310-525: Is a jumble of eroded sandstone outcrops , canyons , blocks, pinnacles , fissures , slabs and ravines . 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. On the Colorado Plateau , it is water that has been the prevailing eroding force. Here, rivers, such as the Colorado , have cut their way over the millennia through

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2420-532: Is a type of desert landscape consisting of a high rocky plateau where the sand has been removed by aeolian processes . Other landforms include plains largely covered by gravels and angular boulders, from which the finer particles have been stripped by the wind. These are called "reg" in the western Sahara, "serir" in the eastern Sahara, "gibber plains" in Australia and "saï" in central Asia. The Tassili Plateau in Algeria

2530-623: Is continuous above 5,600 m (18,400 ft). 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. When rain falls in deserts, as it occasionally does, it is often with great violence. The desert surface is evidence of this with dry stream channels known as arroyos or wadis meandering across its surface. These can experience flash floods , becoming raging torrents with surprising rapidity after

2640-469: Is given by the Köppen climate classification , which treats steppe climates ( BSh and BSk ) as intermediates between desert climates (BW) and humid climates (A, C, D) in ecological characteristics and agricultural potential. Semi-arid climates tend to support short, thorny or scrubby vegetation and are usually dominated by either grasses or shrubs as they usually cannot support forests. To determine if

2750-479: Is higher than that of forests or the sea. 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. Deserts generally receive less than 250 mm (10 in) of precipitation each year. The potential evapotranspiration may be large but (in

2860-551: Is often classified as a cold desert. Other regions of the world have cold deserts, including areas of the Himalayas and other high-altitude areas in other parts of the world. Polar deserts cover much of the ice-free areas of the Arctic and Antarctic. A non-technical definition is that deserts are those parts of Earth's surface that have insufficient vegetation cover to support a human population. Potential evapotranspiration supplements

2970-434: 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. 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 . Polar deserts such as McMurdo Dry Valleys remain ice-free because of the dry katabatic winds that flow downhill from

3080-421: Is only possible for particles less than 0.1 mm (0.0039 in) in diameter. In a dust storm, these fine particles are lifted up and wafted aloft to heights of up to 6 km (3.7 mi). 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). Denser clouds of dust can be formed in stronger winds, moving across

3190-486: Is sufficient precipitation or meltwater from glaciers above. They are usually shallow and saline, and wind blowing over their surface can cause stress, moving the water over nearby low-lying areas. When the lakes dry up, they leave a crust or hardpan behind. This area of deposited clay, silt or sand is known as a playa . 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

3300-657: Is the Atacama Desert . 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. The cold Humboldt Current and the anticyclone of the Pacific are essential to keep the dry climate of the Atacama. The average precipitation in the Chilean region of Antofagasta is just 1 mm (0.039 in) per year. Some weather stations in

3410-406: Is the critically endangered wirebird , of which the plain makes up 10% of its remaining habitat. Prosperous Bay Plain is a biodiversity hotspot , home to an extraordinary concentration of endemic invertebrates, the area being the main evolutionary centre on the island for animals adapted to arid habitats. Some 35–40 species and six genera recorded in this limited area occur nowhere else in

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3520-462: Is the introduced Hottentot fig ( Carpobrotus edulis ). On dry slopes, including those of the Central Basin, there are various chenopods , including the introduced saltbush Atriplex semibaccata . The plain holds a number of important populations of the island's endemic flora. The slopes of Dry Gut hosted the largest remaining group of barn fern ( Ceterach haughtonii ). Dry Gut was also home to

3630-450: Is the key to plant growth. It can only take place during the day as energy from the sun is required, but during the day, many deserts become very hot. 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. Some plants have resolved this problem by adopting crassulacean acid metabolism , allowing them to open their stomata during

3740-431: Is the specializations of mammalian kidneys shown by desert-inhabiting species. Many examples of convergent evolution have been identified in desert organisms, including between cacti and Euphorbia , kangaroo rats and jerboas , Phrynosoma and Moloch lizards. Deserts present a very challenging environment for animals. Not only do they require food and water but they also need to keep their body temperature at

