The amount of turbulence in the ambient air has a major effect upon the rise
and dispersion of air pollutant plumes. The amount of turbulence can be
categorized into defined increments or "stability classes". The
most commonly used categories are the Pasquill stability classes A, B, C, D,
E, and F. Class A denotes the most unstable or most turbulent conditions and
Class F denotes the most stable or least turbulent conditions.
The
Pasquill stability classes are presented below as they are defined by the
prevailing meteorological conditions of: (a) surface windspeed measured at
10 meters above ground level and (b) day-time incoming solar radiation or
the night-time percentage of cloud cover.
Surface
Windspeed
Daytime
Incoming Solar Radiation
Night-time
Cloud Cover
m/s
mi/hr
Strong
Moderate
Slight
>
50%
<
50%
<2
<5
A
A-B
B
E</CENTER
F
2-3
5-7</CENTER
A-B
B
C
E
F
3-5</CENTER
7-11
B
B-C
C
D
E
5-6
11-13</CENTER
C
C-D
D
D
D
>6
>13
C
D
D
D
D
Note:
Class D applies to heavily overcast skies, at any windspeed day or
night.
NOTES:
(1) m/s = meters per second
(2) mi/hr = statute miles per hour
Effect of Altitude on
Windspeeds:
The
winds aloft generally have a higher velocity than the winds at ground level.
In other words, at any given time and place, windspeed usually increases
with altitude. The effect of altitude on windspeed involves two factors:
Øthe degree of turbulent mixing prevailing
in the atmosphere at the given time and place, as characterized by the
Pasquill stability class
Øthe terrain's surface area roughness, which
induces surface friction at the given place