A glance at the world map reveals that most of the desert region is on the western part of the continent for example. Mohave desert in North America, Atacama in Southern America, Namib, Sahara in Africa , Arabian in west Asia. The reasons for such homogeny are:
- The hot desert lie astride the Horse latitudes or the sub tropical high pressure belts where the air is descending, a condition least favourable for precipitation of any kind to take place.
- The rain bearing Trade winds blow off-shore and the westerliesthat are on shore blow outside the desert limits. Whatever winds reach the desert blow from cooler to warmer regions, and their relative humidity is lowered, making condensation almost impossible. The relative humidity reaches to about 30%. This make condition for drought.
- On the western coast, the presence of cold currents give rise to mists and fogs by chilling the on coming air. This air is later warmed by contact with the hot land, and little rain falls. The desiccating effect of the cold peruvian current along the Chilean coast is so pronounced that the mean annual rainfall for the Atacama desert is not more than half an inch.
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