3850-528: 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. 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. Based on precipitation alone, hyperarid deserts receive less than 25 mm (1 in) of rainfall

3960-724: The ABC Islands , the rain shadows of Hispaniola 's mountain ranges in the Dominican Republic and Haiti , parts of the Southwestern United States including California's Central Valley , and sections of South America such as the sertão , the Gran Chaco , and the poleward side of the arid deserts, where they typically feature a Mediterranean precipitation pattern, with generally rainless summers and wetter winters. They are also found in few areas of Europe surrounding

4070-669: The Atacama Desert . Deserts of this type are often long and narrow and bounded to the east by mountain ranges. They occur in Namibia , Chile , southern California and Baja California . 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. In 1961, Peveril Meigs divided desert regions on Earth into three categories according to

4180-586: The Mediterranean Basin . In Europe, BSh climates are predominantly found in southeastern Spain . It can also be found primarily in parts of south Greece but also in marginal areas of Thessaloniki and Chalkidiki in north Greece , most of Formentera , marginal areas of Ibiza and marginal areas of Italy in Sicily , Sardinia and Lampedusa . Cold semi-arid climates (type "BSk") tend to be located in elevated portions of temperate zones generally from

4290-518: The Outback surrounding the central desert regions lies within the hot semi-arid climate region. In South Asia, both India and parts of Pakistan experience the seasonal effects of monsoons and feature short but well-defined wet seasons , but are not sufficiently wet overall to qualify as either a tropical savanna or a humid subtropical climate. Hot semi-arid climates can be also found in parts of North America , such as most of northern Mexico ,

4400-616: The horse latitudes at 30° to 35° North and South. These belts are associated with the subtropical anticyclone and the large-scale descent of dry air. The Sahara Desert is of this type. Mid-latitude deserts occur between 30° and 50° North and South. They are mostly in areas remote from the sea where most of the moisture has already precipitated from the prevailing winds. They include the Tengger and Sonoran Deserts . Monsoon deserts are similar. They occur in regions where large temperature differences occur between sea and land. Moist warm air rises over

4510-414: The rocks , which consequently break in pieces. Although rain seldom occurs in deserts, there are occasional downpours that can result in flash floods. 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. This picks up particles of sand and dust, which can remain airborne for extended periods – sometimes causing

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4620-417: The windward slope of the mountain range . When they descend on the leeward side, they warm and their capacity to hold moisture increases so an area with relatively little precipitation occurs. 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. Other areas are arid by virtue of being a very long way from

4730-486: 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 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

4840-407: The Central Basin. 15°57′40″S 5°39′18″W  /  15.961°S 5.655°W  / -15.961; -5.655 Desert A desert is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation . About one-third of the land surface of

4950-520: The Earth is arid or semi-arid . This includes much of the polar regions , where little precipitation occurs, and which are sometimes called polar deserts or "cold deserts". 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. Deserts are formed by weathering processes as large variations in temperature between day and night put strains on

5060-517: The absence of available water) the actual evapotranspiration may be close to zero. 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 . Most deserts on Earth such as the Sahara Desert , Grand Australian Desert and the Great Basin Desert , occur in low altitudes . One of the driest places on Earth

5170-401: The air but soon fall back to earth, ejecting other particles in the process. Their weight prevents them from being airborne for long and most only travel a distance of a few meters (yards). The sand streams along above the surface of the ground like a fluid, often rising to heights of about 30 cm (12 in). In a really severe steady blow, 2 m (6 ft 7 in) is about as high as

5280-722: The amount of precipitation they received. In this now widely accepted system, extremely arid lands have at least twelve consecutive months without precipitation, arid lands have less than 250 mm (9.8 in) of annual precipitation, and semiarid lands have a mean annual precipitation of between 250 and 500 mm (9.8 and 19.7 in). Both extremely arid and arid lands are considered to be deserts while semiarid lands are generally referred to as steppes when they are grasslands. 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 . Trade wind deserts occur either side of

5390-403: The area in the past, before the introduction of exotic herbivores. Much of the eastern end of Dry Gut has now been filled in to provide a base for the airport runway. Before the discovery of Saint Helena in 1502, the plain was home to seabird breeding colonies . These disappeared after settlement of the island from predation by humans and feral cats . The only endemic land-bird still present

