ABSTRACT Microrelief is an important soil surface feature that significantly affects wind erosion and, therefore, must be simulated for the new USDA-Agricultural Research Service Wind Erosion Research Model to adequately predict wind erosion. A new surface roughness index, the Cumulative Shelter Angle Distribution, which integrates oriented and non-oriented roughness into a single index has been proposed. The objectives of this article are to describe a function which fits the Cumulative Shelter Angle Distribution and to approximate the coefficients of this function based upon well documented soil surface properties. The Cumulative Shelter Angle Distribution is adequately described using a two-parameter Weibull function. TTie Weibull function coefficients are predicted as functions of ridge height and random roughness. Oriented and non-oriented roughness can be modified separately as functions of tillage or weather and the modified values used to estimate the Cumulative Shelter Angle Distribution. An example of the surface roughness estimation is given.