Environmental Engineering Reference
In-Depth Information
pollution episodes. Los Angeles, Denver, Salt Lake City, Mexico City, and industrial-urbanized river
valleys across the continents are examples with frequent inversions, where pollutant concentrations
often exceed air-quality standards, and the population suffers from pollutant-caused respiratory and
other diseases.
When the temperature in the upper layers is much colder than that in the lower layers, upper
air parcels fall downward due to their larger density, and lower air parcels move upward. This
movement generates turbulent or eddy diffusion. The steeper the temperature gradient, the more
the turbulent intensity. This is called an unstable condition. A temperature gradient that is equal
to the dry adiabatic lapse rate is called a neutral condition, and it leads to moderate turbulence. A
temperature gradient that is less steep than the dry adiabatic lapse rate, or even a positive gradient,
is called a stable condition, in which there is minimal or no turbulence at all.
It is a convention to classify the turbulent conditions of the atmosphere into six stability
categories, called Pasquill-Gifford stability categories. They range from A, very unstable (very
turbulent), to F, very stable (little turbulence). Category D is called neutral, with moderate turbu-
lence. The negative temperature gradient (lapse rate) of category D coincides with the dry adiabatic
lapse, about
10
◦
C/km. For categories A, B, and C the magnitude of the negative gradient is greater
than that for D; for E the gradient is smaller than that for D; and for F the gradient is positive.
While the lapse rate of the atmosphere is measured twice daily at the weather stations, the
stability categories can be approximately determined by knowing the surface wind speed, insolation,
and cloud cover. Table 9.5 lists the stability categories. It is seen that in daytime, low wind speeds
and strong insolation lead to unstable categories A or B; high wind speeds and moderate to slight
−
TABLE 9.5
Pasquill-Gifford Stability Categories
Day
Night
Incoming Solar Radiation
Cloud Cover
Surface Wind
(m/s)
Thinly overcast
or
≥
4/8 cloud
Strong
a
Moderate
a
Slight
a
≤
3/8 cloud
<
2
A
A-B
B
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
Stability Category
Stability
dT
/
dz
(
◦
C/100 m)
A
Extremely unstable
≤−
1
.
9
B
Moderately unstable
>
−
1
.
9 but
≤−
1
.
7
C
Slightly unstable
>
−
1
.
7 but
≤−
1
.
5
D
c
Neutral
>
−
1
.
5 but
≤−
0
.
5
E
Slightly stable
>
−
0
.
5 but
≤
1.5
F
Very stable
>
1.5 but
≤
4
.
0
a
Zenith angle under clear skies
>
60
◦
; moderate 35-60
◦
; slight
<
15
◦
.
b
Rural areas
=
F; large urban areas
=
D; small urban areas
=
E.
c
Category D, neutral, applies to heavy overcast, day or night, all wind speeds.
