True or False: The predicted sigma values are unaffected by atmospheric stability class.

True or False: The predicted sigma values are unaffected by atmospheric stability class.

• A. True
• B. False
• C. Not affected by stability class
• D. Only affected by source height

Answer: (B)

Dispersion parameters describe how much the plume spreads as it moves away from the source. In Gaussian dispersion models, the horizontal and vertical spreads are quantified by sigma_y and sigma_z. These sigmas are not fixed numbers; they depend on how turbulent the atmosphere is, which is captured by the atmospheric stability class. Different stability classes represent different levels of turbulence: unstable conditions produce stronger mixing and larger dispersive spreads, so sigma_y and sigma_z grow more quickly with downwind distance. Stable conditions suppress mixing, leading to smaller sigmas and a narrower plume at a given distance. Because of this dependence, the predicted sigma values do change with stability class.

Dispersion parameters describe how much the plume spreads as it moves away from the source. In Gaussian dispersion models, the horizontal and vertical spreads are quantified by sigma_y and sigma_z. These sigmas are not fixed numbers; they depend on how turbulent the atmosphere is, which is captured by the atmospheric stability class. Different stability classes represent different levels of turbulence: unstable conditions produce stronger mixing and larger dispersive spreads, so sigma_y and sigma_z grow more quickly with downwind distance. Stable conditions suppress mixing, leading to smaller sigmas and a narrower plume at a given distance. Because of this dependence, the predicted sigma values do change with stability class.