From Mark Monmonier’s Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather (1999):
“To assess the effects of complex terrain and whimsical weather, planners and environmental engineers use hourly data on wind, temperature, and humidity to simulate the proposed the proposed facility’s operation throughout a more or less typical year. The simulation will treat the region as a huge grid of half-mile-square cells extending ten miles or more outward from the plant—the grid’s resolution and extent depend on terrain as well as the height of the smokestack.—and use one or more air-dispersion models to estimate fallout at each square for every hour of every day throughout the period. In addition to compiling cumulative totals, which are used to make maps of average annual deposition, the simulation will keep track of highest and second-highest concentrations for 1-, 3-, 8-, and 24-hour periods” (108)
How to Create a Toxic Plume Map demonstrates how to construct a, “circular vulnerability zone around a potentially dangerous facility by overlaying on a large-scale topographic or street map the standard plume developed for chlorine by the National Transportation Safety Board using the US Coast Guard’s Hazards Assessment Computer System (HACS)” (102).
Model plumes can be, “‘photocopied onto a transparency sheet in a standard copy machine [and then] laid over a standard 7.5-minute community topographic (quadrant) map from the U.S. Geological Survey.’ The reader can then, ‘place the release point over the source of the chemical, and rotate the model to show a full vulnerability zone.’ Because winds vary, the coalition recommends using the distances in the model plume as a radii for circles describing the threat for all possible wind directions around the industrial plant, railway siding, grade crossing, or other plausible accident site. Concentric circles show relative risk and identify areas where residents have little or no time to close windows or evacuate…circular vulnerability zones are entirely hypothetical” (102-103).
See also: Make Your Own Plume