Desert

In geography it is defined as desert to an area of total the ground surface or almost totally vacated in which the precipitations almost never surpass the 250 millimeters to the year and the terrain he is barren.

A desert is an ecosystem that receives few precipitations. They have reputation to have little life, but that depends on the class of desert; in many abundant life exists, the vegetation adapts to the little humidity and the fauna usually hides during the day to preserve humidity, which means that a desert ecosystem is barren, its characteristic major and therefore, until the technologies of the present, it makes the establishment almost insustentable of social groups. The deserts form the zone surfaced more extensive from the planet: its total area is of 50 million squared kilometers, approximately a third of the ground surface. This is 30% of surfaced grounds, (16% warm deserts and 14% cold deserts).

The deserts have some characteristics in common. The ground of the desert is compound often of sand, and can have dunes. Rocky terrain is typical (a desert whose ground is of rocks or pebbles calls with the Arab word hamada), and reflects the low level of development of the ground, and the shortage of vegetation. The low grounds can be salares. Aeolian trials are major factors in the formation of the landscape.

The deserts can contain valuable mineral tanks that they were formed in the barren environment, or were exposed by the erosion. Due to the dryness of the deserts, they are ideal places for preservation of human and fossil artefacts.

Alternatively, also desert like a depopulated place is defined, not lived by humans nor hardly for being alive some. According to this definition, also other colder climates are desert, like the Arctic, or the tundra.

Great part of the deserts of the world is located in zones characterized by the constant high pressures (to see: anticyclone), condition that does not favor to rain. Between the deserts of these zones they are: the deserts of the Sahara (the most extensive of the planet Ground), Kalahari, Namib and of the Ogaden in Africa; the deserts Sandy, Victory in most of Australia, the deserts of Gobi (or Chamō), Kara Kum, Takla Makán, of Arabia, Rub To the Jalí (OR “The Fourth Barren one”), of Syria, Judea, Sinai-Néguev in Asia and the Mojave, Arizona-Desert deserts de Sonora, Atacama, of Sechura and Dunes of Paraguana in America; it téngase in account that in this list is not had including the “paradojales” desert cold.

Great part of the deserts must to their continentalidad, that is to say, its distance of the sea: for example, the one of Gobi and the others of Central Asia. The humid winds do not arrive until them that come from the oceans.

The deserts of the west coasts of the southern African and South America are affected by the presence of cold ocean currents that cause deshumectación in the atmosphere.

Types of desert

The majority of the classifications is in a combination of number of days of rain per year, the annual pluviométrica amount, temperature, humidity and other factors. In 1953, Peveril Meigs divided the desert Earth regions in three categories, in agreement with the total of rain that receive. By this system, today widely accepted, the extremely barren terrains are those that have at least 12 months consecutive without rain; the barren terrains have 250 millimeters of annual rain less than, and the semi-arid terrains have an annual precipitation average between 250 and 500 millimeters. The barren and extremely barren terrains are the deserts, and the semi-arid terrains covered with gramíneas are called steppes generally.

Nevertheless, the dryness by itself does not provide a precise description than it is a desert. For example: the city of Phoenix, in Arizona, less than receives 250 mm of rain to the year, and immediately is classified it like located in a desert. Nevertheless, some frozen regions of Alaska or Antártida also receive less than 250 mm of rain per year, nevertheless for the vulgar perception they are not desert (is indeed cold or desert deserts nivales: in them the life superior is very difficult but the evapotranspiración and the possibility of finding water fresh is well-known greater than in the classic vulgar notion than it is a desert), to define a desert is warm that is cold one of the parameters that are used is the one of the annual average of precipitations: less than 500 mm/año suppose a semi-arid zone where much hydric stress exists (although this it is possible to be morigerar if the temperatures are low in an order produced to temperatures below 15°C annual average, another ecological definition of desert is the one of a zone that receives 100 equal or less than mm/año of precipitations and without the compenzación of no other natural fresh water source.

The differences of criteria reside in the called phenomenon evapotranspiración. The evapotranspiración is the combination of loss of water by atmospheric evaporation of the water of the ground, along with the loss of water also in the form of steam, through the vital trials of the plants. The evapotranspiración potential is, therefore, the amount of water that evaporates in a given region. The city of Tucson, in Arizona, receives about 300 annual mm of rain, nevertheless, can evaporate about 2,500 mm of water in the period of 1 year. In other words, it means that almost 8 times more water could evaporate of the region of which perpendicularly falls. The rates of evapotranspiración in regions of Alaska are inferiors enough; then, still receiving minimum precipitations, these special regions are much of the basic definition of a desert: a place where the evaporation surpasses the total of the pluviométrica precipitation.

