Frequent Questions

Atmospheric conditions used for off-site consequence analyses

What atmospheric conditions must a source assume when performing the offsite consequence analyses required under 40 CFR Part 68, Subpart B?

 

For the worst-case release analysis, 1.5 meters per second wind speed and F atmospheric stability class must be assumed, unless the stationary source owner or operator can demonstrate that local meteorological data applicable to the stationary source show a higher minimum wind speed or less stable atmosphere at all times during the previous three years (40 CFR §68.22(b)).  For releases of regulated toxic substances, the owner or operator must also use the highest daily maximum temperature and the average humidity for the site during the past three years.  Owners and operators using the RMP Offsite Consequence Analysis Guidance will assume 25 degrees Celsius and 50 percent humidity (40 CFR §68.22(c)).  

For the alternative release scenario, the owner or operator should use the typical meteorological conditions (i.e., wind speed and atmospheric stability) for the stationary source (40 CFR §68.22(b)), as well as typical temperature and humidity data gathered at the stationary source or at a local meteorological station (40 CFR §68.22(c)).  Owners and operators using the RMP Offsite Consequence Analysis Guidance will assume 3 meters per second wind speed, D atmospheric stability class, 25 degrees Celsius and 50 percent humidity.

 

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