Page 336 - Fundamentals of Air Pollution 3E

292 19. Transport and Dispersion of Air Pollutants

zontal thermal structure of the atmosphere may exert a dominating influ-
ence at higher altitudes, such that the wind will shift counterclockwise
(back) with additional height. Cold air advection in an air layer will cause
the wind to back with height through that layer. Warm air advection will
cause veering with height.

B. Wind Speed
Wind speed generally increases with height. A number of expressions
describe the variation of wind speed in the surface boundary layer. A power
law profile has frequently been used in air pollution work:

where u(z) is the wind speed at height z, w(z a) the wind speed at the
anemometer measurement height z a, and p an exponent varying from about
0.1 to 0.4. Figure 19-1 gives the measured wind speed variation with height
for specific instances for five locations. The result of using the power law
profile (Eq. 19-1) is also shown (open circles and dashed lines) using a
value of p of 1/7. It should be noted that the power law wind profiles do
not necessarily represent the data well. The exponent actually varies with
atmospheric stability, surface roughness, and depth of the layer (1).

Fig. 19-1. Wind variation with height—measured (solid lines) and one-seventh power
law (dashed lines).