Said this, different forms from deserts exist. The cold deserts can be snow protections; these places do not receive much rain, and the one that falls she remains congealed like compact snow. These areas are called tundra commonly, when in them one short station with temperatures over zero Celsius degrees exists and blooms something of vegetation in that period; or polar regions, if the temperature throughout remains under the freezing point the year, leaving the ground practically without ways of life.

The majority of the deserts not-polestars forms because they receive very little water. The water tends to refresh, or at least to moderate, the effects of the climate in which he is abundant. In some parts of the world, the deserts arise due to the existence from barriers to rain, when the air masses most of unload their humidity on a mountain range; other areas are barren because they are very distant of humidity sources.

The deserts also classify by their geographic location and predominant climatic landlord, like tradewinds, average latitudes, barriers anti-rains, slabs, of monsoon, and polestars. Some desert areas old presents in not-barren regions form the paleodesiertos calls. Aside from this, also the extraterrestrial deserts exist.

Deserts in tradewinds regions

The tradewinds take place in two strips of the balloon divided by the Line of Ecuador, and they form by the heating of the air in the equatorial region. These winds droughts dissipate the cloud cover, allowing that is warmed up plus the ground by the light of the Sun. The majority of the great Earth deserts is in regions crossed by tradewinds. The desert major of our planet, the Sahara, to North Africa, that sometimes undergoes temperatures of more of 57° C, is a tradewinds desert.

Deserts of average latitudes

The deserts of average latitudes are also located between the parallels 30° and 50° N. and in the same strip in the South hemisphere, in subtropical zones of atmospheric high pressure. These deserts are in river basins of drainage separated from the oceans and have great variations of annual temperatures. The desert of Sonant, in the southwest of North America is a typical desert of average latitude. The desert of Tengger, in China, is another example.

Deserts due to barriers to the humid air

The deserts of this type form due to great mountainous barriers that prevent the humid cloud arrival in the areas to leeward (that is, protected of the wind, that brings the humidity). To size in which the air raises by the mountain, the water hurries and the air loses its humid content. Thus, a desert in the opposite side forms. The desert of Judea in Israel and Palestine, and the one of Whose in Argentina, they are an example.

Coastal deserts

The coastal deserts are located generally in the western edges of continents next to the Tropical of Cancer and Capricorn. They are influenced by cold coastal ocean currents, that reason parallel to the coast. Due to the local wind systems that dominate tradewinds, these deserts are less stable than those of another type. During the winter, the fog, produced by ascending cold currents, frequently covers the coastal deserts with a white mantle that the solar radiation blocks. The coastal deserts are relatively complex, because they are the product of ground systems, oceanic and atmospherics. A coastal desert, the one of Atacama, in Chile, is driest of the Earth. In him, a possible rain of being measured - it is to say, of 1 mm or it can more take place each once 5, 20 or until every 400 years.

Deserts of monsoon

Monsoon, derived from an Arab word that it means “climatic station”, talks about to a wind system with a marked azonal reversion. The monsoons the continents and the oceans are developed in response to variations of temperature between. The tradewinds of the south of the Indian Ocean, for example, clear rains in India at which they arrive at the coast. In agreement the monsoon crosses the India of south-east to the northwest, by the Talweg call of the Monsoon (approximately the valley of the Ganges river) and hits against high mountains of the Himalayas loses its humidity in copiosísimas rains and snowed until the point that in the Eastern side of the mountainous imprisonment Aravalli the wind already is dry and with effect they foen (that is to say by adiabatic heating). The deserts of the Rajastán and Cholistán in the northwest of India, and the desert of Thar between Pakistan and India, are part of a region of desert of monsoon to the west of the mountainous imprisonment.

Cold deserts

The polar deserts are areas with an annual precipitation superior to 250 mm and one average temperature of the warmest month of the year inferior to 10° C. The polar deserts of the planet cover almost 90 million km ² and are mainly rock beds or gravel plains. The sand dunes are not typical of these deserts, nevertheless the snow dunes (sastruguis) form commonly in areas where the local precipitation is more abundant. The exchanges of temperature in the polar deserts frequently surpass the freezing point of the water. This rotation ice-I melt leaves characteristic marks in the ground, that arrive at half meter of diameter. Examples of cold deserts are the one of Gobi in Mongolia and China, the one of Tibet, the one of the Great Snow-covered River basin and the one of the Puna and the Altoandino Desert.

