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141-623: The Tottori Sand Dunes ( 鳥取砂丘 , Tottori sakyū ) are sand dunes located outside the city center of Tottori in Tottori Prefecture , Japan . At a length of 9 miles (14 km) and less than 1.5 miles (2.4 km) wide, it is the largest sand dune in Japan. The sand dunes are part of San'in Kaigan Geopark , which is part of The UNESCO Global Geoparks . The sand formations were created when sediment deposits carried from

282-530: 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

423-455: 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

564-456: 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

705-428: 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

846-438: 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

987-505: 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

1128-562: A dune forms, plant succession occurs. The conditions on an embryo dune are harsh, with salt spray from the sea carried on strong winds. The dune is well drained and often dry, and composed of calcium carbonate from seashells. Rotting seaweed , brought in by storm waves adds nutrients to allow pioneer species to colonize the dune. For example, in the United Kingdom these pioneer species are often marram grass , sea wort grass and other sea grasses. These plants are well adapted to

1269-590: A few tens of metres except at their nose, where vegetation stops or slows the advance of accumulating sand. Simple parabolic dunes have only one set of arms that trail upwind, behind the leading nose. Compound parabolic dunes are coalesced features with several sets of trailing arms. Complex parabolic dunes include subsidiary superposed or coalesced forms, usually of barchanoid or linear shapes. Parabolic dunes, like crescent dunes, occur in areas where very strong winds are mostly unidirectional. Although these dunes are found in areas now characterized by variable wind speeds,

1410-415: 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

1551-407: A low-lying pan within the basin where 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

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1692-484: 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

1833-423: 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

1974-412: A significant role in minimizing wave energy as it moves onshore. As a result, coastal dunes, especially those in the foredune area affected by a storm surge , will retreat or erode. To counteract the damage from tropical activity on coastal dunes, short term post-storm efforts can be made by individual agencies through fencing to help with sand accumulation. How much a dune erodes during any storm surge

2115-461: 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

2256-462: A star dune superimposed on its crest is the most common complex dune. Simple dunes represent a wind regime that has not changed in intensity or direction since the formation of the dune, while compound and complex dunes suggest that the intensity and direction of the wind has changed. The sand mass of dunes can move either windward or leeward, depending on if the wind is making contact with the dune from below or above its apogee. If wind hits from above,

2397-439: 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

2538-584: 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

2679-485: 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

2820-632: 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

2961-819: A unidirectional wind. In the sheltered troughs between highly developed seif dunes, barchans may be formed, because the wind is constrained to be unidirectional by the dunes. Seif dunes are common in the Sahara. They range up to 300 m (980 ft) in height and 300 km (190 mi) in length. In the southern third of the Arabian Peninsula, a vast erg , called the Rub' al Khali or Empty Quarter, contains seif dunes that stretch for almost 200 km (120 mi) and reach heights of over 300 m (980 ft). Linear loess hills known as pahas are superficially similar. These hills appear to have been formed during

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3102-400: 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

3243-472: 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

3384-538: 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

3525-485: Is a type of sandstone that is formed when a marine or aeolian sand dune becomes compacted and hardened. Once in this form, water passing through the rock can carry and deposit minerals, which can alter the colour of the rock. Cross-bedded layers of stacks of lithified dunes can produce the cross-hatching patterns, such as those seen in Zion National Park in the western United States. A slang term, used in

3666-542: 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

3807-442: Is a very large aeolian landform, with a length of several kilometers and a height of tens to hundreds of meters, and which may have superimposed dunes. Dunes are made of sand-sized particles, and may consist of quartz, calcium carbonate, snow, gypsum, or other materials. The upwind/upstream/upcurrent side of the dune is called the stoss side; the downflow side is called the lee side. Sand is pushed (creep) or bounces ( saltation ) up

3948-410: Is blown along the beach. Dunes form where the beach is wide enough to allow for the accumulation of wind-blown sand, and where prevailing onshore winds tend to blow sand inland. The three key ingredients for coastal dune formation are a large sand supply, winds to move said sand supply, and a place for the sand supply to accumulate. Obstacles—for example, vegetation, pebbles and so on—tend to slow down

