A STUDY OF WINDBORNE SAND AND DUST IN DESERT AREAS

Severe sand and dust storms occur at a rate of less than 2 per year on the average in the des ert areas of southwestern United States. Less severe storms average about 4 per year. Most storms occur in late winter or spring and last for a period of 1 to 3 days. Storm incidence is higher in desert areas that have been dis turbed by man's activities, such as agrrgul ture or large- scale military maneuvers. During a storm, sand and dust in militarily significant amounts are carried a few miles at most, but generally only a few hundreds or thousands of feet. Wind-driven sand is carried mostly within 2 feet of the ground, with 6 feet as a maximum height in all but extremely high winds. Critical pick-up velocities of winds vary according to the type of desert surface, the grain size and coherency of the surface materials, and whether or not the surface has been disturbed artifi cially. In dune areas, winds of 10 to 15 miles per hour will innmovement, and on other sandy terrain, winds of 20 miles per hour will be necessary for this. Fine materials on desert flats will be set in motion at 20 to 25 miles per hour, and on alluvial fans and playas at 30 to 35 miles per hh r. No wind-blown mate rial will be derived from desert pavements unless the surface has been broken, and on all other above-mentioned types, disturbing the surface will lower critical pick-up velocities by as much as 5 miles per hour. (Author)