Settling velocities in air were measured for natural quartz sand grains selected to provide a range of grain diameters and shapes (roundness and sphericity). The measured settling velocities W m , obtained in an 11.2 m-long settling tube, are significantly lower than the settling velocity W s of a sphere having the same weight. The two are empirically related by the equation W m = 1.10W s (super 0.90) with R 2 = 0.84. Although this departure of the settling velocities of natural grains from the settling rates of spheres is most likely due to grain shape effects, no dependence could be found on shape measures such as the Corey shape factor. Simple Gibbs-type empirical equations are also obtained for sand grains settling in air, one equation for the settling of spheres and a second for the irregular natural sand grains.--Modified journal abstract.

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