AirWare   On-line Reference Manual

Release Level 5.3.0 beta
Release Date 2007 06
Revision Level 1.1

Pasquill Stability Classes

The tendency of the atmosphere to resist or enhance vertical motion and thus turbulence is termed stability. Stability is related to both the change of temperature with height (the lapse rate)driven by the boundary layer energy budget, and wind speed together with surface characteristics (roughness).

A neutral atmosphere neither enhances nor inhibits mechanical turbulence. An unstable atmosphere enhances turbulence, whereas a stable atmosphere inhibits mechanical turbulence.

The turbulence of the atmosphere is by far the most important parameter affecting dilution of a pollutant. The more unstable the atmosphere, the greater the dilution.

Stability classes are defined for different meteorological situations, characterised by wind speed and solar radiation (during the day) and cloud cover during the night. The so called Pasquill-Turner stability classes (based on D. Bruce Turners Workbook of Atmospheric Dispersion Estimates include six stability classes:

    1 A very unstable
    2 B unstable
    3 C slightly unstable
    4 D neutral
    5 E stable
    6 F very stable

    The stability classes can be (roughly and empirically) related to the driving forces wind and boundary layer energy budget through wind speed, solar radition, and cloud cover:

    Wind speed (m/s) DAY
    Incoming solar radiation
    NIGHT
    Strong Moderate Slight > 4/8 cloud < 3/8 cloud
    < 2 A A - B B    
    2 - 3 A - B B C E F
    3 - 5 B B - C C D E
    5 - 6 C C - D D D D
    > 6 C D D D D

Modern approaches (e.g., in AERMET/AERMOD, CAMx) tend to use parameters of boundary layer physics related to the energy budget and surface roughness more directly, to derive continuous rather than discrete, classified turbulence parameters.


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