4089-445: Is by saltation , where sand particles skip along the ground like a bouncing ball . When these skipping particles land, they may knock into other particles and cause them to move as well, in a process known as creep . With slightly stronger winds, particles collide in mid-air, causing sheet flows . In a major dust storm , dunes may move tens of metres through such sheet flows. Also as in the case of snow, sand avalanches , falling down

4230-633: 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

4371-428: Is due to a number of pressures related to their proximity to the ocean and confinement to growth on sandy substrates. These include: Plants have evolved many adaptations to cope with these pressures: 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

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4512-479: 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

4653-560: 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

4794-434: 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

4935-421: 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

5076-404: Is related to its location on the coastal shoreline and the profile of the beach during a particular season. In those areas with harsher winter weather, during the summer a beach tends to take on more of a convex appearance due to gentler waves, while the same beach in the winter may take on more of a concave appearance. As a result, coastal dunes can get eroded much more quickly in the winter than in

5217-486: 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

5358-682: 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

5499-450: 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

5640-431: 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

5781-528: 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

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5922-641: 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

6063-574: The Chūgoku Mountains by the Sendai River were thrown away into the Sea of Japan . Strong winds then shaped the dunes over a span of 100,000 years. The area of the dunes has been steadily decreasing due to a government reforestation program following World War II . Concrete barriers have been built along the coast to prevent the formations from eroding. Authorities have adopted measures to stop

6204-523: The Western Desert of Egypt . The largest crescentic dunes on Earth, with mean crest-to-crest widths of more than three kilometres, are in China's Taklamakan Desert . Abundant barchan dunes may merge into barchanoid ridges, which then grade into linear (or slightly sinuous) transverse dunes, so called because they lie transverse, or across, the wind direction, with the wind blowing perpendicular to

6345-621: 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

6486-414: 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

6627-414: 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

6768-480: The slipface of the dunes—that face away from the winds—also move the dunes forward. 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 the Earth is arid or semi-arid . This includes much of

6909-459: The southwest US , for consolidated and hardened sand dunes is "slickrock", a name that was introduced by pioneers of the Old West because their steel-rimmed wagon wheels could not gain traction on the rock. Sand dunes can have a negative impact on humans when they encroach on human habitats. Sand dunes move via a few different means, all of them helped along by wind. One way that dunes can move

7050-659: The stoss (upflow) side, where the sand is pushed up the dune, and have a shorter slip face in the lee side. The valley or trough between dunes is called a dune slack . Dunes are most common in desert environments, where the lack of moisture hinders the growth of vegetation that would otherwise interfere with the development of dunes. However, sand deposits are not restricted to deserts, and dunes are also found along sea shores, along streams in semiarid climates, in areas of glacial outwash , and in other areas where poorly cemented sandstone bedrock disintegrates to produce an ample supply of loose sand. Subaqueous dunes can form from

7191-417: 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

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7332-492: 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

7473-532: 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

7614-551: The action of water flow ( fluvial processes) on sand or gravel beds of rivers , estuaries , and the sea-bed. Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach . In most cases, the dunes are important in protecting the land against potential ravages by storm waves from the sea . Artificial dunes are sometimes constructed to protect coastal areas. The dynamic action of wind and water can sometimes cause dunes to drift, which can have serious consequences. For example,

7755-401: 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

7896-722: 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

8037-411: 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

8178-457: 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

8319-523: 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

8460-409: The coastal environment of the sand dune vital to their species' survival. Over the course of time coastal dunes may be impacted by tropical cyclones or other intense storm activity, dependent on their location. Recent work has suggested that coastal dunes tend to evolve toward a high or low morphology depending on the growth rate of dunes relative to storm frequency. During a storm event, dunes play

8601-615: 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

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8742-597: 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

8883-586: 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

9024-608: The concave sides of the dunes. These dunes form under winds that blow consistently from one direction (unimodal winds). They form separate crescents when the sand supply is comparatively small. When the sand supply is greater, they may merge into barchanoid ridges, and then transverse dunes (see below). Some types of crescentic dunes move more quickly over desert surfaces than any other type of dune. A group of dunes moved more than 100 metres per year between 1954 and 1959 in China 's Ningxia Province , and similar speeds have been recorded in