5500-411: 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. Deserts, both hot and cold, play a part in moderating Earth's temperature, because they reflect more of the incoming light and their albedo

5610-457: The capture of large quantities of solar energy . 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". The correlation between aridity and sparse population is complex and dynamic, varying by culture, era, and technologies; thus

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5720-516: The cellular structure of which has been modified to allow them to store water. 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. The giant saguaro cacti of the Sonoran Desert form "forests", providing shade for other plants and nesting places for desert birds. Saguaro grows slowly but may live for up to two hundred years. The surface of

5830-615: The coldest part of the year. Animals adapted to live in deserts are called xerocoles . There is no evidence that body temperature of mammals and birds is adaptive to the different climates, either of great heat or cold. In fact, with a very few exceptions, their basal metabolic rate is determined by body size, irrespective of the climate in which they live. 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

5940-597: The collision between airborne particles and by the impacts of saltating sand grains landing on the ground. 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. Deserts and semi-deserts are home to ecosystems with low or very low biomass and primary productivity in arid or semi-arid climates. They are mostly found in subtropical high-pressure belts and major continental rain shadows . Primary productivity depends on low densities of small photoautotrophs that sustain

6050-586: The color of the underlying surface. The desert lark takes frequent dust baths which ensures that it matches its environment. Water and carbon dioxide are metabolic end products of oxidation of fats, proteins, and carbohydrates. 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. The kangaroo rat for example makes use of this water of metabolism and conserves water both by having

6160-468: The concept of desert, without the characteristic of aridity, not lacking water, but instead lacking life. Such places can be so-called "ocean deserts", which are mostly at the centers of gyres , but also hypoxic or anoxic waters such as dead zones . Deserts usually have a large diurnal and seasonal temperature range, with high daytime temperatures falling sharply at night. The diurnal range may be as much as 20 to 30 °C (36 to 54 °F) and

6270-503: The desert floor to scarify the seed coat. The seed of the mesquite tree, which grows in deserts in the Americas, is hard and fails to sprout even when planted carefully. 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. The stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect

6380-500: The desert need special adaptations to survive in the harsh environment. Plants tend to be tough and wiry with small or no leaves, water-resistant cuticles , and often spines to deter herbivory . 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. Animals need to keep cool and find enough food and water to survive. Many are nocturnal , and stay in

6490-587: The driest season, and others curl their leaves up to reduce transpiration. Others, such as aloes , store water in succulent leaves or stems or in fleshy tubers. 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. The saltbush in Australia has succulent leaves and secretes salt crystals, enabling it to live in saline areas. In common with cacti, many have developed spines to ward off browsing animals. Some desert plants produce seed which lies dormant in

6600-455: The external temperature. 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. Kangaroos keep cool by increasing their respiration rate, panting, sweating and moistening the skin of their forelegs with saliva . Mammals living in cold deserts have developed greater insulation through warmer body fur and insulating layers of fat beneath

6710-561: The form of springs , aquifers , underground rivers or lakes. Where these lie close to the surface, wells can be dug and oases may form where plant and animal life can flourish. The Nubian Sandstone Aquifer System under the Sahara Desert is the largest known accumulation of fossil water . The Great Man-Made River is a scheme launched by Libya's Muammar Gaddafi to tap this aquifer and supply water to coastal cities. Kharga Oasis in Egypt

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6820-407: The formation of sand storms or dust storms . Wind-blown sand grains striking any solid object in their path can abrade the surface. Rocks are smoothed down, and the wind sorts sand into uniform deposits. The grains end up as level sheets of sand or are piled high in billowing sand dunes . Other deserts are flat, stony plains where all the fine material has been blown away and the surface consists of

6930-458: The fragmentation of harder granites , limestone and sandstone . 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. As the mountains are eroded, more and more sand is created. At high wind speeds, sand grains are picked up off the surface and blown along, a process known as saltation . The whirling airborne grains act as

7040-439: 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. Cold deserts often have high concentrations of salt in the soil. Grasses and low shrubs are the dominant vegetation here and the ground may be covered with lichens . Most shrubs have spiny leaves and shed them in

