182 Desert Plants 6(4) What is a Desert?

What is a desert? A quick answer that comes to mind is that it is a region where there is little precipitation. This may be satisfactory as far as it goes, but why is there little rain or snow? Aridity arises from a number of general causes acting indi-vidually or in combination. One of these is the separation by topography or distance from oceanic moisture sources. One of the causes of aridity in the Takla-Makan and Turkestan Gobi deserts in central Asia, for example, is their great dis-tance from major moisture sources. Another cause is the drying effect of air masses moving over a mountain barrier, as is the case with the South Ameri-can Patagonia desert in the lee of the Andes, as well as the Great Basin desert in North America cut off from the Pacific Ocean by high mountains