9165-468: 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

9306-405: The corridors between individual dunes. Because all dune arms are oriented in the same direction, and, the inter-dune corridors are generally swept clear of loose sand, the corridors can usually be traversed in between the trailing arms of the dune. However to cross straight over the dune by going over the trailing arms, can be very difficult. Also, traversing the nose is very difficult as well because

9447-508: 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

9588-500: 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

9729-439: The direction (s) of prevailing winds, are known as lunettes, source-bordering dunes, bourrelets and clay dunes. They may be composed of clay, silt, sand, or gypsum, eroded from the basin floor or shore, transported up the concave side of the dune, and deposited on the convex side. Examples in Australia are up to 6.5 km long, 1 km wide, and up to 50 metres high. They also occur in southern and West Africa , and in parts of

9870-682: The distance between the water line and where vegetation can grow. Coastal dunes can be classified by where they develop, or begin to take shape. Dunes are commonly grouped into either the Primary Dune Group or the Secondary Dune Group. Primary dunes gain most of their sand from the beach itself, while secondary dunes gain their sand from the primary dune. Along the Florida Panhandle, most dunes are considered to be foredunes or hummocks. Different locations around

10011-587: 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

10152-604: The dune and underlying soils . The stability of the dunes was once attributed to the vegetative cover but recent research has pointed to water as the main source of parabolic dune stability. The vegetation that covers them—grasses, shrubs, and trees—help anchor the trailing arms. In inland deserts, parabolic dunes commonly originate and extend downwind from blowouts in sand sheets only partly anchored by vegetation. They can also originate from beach sands and extend inland into vegetated areas in coastal zones and on shores of large lakes. Most parabolic dunes do not reach heights higher than

10293-513: The dunes and provide horticultural benefits, but instead spread taking land away from native species. Ammophila arenaria , known as European beachgrass, has a similar story, though it has no horticulture benefits. It has great ground coverage and, as intended, stabilized the dunes but as an unintended side effect prevented native species from thriving in those dunes. One such example is the dune field at Point Reyes, California . There are now efforts to get rid of both of these invasive species. As

10434-598: The dunes, washing humus into the slacks, and the slacks may be much more developed than the exposed tops of the dunes. It is usually in the slacks that more rare species are developed and there is a tendency for the dune slacks' soil to be waterlogged where only marsh plants can survive. In Europe these plants include: creeping willow, cotton grass, yellow iris , reeds, and rushes. As for vertebrates in European dunes, natterjack toads sometimes breed here. Dune ecosystems are extremely difficult places for plants to survive. This

10575-592: The dunes. Typically these are heather , heaths and gorses . These too are adapted to the low soil water content and have small, prickly leaves which reduce transpiration. Heather adds humus to the soil and is usually replaced by coniferous trees, which can tolerate low soil pH , caused by the accumulation and decomposition of organic matter with nitrate leaching. Coniferous forests and heathland are common climax communities for sand dune systems. Young dunes are called yellow dunes and dunes which have high humus content are called grey dunes . Leaching occurs on

10716-401: The effective winds associated with the growth and migration of both the parabolic and crescent dunes probably are the most consistent in wind direction. The grain size for these well-sorted, very fine to medium sands is about 0.06 to 0.5 mm. Parabolic dunes have loose sand and steep slopes only on their outer flanks. The inner slopes are mostly well packed and anchored by vegetation, as are

10857-455: 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

10998-572: 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

11139-407: 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

11280-458: 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

11421-519: The geological record . All these dune shapes may occur in three forms: simple (isolated dunes of basic type), compound (larger dunes on which smaller dunes of same type form), and complex (combinations of different types). Simple dunes are basic forms with the minimum number of slipfaces that define the geometric type. Compound dunes are large dunes on which smaller dunes of similar type and slipface orientation are superimposed. Complex dunes are combinations of two or more dune types. A crescentic dune with

