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Pasquill Stability Classes

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A: Extremely unstable conditions D: Neutral conditions
B: Moderately unstable conditions E: Slightly stable conditions
C: Slightly unstable conditions F: Moderately stable conditions
G: Extremely Stable

Meteorological conditions defining Pasquill stability classes.

Daytime insolation Night-time conditions
Surface wind speed (m/s) Strong Moderate Slight Thin overcast or > 4/8 low cloud <= 4/8 cloudiness
< 2 A A - B B E F
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

Source: Pasquill, 1961.

1. Strong insolation corresponds to sunny midday in midsummer in England; slight insolation to similar conditions in midwinter.

2. Night refers to the period from 1 hour before sunset to 1 hour after sunrise.

3. The neutral category D should also be used, regardless of wind speed, for overcast conditions during day or night and for any sky conditions during the hour preceding or following night as defined above.

PG classes for fluctuations in wind direction and the vertical temperature gradient.

Pasquill Class Sigma Theta (degrees) Delta T/Delta Z (Deg C/100 m)
A 25 -1.9
B 20 -1.9 to -1.7
C 15 -1.7 to -1.5
D 10 -1.5 to -0.5
E 5 -0.5 to 1.5
F 2.5 1.5 to 4.0
G 1.7 >4.0
Modified: December 4, 2019
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