7150-474: The ground have a marked effect on the degree of aridity and the plant and animal life that can be sustained. 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. A semi-arid desert or a steppe is a version of the arid desert with much more rainfall, vegetation and higher humidity. These regions feature

7260-432: The ground on its long legs, stopping periodically to snatch up insects. Like other desert birds, it is well-camouflaged by its coloring and can merge into the landscape when stationary. 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. Some small diurnal birds are found in very restricted localities where their plumage matches

7370-427: The ground rises quickly through a small pocket of cooler, low-pressure air above forming a whirling column of particles, a dust devil . Sandstorms occur with much less frequency than dust storms. They are often preceded by severe dust storms and occur when the wind velocity increases to a point where it can lift heavier particles. These grains of sand, up to about 0.5 mm (0.020 in) in diameter are jerked into

7480-429: The high desert floor, creating canyons that are over a mile (6,000 feet or 1,800 meters) deep in places, exposing strata that are over two billion years old. Sand and dust storms are natural events that occur in arid regions where the land is not protected by a covering of vegetation. Dust storms usually start in desert margins rather than the deserts themselves where the finer materials have already been blown away. As

7590-698: The high elevations of the Tabernas Desert (and some parts of the Spanish Plateu ), 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 . They usually feature BSh (hot steppe) or BSk (temperate steppe) in the Köppen climate classification . Coastal deserts are mostly found on

7700-450: The import of water from an outside source. Many trade routes have been forged across deserts, especially across the Sahara , and traditionally were used by caravans of camels carrying salt, gold, ivory and other goods. Large numbers of slaves were also taken northwards across the Sahara. Some mineral extraction also takes place in deserts, and the uninterrupted sunlight gives potential for

7810-523: The lack of heat to aid in the evaporation process. Deserts are sometimes classified as "hot" or "cold", "semiarid" or "coastal". 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. Winter temperatures vary considerably between different deserts and are often related to

7920-479: The land with a billowing leading edge. The sunlight can be obliterated and it may become as dark as night at ground level. 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). The mean particle size was 1.44 μm. A much smaller scale, short-lived phenomenon can occur in calm conditions when hot air near

8030-469: The land, deposits its water content and circulates back to sea. Further inland, areas receive very little precipitation. The Thar Desert near the India/Pakistan border is of this type. In some parts of the world, deserts are created by a rain shadow effect. Orographic lift occurs as air masses rise to pass over high ground. In the process they cool and lose much of their moisture by precipitation on

8140-692: The last ice age when the climate was colder and wetter. These include the Great Salt Lake , Utah Lake , Sevier Lake and many dry lake beds. 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 . Deserts have been defined and classified in

8250-510: 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 mm (12 in) of rain per year, however about 2,500 mm (98 in) of water could evaporate over the course of a year. In other words, about eight times more water could evaporate from the region than actually falls as rain. Rates of evapotranspiration in cold regions such as Alaska are much lower because of

8360-412: The location of the desert on the continental landmass and the latitude. 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. 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. Some cold deserts are far from

8470-404: The measurement of precipitation 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 the amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric evaporation and through

8580-525: The mid-30s to low 50s latitudes, typically bordering a humid continental climate or a Mediterranean climate . They are also typically found in continental interiors some distance from large bodies of water. Cold semi-arid climates usually feature warm to hot dry summers, though their summers are typically not quite as hot as those of hot semi-arid climates. Unlike hot semi-arid climates, areas with cold semi-arid climates tend to have cold and possibly freezing winters. These areas usually see some snowfall during

8690-470: The most abundant remaining population of the endemic boneseed, ( Osteospermum sancta-helenae ). Annual flushes of endemic babies'-toes ( Hydrodea cryptantha ), goosefoot ( Chenopodium helenense ) and neglected sedge ( Bulbostylis neglecta ) occur across the plain. There are a few scattered scrubwoods ( Commidendrum rugosum ), teaplants ( Frankenia portulacifolia ) and salad plant ( Hypertelis acida ), threatened endemics which may have grown more plentifully in

