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Atmopsheric Stability

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) and wind speed.

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

  DAY
Incoming solar radiation
NIGHT
Wind speed (m/s) 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



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