When air blows across the surface of dry, loose sand, a critical shear velocity (fluid threshold, ut), must be achieved to initiate motion. However, since most natural sediments consist of a range of grain sizes, fluid threshold for any sediment cannot be defined by a finite value but should be viewed as a threshold range which is a function of the size, shape, sorting and packing of the surface sediment. In order to investigate the initiation of particle movement by wind a series of wind-tunnel tests was carried out on a range of pre-screened fluvial sands and commercially available glass beads with differing mean sizes and sorting characteristics. A sensitive laser-monitoring system was used in conjunction with a high speed counter to detect initial grain motion and to count individual grain movements. Test results indicate that when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first entrained by the fluid drag and lift forces either in surface creep (rolling) or in saltation (bouncing or hopping downwind). As velocity continues to rise, larger or less exposed grains may also be moved by fluid drag. On striking the surface saltating grains impart momentum to stationary grains. This impact may result in the rebound of the original grain as well as the ejection of one or more stationary grains into the air stream at shear velocities lower than that required to entrain a stationary particle by direct fluid pressure.

As a result, there is a cascade effect with a few grains of varying size initially moving over a range of shear velocities (the fluid threshold range) and setting in motion a rapidly increasing number of grains. Results of the tests showed that the progression from fluid to dynamic threshold, based on grain movement, can be characterized by a power function, the coefficients of which are directly related to the mean size and sorting characteristics of the sediment.