8800-597: The nearest available sources of moisture. 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 . Many locations within this category have elevations exceeding 3,000 m (9,800 ft) and the thermal regime can be hemiboreal . These places owe their profound aridity (the average annual precipitation

8910-409: The night to allow CO 2 to enter, and close them during the day, or by using C4 carbon fixation . Many desert plants have reduced the size of their leaves or abandoned them altogether. Cacti are present in both North and South America with a post-Gondwana origin. The genus is desert specialist, and in most species, the leaves have been dispensed with and the chlorophyll displaced into the trunks,

9020-514: 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. The largest of these deserts are found in Central Asia. Others occur on the eastern side of the Rocky Mountains , the eastern side of the southern Andes and in southern Australia. Polar deserts are a particular class of cold desert. The air

9130-426: The outer surfaces of rocks split off in flat flakes. 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. Chemical weathering processes probably play a more important role in deserts than was previously thought. The necessary moisture may be present in the form of dew or mist. Ground water may be drawn to

9240-466: The pavement is formed. 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. Very little further erosion takes place after the formation of a pavement, and the ground becomes stable. Evaporation brings moisture to the surface by capillary action and calcium salts may be precipitated, binding particles together to form

9350-465: The plain have been affected by many factors, including the systematic removal of loose and portable rocks for construction purposes, the formation of vehicle tracks, and the building of structures such as forts. The main threat to the invertebrate communities is the Saint Helena Airport , now completed, which, together with access roads, involved destruction of part of the plain, including part of

9460-413: The plain there is a 60 hectare (0.6 km) depression known as the Central Basin, with a level dusty base. This forms a miniature mature desert ecosystem. It falls within the district of Longwood . The climate of the plain is arid and plants are few, scattered and low-growing. Dusty areas and gullies are dominated by the native samphire Suaeda fruticosa . In rocky areas the more dominant plant

9570-438: The rock surface experiences even greater temperature differentials. 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. In hot deserts, the temperature during daytime can exceed 45 °C (113 °F) in summer and plunge below freezing point at night during winter. Such large temperature variations have

9680-503: The sand stream can rise as the largest sand grains do not become airborne at all. They are transported by creep, being rolled along the desert floor or performing short jumps. During a sandstorm, the wind-blown sand particles become electrically charged . Such electric fields , which range in size up to 80 kV/m, can produce sparks and cause interference with telecommunications equipment. They are also unpleasant for humans and can cause headaches and nausea. The electric fields are caused by

9790-435: The shade or underground during the heat of the day. They tend to be efficient at conserving water, extracting most of their needs from their food and concentrating their urine . Some animals remain in a state of dormancy for long periods, ready to become active again during the rare rainfall. They then reproduce rapidly while conditions are favorable before returning to dormancy. People have struggled to live in deserts and

9900-441: 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. 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. Small ripples form on the sand sheet when the wind exceeds 24 km/h (15 mph). They form perpendicular to

10010-545: The skin and various thermoregulatory strategies to maintain its body temperature in one of the harshest environments on Earth. Semi-arid climate A semi-arid climate , semi-desert climate , or steppe climate is a dry climate sub-type. It is located on regions that receive precipitation below potential evapotranspiration , but not as low as a desert climate . There are different kinds of semi-arid climates, depending on variables such as temperature, and they give rise to different biomes . A more precise definition

10120-412: The skin. 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. 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. The emperor penguin has dense plumage, a downy under layer, an air insulation layer next to

10230-466: The soil until sparked into growth by rainfall. 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. 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. Some seed will not germinate until it has been blown about on

10340-464: The sparse vegetation cover and the activities of large mammals and people. 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. Competitive interactions are weak. Plants face severe challenges in arid environments. Problems they need to solve include how to obtain enough water, how to avoid being eaten and how to reproduce. Photosynthesis

10450-440: The surface by evaporation and the formation of salt crystals may dislodge rock particles as sand or disintegrate rocks by exfoliation. Shallow caves are sometimes formed at the base of cliffs by this means. As the desert mountains decay, large areas of shattered rock and rubble occur. The process continues and the end products are either dust or sand. Dust is formed from solidified clay or volcanic deposits whereas sand results from

