In protective action distance modeling, what primarily determines the extent of the downwind hazard footprint?

• A. Wind direction and worst-case wind speed
• B. Temperature gradient
• C. Humidity
• D. Altitude

Answer: (A)

The extent of the downwind hazard footprint is driven mainly by how the plume is carried by the wind. The plume travels in the downwind direction, and the speed of the wind determines how far it can spread before concentrations fall below hazardous levels. Using a worst-case wind speed provides a conservative estimate of the maximum reach, which is essential for planning protective actions. Temperature gradients, humidity, and altitude do influence plume behavior—through stability, buoyancy, and mixing—but they do not control how far the plume travels as directly as wind direction and wind speed do.