Gas escaping from a pressurized source forms a plume that spreads downwind, diluting as it travels due to atmospheric turbulence. Wider, faster-diluting plumes occur under unstable conditions (class A); narrow, slow-to-dilute plumes occur under stable conditions (class F) — the same leak rate can be far more hazardous on a stable, calm night.
C(x,y,z) = Q / (2π·u·σy·σz) · exp(-y²/2σy²) · [exp(-(z-H)²/2σz²) + exp(-(z+H)²/2σz²)]
σy, σz grow with downwind distance x and depend on stability class
- Leak rate — sets the source strength Q; higher rate raises peak concentration everywhere downwind.
- Wind speed — faster wind dilutes the plume (lower concentration) but carries it further, downwind.
- Stability class — controls plume spread: unstable (A) spreads and dilutes fast, stable (F) stays narrow and travels far at high concentration.
- Evacuation zone toggle — shows/hides the ground-level hazard radius ring computed from the current concentration threshold.
Emergency responders use this exact Gaussian plume model (e.g. ALOHA software) to set evacuation perimeters after industrial chemical releases.