To be transported far from their place of origin, dust particles must have terminal sedimentation velocities (corresponding to particle size, shape, and density) that are small compared to the root-mean-square velocity fluctuations of the supporting air. Measurements of size distribution changes with height above eroding soil surfaces confirm this. Size distribution of dust obtained by aircraft in the southwestern United States, hundreds of kilometres from the source region, show that the diameters of the largest particles range between 0.02 and 0.1 mm.
In studying the production of dust, both aerodynamic and soil relationships must be considered. The partitioning of stress to nonerodible elements, for example, is an important physical mechanism in determining the threshold wind speed at which soil is eroded. Another important factor in the initiation of wind erosion is the strength of the aggregation of the soil.
Measurements of the vertical flux of particles smaller than 0.02 mm above eroding soils show a sharp increase of dust production with wind speed. Soils that contain fine material (silt and clay) tend to produce more fine dust than coarse soils do for an equal amount of total soil erosion. For fine-textured soils, the history of the soil is important in determining fine-particle emissions by erosion. For example, aggregates formed in a clay soil that had been water-soaked and dried were very resistant to breakage, whereas aggregates in the same soil that had been subjected to high winter winds and drouth were more easily broken.
Field observations of fine particle emissions are related to total soil movement and, by Agricultural Research Service empirical formula, to soil and wind conditions. Wind-tunnel simulations of erosion as well as high-speed measurements of concentration versus time in observed wind erosion are used to illuminate the problem of wind erosion of arid soils.