Environmental Engineering Reference
In-Depth Information
Real
source
Point of interest
z - z s
z
z s
z 1
Reflected
plume
Surface
x
z = 0
- z 1
- z s
Virtual
source
FIGURE 6.31 Reflection of a gaseous plume of a pollutant mathematically analyzed using
an imaginary source.
σ z have been correlated to both mechanical turbu-
lence (wind speed at 10 m height) and buoyant turbulence (time of day). These give
rise to what are called Pasquill stability categories given in Table 6.6. Based on the
stability class, appropriate standard deviations for any distance x downwind of the
source are obtained (Figure 6.32).
The standard deviations
σ y and
TABLE 6.6
Pasquill Stability Categories
Day
Night
Incoming Solar Radiation
Cloud Cover
Wind Speed at Mostly Mostly
10 m Height (m/s) Strong Moderate Slight Overcast Clear
< 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
Note: Class A is the most unstable and Class F is the most stable. Class B is moderately unstable
and Class E is slightly stable. Class C is slightly unstable. Class D is neutral stability and
should be used for overcast conditions during day or night.
 
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