AIR-EIA was funded under INFO2000
by the European Commssion (PUB-1221).


air pollution and environmental impact assessment:
the multimedia information source

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

Incoming solar radiation
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

Related links:

© Copyright 1995-2016 by:   ESS   Environmental Software and Services GmbH AUSTRIA | print page