Polar deserts

They are characterized by two desertizantes factors: the atmospheric high pressures (anticyclone constant presence) and, especially, the low or null index of precipitations to the year since when being the temperature constantly under 0°C the water is naturally in solid state (except for rare exceptions), the major of these polar deserts is also nival and it is located in the inner areas of the Antártida (in spite of it, the usual perception is the one about which it is not a desert since in such region the water abounds, but in the form of ice, without by sustaining to organic life superior), similar although less extensive it is the correlative desert to the inlandsis of the center of Greenland.

Paleodesiertos (desert “fossils”)

The investigations in sand seas (vast dune regions) old, archaeological exchanges in marshy river basins, analyzes and of vegetation indicate that the climatic conditions changed considerably in main areas of the planet in a recent geologic past. During last the 12,500 years, for example, parts of some deserts already were very barren. Near a 10% of the terrain located between the latitude 30° N. and 30° S. today are covered by deserts. Nevertheless, 18,000 years ago, the deserts (that formed two immense belts) occupied only a 50% of this area. As it happens today, the tropical forests and the savannahs occupied the zone between these two strips.

Fossil sediments of deserts of up to 500 million years of age in many parts of the balloon have been. The landlords of sediments dunares were in areas that today are not desert. Those same areas receive today between 80 and 150 mm of rain per year. Some old regions dunares are occupied at present by humid tropical forests.

The sand mountains called Sand Hills (= Sand Cholines) are dune a dead field of 57,000 km ² in center of Nebraska. The greater sand sea of the Western Hemishpere today is stabilized by the vegetation, and receives near 500 mm of rain per year. The dunes of Sand Hills arrive at the 120 ms of height. The desert of the Kalahari also is paleodesierto.

Desert climates

The deserts are distributed between different zones:

• Semi-arid zones: They have an average of precipitations of 200 to 500 mm annual. Usually they are located in the edges of the deserts and include around 15 % of the land surface of the planet
• Arid areas: they are annual precipitations of 25 to 200 mm and includes 16 % of the land surface of the planet
• Hyperbarren zones: they are so dry that sometimes it does not rain during years. These include 4 % of the ground surface.

Flora and fauna

As much plants (xerófitas), like animal, survive if they manage to adapt to the rigor of the conditions of this bioma.

The people also have adapted to the conditions of the desert, looking for forms to take advantage of the water available, for example, living in oasis, near springs, or excavating wells in I milk of rivers droughts to take advantage of the water supplies. Also, in this surroundings usually they use the solar energy, to even cook (it cooks to pave).

The few amphibians that exist between the animal of the desert are able to be long time in lethargy during the periods droughts. When rains arrive, they mature quickly, ova mate and put. Many birds and rodents only reproduce during the periods following to winter rains, time at which the growth of the vegetation takes place. Some rodents of the desert, like the rat North American kangaroo and jerbillo African, feed themselves on dry seeds; their metabolic trials are extremely effective to conserve and to recycle water, and its urine highly is concentrated. Several mammals of the desert, like the camel, are able to support a considerable dehydration. The majority of the reptiles and mammals is nocturnes and remains in fresh madrigueras or in the shade during the day. Some reptiles, like the cornudo lizard, can control the heat production of their body, varying the tempo of the beats of their heart and its metabolism. Mammals like órix, own the capacity to make oscillate the temperatures of their bodies catching heat during the day and releasing it during the night.

The greatest Earth deserts

• It is seen: Annex: Greater deserts of the world

Desert places in other planets

Mars is the unique one of planets of the Solar System in which already aeolian phenomena have been identified. In spite of their superficial atmospheric pressure (only 1/100 of the ground one), the landlords of atmospheric circulation in Mars have formed a circumpolar sand sea with more than 5 million km ², much greater than the ground deserts.

The Martian deserts consist mainly of dunes in the form of average-moon of planar areas next to ice the permanent layer of the North Pole of the planet. The smaller dune fields occupy the fund of many craters in the Martian polar regions.

It is also seen

• Desertización
• Desertificación
• Oasis
• Erosion

External Links

• Wikimedia Commons lodges content multimedia on Desierto.Commons
• Scientific consensus on the Desertificación, summary realized by GreenFacts of a report of the Evaluation of Ecosystems of the Millenium

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