The principal desert areas of the world are found in the two subtropical belts between latitudes 10� and 35� north and south of the equator. Very little land in the Southern Hemisphere lies between these latitudes, with the exception of the southernmost tip of the African sub-continent arid the narrow tail of the South American island continent, and therefore the major deserts of the world lie in the Northern Hemisphere. Desert conditions are characterized by extreme dryness; annual precipitation is less than 25 centimetres, and the sun evaporates all water that falls as rain.
The lack of moisture reaching desert areas is due to a number of factors, The principal one is the descent of dry air from the upper atmosphere, a feature of the global circulation pattern that is typical of these latitudes. Air is drawn to the low-pressure zones in equatorial regions, where it is heated and forced to rise. In the upper reaches of the atmosphere the air spreads out from the equator, cools gradually and descends, reaching the ground in desert areas, where it has a very low water content. In some cases a desert may owe its existence to being situated in the heart of a continent, far from any marine influence or moist wind. Allied to this is the rain-shadow phenomenon, found in areas where air currents from the sea cross high mountains, dropping their rain on the seaward side as they rise. On the other side of the mountain the air descends, completely dry, giving rise to a desert area.
The intensity of solar radiation in the desert is very great in comparison with other areas. In moist regions up to 60 per cent may be reflected away by clouds, atmospheric dust, water and plant
surfaces, but in deserts only ten per cent of the total radiation is reflected in this way. On the other hand, due to the lack of insulating cloud cover, up to 90 per cent of the accumulated heat of the day is lost at night through radiation. The result is an extremely large temperature difference between day and night, which may be as great as 40�C.
The worldwide extent of deserts is now much smaller than was the case during the Age of Man. For one thing there is a smaller area of land lying within the desert belts than there was then - due largely to the movement of the Australian continent northwards out of the desert belt. Also in man's time, inefficient agricultural techniques and widespread grazing of domestic animals on poor land artificially enlarged the desert regions and was one of the factors that led to man's ultimate decline; with the desert becoming more extensive year by year, the area of the earth's surface suitable for cultivation decreased. After man's disappearance the earth's natural habitats re-established themselves and the deserts resumed their natural proportions.
Life in the desert has to cope with many hostile factors, such as lack of water and extremes of temperature. Even so a large number of animals and plants have evolved to cope with them successfully. Similar compensating adaptations - such as highly efficient kidneys producing particularly concentrated urine, large ears to dissipate heat and the ability to burrow to depths at which conditions are less severe - have been developed independently in many widely separated groups of animals in different areas of desert.