11562-595: The globe have dune formations unique to their given coastal profile. Coastal sand dunes can provide privacy and/or habitats to support local flora and fauna. Animals such as sand snakes, lizards, and rodents can live in coastal sand dunes, along with insects of all types. Often the vegetation of sand dunes is discussed without acknowledging the importance that coastal dunes have for animals. Further, some animals, such as foxes and feral pigs can use coastal dunes as hunting grounds to find food. Birds are also known to utilize coastal dunes as nesting grounds. All these species find

11703-439: 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

11844-474: 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

11985-432: 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

12126-427: 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

12267-408: The harsh conditions of the foredune, typically having deep roots which reach the water table , root nodules that produce nitrogen compounds, and protected stoma , reducing transpiration . Also, the deep roots bind the sand together, and the dune grows into a foredune as more sand is blown over the grasses. The grasses add nitrogen to the soil, meaning other, less hardy plants can then colonize

12408-429: 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

12549-783: 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

12690-454: 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

12831-523: 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

12972-431: 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

13113-476: 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

13254-486: The last ice age under permafrost conditions dominated by sparse tundra vegetation. Star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally. They dominate the Grand Erg Oriental of the Sahara. In other deserts, they occur around

13395-723: 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

13536-510: 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

13677-412: 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

13818-521: The margins of the sand seas , particularly near topographic barriers. In the southeast Badain Jaran Desert of China, the star dunes are up to 500 metres tall and may be the tallest dunes on Earth. Oval or circular mounds that generally lack a slipface. Dome dunes are rare and occur at the far upwind margins of sand seas. Fixed crescentic dunes that form on the leeward margins of playas and river valleys in arid and semiarid regions in response to

13959-404: 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

14100-617: 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

14241-409: 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,

14382-658: The nose is usually made up of loose sand without much if any vegetation. A type of extensive parabolic dune that lacks discernible slipfaces and has mostly coarse grained sand is known as a zibar . The term zibar comes from the Arabic word to describe "rolling transverse ridges ... with a hard surface". The dunes are small, have low relief, and can be found in many places across the planet from Wyoming (United States) to Saudi Arabia to Australia. Spacing between zibars ranges from 50 to 400 metres and they do not become more than 10 metres high. The dunes form at about ninety degrees to

14523-527: 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

14664-426: 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

14805-466: 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

14946-440: The prevailing wind which blows away the small, fine-grained sand leaving behind the coarser grained sand to form the crest. Occurring wherever winds periodically reverse direction, reversing dunes are varieties of any of the above shapes. These dunes typically have major and minor slipfaces oriented in opposite directions. The minor slipfaces are usually temporary, as they appear after a reverse wind and are generally destroyed when

15087-484: The resultant direction of sand movement (hence the name "longitudinal"). Some linear dunes merge to form Y-shaped compound dunes. Formation is debated. Ralph Bagnold , in The Physics of Blown Sand and Desert Dunes , suggested that some seif dunes form when a barchan dune moves into a bidirectional wind regime, and one arm or wing of the crescent elongates. Others suggest that seif dunes are formed by vortices in

15228-479: The ridge crest. Seif dunes are linear (or slightly sinuous) dunes with two slip faces. The two slip faces make them sharp-crested. They are called seif dunes after the Arabic word for "sword". They may be more than 160 kilometres (100 miles) long, and thus easily visible in satellite images (see illustrations). Seif dunes are associated with bidirectional winds. The long axes and ridges of these dunes extend along

15369-438: 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

15510-415: The sand in the dune migrates forward. In plan view, these are U-shaped or V-shaped mounds of well-sorted, very fine to medium sand with elongated arms that extend upwind behind the central part of the dune. There are slipfaces that often occur on the outer side of the nose and on the outer slopes of the arms. These dunes often occur in semiarid areas where the precipitation is retained in the lower parts of

15651-425: The sand particles move leeward; the leeward flux of sand is greater than the windward flux. Conversely, if sand hits from below, sand particles move windward. Further, if the wind is carrying sand particles when it hits the dune, the dune's sand particles will saltate more than if the wind had hit the dune without carrying sand particles. Coastal dunes form when wet sand is deposited along the coast and dries out and

15792-503: 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

15933-438: The sea. A nabkha , or coppice dune, is a small dune anchored by vegetation. They usually indicate desertification or soil erosion, and serve as nesting and burrow sites for animals. Sub-aqueous ( underwater ) dunes form on a bed of sand or gravel under the actions of water flow. They are ubiquitous in natural channels such as rivers and estuaries, and also form in engineered canals and pipelines. Dunes move downstream as