10560-464: The surface of the ground. Dunes are sometimes solitary, but they are more often grouped together in dune fields. When these are extensive, they are known as sand seas or ergs . The shape of the dune depends on the characteristics of the prevailing wind. Barchan dunes are produced by strong winds blowing across a level surface and are crescent-shaped with the concave side away from the wind. When there are two directions from which winds regularly blow,

10670-561: The surrounding mountains. Former desert areas presently in non-arid environments, such as the Sandhills in Nebraska , are known as paleodeserts. In the Köppen climate 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 climates. Polar deserts are

10780-597: The surrounding semi-arid lands for millennia. Nomads have moved their flocks and herds to wherever grazing is available, and oases have provided opportunities for a more settled way of life. The cultivation of semi-arid regions encourages erosion of soil and is one of the causes of increased desertification. 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

10890-399: The tallest type of dune. Rounded mounds of sand without a slip face are the rare dome dunes, found on the upwind edges of sand seas. In deserts where large amounts of limestone mountains surround a closed basin , such as at White Sands National Park in south-central New Mexico , occasional storm runoff transports dissolved limestone and gypsum into a low-lying pan within the basin where

11000-407: 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. Other xerophytic plants have developed similar strategies by a process known as convergent evolution . They limit water loss by reducing the size and number of stomata, by having waxy coatings and hairy or tiny leaves. Some are deciduous, shedding their leaves in

11110-496: The use of the word desert can cause confusion. 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). 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

11220-416: The water evaporates, depositing the gypsum and forming crystals known as selenite . The crystals left behind by this process are eroded by the wind and deposited as vast white dune fields that resemble snow-covered landscapes. These types of dune are rare, and only form in closed arid basins that retain the highly soluble gypsum that would otherwise be washed into the sea. A large part of the surface area of

11330-476: The western edges of continental land masses in regions where cold currents approach the land or cold water upwellings rise from the ocean depths. 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. 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

11440-404: The wind blows, saltation and creep take place on the windward side of the dune and individual grains of sand move uphill. When they reach the crest, they cascade down the far side. 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. As this wind-induced movement of sand grains takes place, the dune moves slowly across

11550-511: The wind direction and gradually move across the surface as the wind continues to blow. The distance between their crests corresponds to the average length of jumps made by particles during saltation. The ripples are ephemeral and a change in wind direction causes them to reorganise. Sand dunes are accumulations of windblown sand piled up in mounds or ridges. They form downwind of copious sources of dry, loose sand and occur when topographic and climatic conditions cause airborne particles to settle. As

11660-796: The winter, though snowfall is much lower than at locations at similar latitudes with more humid climates. Areas featuring cold semi-arid climates tend to have higher elevations than areas with hot semi-arid climates, and tend to feature major temperature swings between day and night, sometimes by as much as 20 °C (36 °F) or more. These large diurnal temperature variations are seldom seen in hot semi-arid climates. Cold semi-arid climates at higher latitudes tend to have dry winters and wetter summers, while cold semi-arid climates at lower latitudes tend to have precipitation patterns more akin to Mediterranean climates , with dry summers, relatively wet winters, and even wetter springs and autumns. Cold semi-arid climates are most commonly found in central Asia and

11770-450: The world's deserts consists of flat, stone-covered plains dominated by wind erosion. In " eolian deflation ", the wind continually removes fine-grained material, which becomes wind-blown sand. 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. Different theories exist as to how exactly

11880-470: 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. Where sand does occur, it is usually in large quantities in the form of sand sheets or extensive areas of dunes . 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. The structure of

11990-406: The world. The Saint Helena giant earwig used to inhabit the plain, and may still, although there have been no live records of it since 1967. The Saint Helena giant beetle may also be extinct. The mollusc Nesopupa turtoni in the whorl snail family Vertiginidae , previously known only as a fossil and long presumed to be extinct, was discovered alive in a 2003 survey Invertebrate habitats on

12100-459: Was the sparse population. Deserts occupy about one third of Earth's land surface. Bottomlands may be salt -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 rather than rain. Antarctica is the world's largest cold desert (composed of about 98% thick continental ice sheet and 2% barren rock). Some of

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