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Deserts are varied and variable, and it is impossible to arrive at a concise definition that satisfies every case. However, their most fundamental characteristic is a shortage of available moisture for plants, resulting from an imbalance between and . This situation is by considerable variability in the timing of rainfall, low atmospheric , high daytime temperatures, and winds.
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Averageprecipitation ranges from almost zero in some South American coastal deserts and Libyan deserts to about 600 millimetres (24 inches) in deserts in , although most recognized deserts have an annual rainfall below 400 millimetres. Some authorities consider 250 millimetres the upper limit for mean annual precipitation for true deserts, describing places with a mean annual rainfall of between 250 and 400 millimetres as semideserts. Regions this dry are barely arable and contribute to human food production only by providing grazing lands for livestock.
The arid conditions of the major desert areas result from their position in subtropical regions to either side of the moist equatorial belt. Thepattern known as the plays an important role in desert . In areas close to the Equator, where the amount of incoming per unit surface area is greatest, air near the ground is heated, then rises, expands, and cools. This process leads to the condensation of moisture and to precipitation. At high levels in the , the risen air moves away from the equatorial to descend eventually in the subtropics as it cools; it moves back toward the at low altitudes, completing the Hadley cell circulation pattern. The air descending over the subtropics has already lost most of its moisture as formed during its previous ascent near the Equator. As it descends, it becomes compressed and warmer, its falling further. (For further discussion of relative humidity, see .) Hot deserts occur in those regions to the north and south of the equatorial belt that lie beneath these descending, dry air masses. This pattern may be interrupted where local precipitation is increased, especially on the east sides of continents where winds blow onshore, carrying moisture picked up over the . Conversely, deserts may be found elsewhere, as in the lee of ranges, where air is forced to rise, cool, and lose moisture as rain falling on the windward slopes.
Rainfall in deserts is usually meagre. In some cases several years may pass without rain; for example, at Cochones,, no rain fell at all in 45 consecutive years between 1919 and 1964. Usually, however, rain falls in deserts for at least a few days each year—typically 15 to 20 days. When precipitation occurs, it may be very heavy for short periods. For instance, 14 millimetres fell at Mashʾabe Sade, , in only seven minutes on October 5, 1979, and in southwestern the entire annual rainfall commonly occurs as heavy showers falling within a single month. Such rainfall usually occurs only over small areas and results from local convectional cells, with more widespread frontal rain being restricted to the southern and northern fringes of deserts. In some local desert showers, the rain falling from clouds evaporates before it reaches the ground. Regions near the equatorial margins of hot deserts receive most of their rain in summer—June to in the Northern Hemisphere and December to February in the Southern Hemisphere—while those near the temperate margins receive most of their rainfall in winter. Rain is particularly erratic and equally unlikely to occur in all seasons in intermediate regions.
In some deserts that are located near coasts, such as theDesert of southwestern and those of the west coasts of the Americas in and , is an important source of moisture that is otherwise scarce. Moisture droplets settle from the fog onto plants and then may drip onto the soil or be absorbed directly by plant shoots. also may be significant, although not in deserts in from the central parts of continents where atmospheric humidity is consistently very low.
In most desert regions atmospheric humidity is usually too low to permitof fog or dew to any significant extent. Potential rates (the rate of evaporation that would occur if water were continually present) are correspondingly high, typically 2,500 to 3,500 millimetres per year, with as much as 4,262 millimetres potential evaporation per year having been recorded in in California. are not unusually strong or frequent in comparison with environments, but the general lack of vegetation in deserts the effect of wind at ground level. Winds can induce the of fine materials and the evaporation of moisture and thereby help determine which plants survive in the desert.
Hot deserts, as their name indicates, experience very highby day, especially in summer. Absolute maximum air temperatures in all hot deserts exceed 40 °C (104 °F), and the highest value recorded, in , is 58 °C (136.4 °F). The of the soil surface can rise even beyond that of the air, with values as high as 78 °C (172 °F) recorded in the . However, night temperatures can fall dramatically, because the same lack of cloud cover that admits high levels of incoming during the day also allows rapid loss of energy through long-wave radiation to the sky at night. Absolute minimum temperatures, except in desert areas close to the sea, are generally below the . Typical mean annual temperatures are between 20 °C (68 °F) and 25 °C (77 °F).
Temperate oroccur in temperate regions at higher latitudes—and therefore colder temperatures—than those at which hot deserts are found. These dry environments are caused by either remoteness from the , which results in low atmospheric humidity from a lack of onshore winds, or the presence of high mountains separating the desert from the coast. The largest area of temperate desert lies in , with smaller areas in western , southeastern , and southern . While they experience lower temperatures than the more typical hot deserts, temperate deserts are similar in aridity and consequent environmental features including landforms and soils.
The peculiar climaticof deserts has favoured the development of certain characteristic landforms. Stony plains called or plains are widespread, their surface covered by consisting of coarse gravel and stones coated with a of dark “desert varnish” (a glossy dark surface cover consisting of oxides of iron). Rocky, boulder-strewn plateaus cut by dry, usually steep-sided valleys called are also found in deserts in many parts of the world. The local topographic and microclimatic variations produced by this rugged surface, and the opportunities for runoff—and in a few places surface accumulation—of rainwater, are important in providing localized habitats for plants and animals. Large areas of loose, mobile sand provide the harshest and poorest of the major desert types.
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Desertare mainly immature, weakly developed in terms of their soil profiles, and mostly alkaline. Sands, sandy or gravelly loams, shallow stony soils, and alluvium (material deposited by rivers and streams) and scree-derived deposits (rocky material at the base of cliffs) predominate. Although almost always dry, these soils may support well-developed microbial , particularly in association with roots. Domestic animals, however, can have a impact by trampling and compacting the soil; this activity can reduce the infiltration of water and damage vegetation, leading to erosion and redistribution of soil materials.
Deserts 101 | National Geographic
Deserts are diverse ecosystems that occur on all seven continents. Learn about the four major types of deserts, the surprising amount of wildlife some of them contain, and how new desert areas are beginning to form.
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