Question from John Pine:
From your paper posted on the Hurricane and Chemical Hazards web site you show two tables reflecting wind and distance of the release. If I understand the two graphics correctly, as the wind increases, the vulnerability zone decreases. Is this correct? What in your opinion accounts for this?

Response from Erno Sajo, Associate Professor, LSU
The explanation of shorter footprint with increasing wind speed is as follows: Atmospheric turbulence is proportional to air speed. The extent of the turbulence determines the rate of dilution of the pollutant. The higher the turbulence, the quicker the pollutant disperses and thus gives rise to a lower concentration. Hence, the footprint that indicates the area of impact by a given concentration (say IDLH) will be gradually smaller as the turbulence increases and the concentration decreases. The famous Pasquill-Gifford chart of atmospheric stability is related to turbulence. In that table the stability categories are listed as function of wind speed and solar input. High wind, high solar input gives rise to a C stability (slightly unstable), while low wind night time conditions are associated with F stability (strongly stable). Overcast skies coupled with above 5 m/s winds always fall in neutral stability, D (this is the predominant condition in case of a hurricane). Under this condition, atmospheric turbulence becomes dominated by the wind as there is insignificant solar heat-generated air motion. If the turbulence has a large scale (i.e. physically large) it tends to make a plume meander. A small scale turbulance tends to tear the plume apart. In case of a hurricane both turbulence scales are present, but current air dispersion models can only account for the small turbulences and shear stresses.