Dispersion Conditions and Winds

There are six categories of “dispersion conditions” which are calculated by the Oklahoma Dispersion Model (ODM): Excellent (EX), Good (G), Moderately Good (MG), Moderately Poor (MP), Poor (P), and Very Poor (VP). The first three categories appear in various shades of green on the dispersion maps, while the last three range from beige to orange to red. A numbered system is also used to represent the six dispersion categories: 1=VP, 2=P, 3=MP, 4=MG, 5=G, and 6=EX. The ODM estimates ground-level dispersion, which is the ability of the atmosphere to dilute and disperse a compound such as smoke as it travels downwind. The calculated dispersion categories are valid for downwind distances of 1/4 mile to several miles. The categories can be interpreted as follows – for a given downwind distance (e.g., 1 mile), the smoke concentration near the plume centerline will be greatest under VP conditions and the lowest under EX conditions. Thus, as a general guideline for prescribed burning, Moderately Good (MG) or better (G, EX) conditions should be present during the burn to avoid smoking out potential sensitive areas downwind. In this case, in the forecast element table on the Fire Prescription Planner, one would set a “Lower Limit” for Dispersion Conditions of “Moderately Good.” Note that since the dispersion calculations are a function of the forecast weather conditions, inaccuracies in the NAM weather forecast will lead to inaccuracies in the dispersion forecast.