Exam 18: Deserts and the Work of Winds

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Wind cannot lift a loose particle directly from the ground. To move any particle, wind must reach a velocity able to overcome the inertia of the largest grains. As these grains begin to move, rolling at first, they collide with other particles, propelling them upward. Once airborne, particles are carried forward by the wind in parabolic paths until gravity pulls them down. When these grains fall back to the surface, they strike other grains and in turn set them in motion. If wind velocity drops below the level required to move large grains, transport of mid- sized grains continues until the wind dies down further. Then even the smallest grains no longer saltate, ending the chain reaction that set the particles in motion.

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Deserts are defined by their aridity, or dryness. There are two guidelines that define a region as being dry enough to be a desert. The region must receive less than 25 centimeters (10 inches) of precipitation annually. The second guideline is more useful, and identifies deserts by means of an aridity index, that is, a ratio of a region's potential annual evaporation to its recorded average annual precipitation. An area is considered a desert if its aridity index is greater than 4.0, which means that its potential evaporation is at least four times greater than the average annual precipitation.