Vapor Dispersion Calculator

In this simplified model, a faster wind disperses and dilutes the released vapor more quickly, spreading the same total release over a shorter downwind distance before it drops below the Target Concentration threshold. This reflects the general behavior of real plume dispersion, though a real incident also involves the direction and gustiness of wind, which this single-value estimate can't capture.

Yes — beyond setting Vapor Density and Heavier Than Air directly, Molecular Weight also factors into the concentration conversion used for Hazard Distance, since a heavier gas represents more mass per unit of parts-per-million concentration. Release Rate and Wind Speed, by contrast, have no effect on Vapor Density or Heavier Than Air at all — those two outputs depend on Molecular Weight alone.

It's a simplified approximation inspired by that framework, not a literal implementation of it. The real Pasquill-Gifford sigma-y and sigma-z curves — documented, for example, in NOAA/EPA's ALOHA Technical Documentation, which implements the same Pasquill-Gifford-Turner stability typing scheme for its own Gaussian dispersion model — are piecewise power-law functions of downwind distance that differ by stability class in a more complex way than the constant dispersion coefficient used here, which assumes plume spread grows roughly linearly with distance. The direction of the effect — unstable conditions dispersing faster, stable conditions concentrating more — matches real atmospheric behavior, but the specific coefficients are illustrative, not a certified match to published values.

No — this is a planning-level screening tool for training and general situational awareness, not a certified emergency-response instrument. Real protective-action decisions during an actual chemical release should come from validated dispersion software (such as ALOHA/CAMEO), site-specific data, and a qualified HAZMAT responder or emergency manager, never from a single simplified distance figure.