Different meteorological factors are important in modeling a release, depending on its magnitude and type. Initially, the wind speed and direction at the release location control the plume's path. Accurately determining the winds at the accident site is paramount to producing a credible result. If the release involves a powerful explosion or fire, the winds thousands of feet above the ground may determine the plume's transport, and its vertical extent may be limited by a temperature inversion in the atmosphere. As the plume travels downwind, turbulence dilutes the plume by mixing the material vertically and horizontally, which the modeler must estimate.
Mountain ridges can block flows, and valleys are channels for winds.
Furthermore, material may be removed from the plume by being deposited on the ground or washed out by precipitation. If the event needs to be projected several days into the future, ARAC may run its own version of the Navy's Coupled Ocean-Atmosphere Mesoscale Prediction System model to generate detailed wind data. For example, it might show how hills deflect the oncoming wind flow and how wind is channeled through passes. LODI, the third code, simulates the release, transport, diffusion, and deposition of a release by using numerical marker particles, that is, the positions in space representative of the released material. Finally, a graphics code converts the data to plots of interest, such as doses for radiological accidents, air concentrations for chemical releases, or ground deposition.