16074-435: 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

16215-441: 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

16356-590: The shrinkage of the dunes, partly because they attract a significant amount of tourism to the area. Sand dunes A dune is a landform composed of wind- or water-driven sand . It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex . A large dune complex is called a dune field , while broad, flat regions covered with wind-swept sand or dunes, with little or no vegetation, are called ergs or sand seas . Dunes occur in different shapes and sizes, but most kinds of dunes are longer on

16497-412: 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

16638-466: 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

16779-464: 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

16920-402: The steeper slip face facing downstream. Ripple marks preserved in sedimentary strata in the geological record can be used to determine the direction of current flow, and thus an indication of the source of the sediments. Dunes on the bed of a channel significantly increase flow resistance, their presence and growth playing a major part in river flooding . A lithified (consolidated) sand dune

17061-604: The stoss side, and slides down the lee side. A side of a dune that the sand has slid down is called a slip face (or slipface). The Bagnold formula gives the speed at which particles can be transported. Five basic dune types are recognized: crescentic, linear, star, dome, and parabolic. Dune areas may occur in three forms: simple (isolated dunes of basic type), compound (larger dunes on which smaller dunes of same type form), and complex (combinations of different types). Barchan dunes are crescent-shaped mounds which are generally wider than they are long. The lee-side slipfaces are on

17202-679: The summer. The converse is true in areas with harsher summer weather. There are many threats to these coastal communities. Some coastal dunes, for example ones in San Francisco, have been completely altered by urbanization; reshaping the dune for human use. This puts native species at risk. Another danger, in California and places in the UK specifically, is the introduction of invasive species. Plant species, such as Carpobrotus edulis , were introduced from South Africa in an attempt to stabilize

17343-440: 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

17484-464: 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,

17625-577: 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

17766-551: 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

17907-399: 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

18048-509: The town of Eucla, Western Australia , had to be relocated in the 1890s because of dune drift. The modern word "dune" came into English from French around 1790, which in turn came from Middle Dutch dūne . A universally precise distinction does not exist between ripples, dunes, and draas , which are all deposits of the same type of materials. Dunes are generally defined as greater than 7 cm tall and may have ripples, while ripples are deposits that are less than 3 cm tall. A draa

18189-407: 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

18330-471: The upstream slope is eroded and the sediment deposited on the downstream or lee slope in typical bedform construction. In the case of sub-aqueous barchan dunes, sediment is lost by their extremities, known as horns. These dunes most often form as a continuous 'train' of dunes, showing remarkable similarity in wavelength and height. The shape of a dune gives information about its formation environment. For instance, rivers produce asymmetrical ripples, with

18471-496: 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

18612-416: 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

18753-534: The western United States, especially Texas. U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. These dunes are formed from blowout dunes where the erosion of vegetated sand leads to a U-shaped depression. The elongated arms are held in place by vegetation; the largest arm known on Earth reaches 12 km. Sometimes these dunes are called U-shaped, blowout , or hairpin dunes, and they are well known in coastal deserts. Unlike crescent shaped dunes, their crests point upwind. The bulk of

18894-476: 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

19035-402: The wind and lead to the deposition of sand grains. These small "incipient dunes or "shadow dunes" tend to grow in the vertical direction if the obstacle slowing the wind can also grow vertically (i.e., vegetation). Coastal dunes expand laterally as a result of lateral growth of coastal plants via seed or rhizome . Models of coastal dunes suggest that their final equilibrium height is related to

19176-404: 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

19317-511: 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

19458-483: The wind next blows in the dominant direction. Draas are very large-scale dune bedforms; they may be tens or a few hundreds of metres in height, kilometres wide, and hundreds of kilometres in length. After a draa has reached a certain size, it generally develops superimposed dune forms. They are thought to be more ancient and slower-moving than smaller dunes, and to form by vertical growth of existing dunes. Draas are widespread in sand seas and are well-represented in

19599-450: 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

19740-477: 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

19881-459: 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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