4.4 CALM AND MISSING WIND CONDITIONS

        The MEPAS atmospheric component uses six classes of atmospheric stability to characterize the dispersion rates. The atmospheric stability classes are designated by the letters A to F (Slade 1968) and are commonly referred to as the Pasquill Stability Categories (Pasquill and Smith 1983).

        In the characterization of frequency of winds in each of these stability classes, often some fraction of the reported cases are for calm or zero wind speed conditions. Since the formulations for the climatological atmospheric component require a non-zero wind speed, a method was adopted for handling the reported calm conditions based on assigning these conditions a low, non-zero wind speed in a manner that reflects the occurrences of winds with low wind speeds. This procedure is appropriate because calm conditions are more correctly defined as conditions when the winds are less than the starting speed of the anemometer--the atmosphere is very seldom really calm with no air movements.

        The occurrences of calm conditions in each stability class are distributed in a special wind speed class. The relative frequencies of occurrence of winds in the lowest wind speed class are used to distribute calms as a function of direction. If the lowest wind speed class has no entries, then the calms are distributed equally over the 16 direction sectors. Calm conditions are modeled with a wind speed of 0.5 m/s.

        The input table of dispersion conditions is normalized to represent all conditions. That is, missing conditions are distributed according to the input frequencies of occurrence.