Central Campus Air Quality Model (CCAQM) Instructions

Atmospheric Stability

Stability is the tendency to resist or enhance vertical motion (turbulence) in the atmosphere.  A neutral atmosphere does not resist or enhance mechanical turbulence.  A stable atmosphere resists turbulence, but an unstable atmosphere enhances mechanical turbulence.  Furthermore, the mechanical turbulence in the atmosphere greatly affects the concentration of a pollutant.  Thus, the more unstable the atmosphere is, the greater the dilution of the pollutant.

Stability is determined using both the change of temperature with height (which can be termed the adiabatic lapse rate) and wind speed.   Stability classes are determined for different meteorological conditions, which are dependent on wind speed and solar insolation during the day and cloud cover during the night.  The classes A, B, and C stand for very unstable, unstable, and slightly unstable conditions, respectively; D stands for a neutral condition; and E and F stand for stable and very stable conditions, respectively.  Class G represents the most stable atmosphere possible, and is used for the worst-case simulations.

Criteria for classifying atmospheric stability categories are listed in the following table:

Atmospheric Stability Classification System

Strong daily solar insolation is a clear summer day with the sun higher than 60° above the horizon.  Moderate solar insolation is a summer day with few broken clouds with sun between 35° and 60°.  Slight insolation corresponds to a fall afternoon, or cloudy summer day, or clear summer day with the sun less that 15° above the horizon.