Page 311 - Fundamentals of Air Pollution

IV. Local Wind Systems 267

Fig. 17-21. Urban heat island (light regional wind).

area. This circulation is referred to as the urban heat island. The strength of
the resulting flow is dependent on the difference in temperature between
the urban center and its surroundings.
When the regional wind allows the outflow to take place in primarily
one direction and the rising warm urban air moves off with this regional
flow, the circulation is termed the urban plume (Fig. 17-22). Under this
circumstance, the inflow to the urban center near the surface may also be
asymmetric, although it is more likely to be symmetric than the outflow at
higher altitudes.
The urban area also gives off heat through the release of gases from
combustion and industrial processes. Compared to the heat received
through solar radiation and subsquently released, the combustion and
process heat is usually quite small, although it may be 10% or more in
major urban areas. It can be of significance in the vicinity of a specific local
source, such as a steam power plant (where the release of heat is large
over a small area) and during light-wind winter conditions.

D, Flow around Structures

When the wind encounters objects in its path such as an isolated struc-
ture, the flow usually is strongly perturbed and a turbulent wake is formed
in the vicinity of the structure, especially downwind of it. If the structure
is semistreamlined in shape, the flow may move around it with little distur-
bance. Since most structures have edges and corners, generation of a turbu-
lent wake is quite common. Figure 17-23 shows schematically the flow in
the vicinity of a cubic structure. The disturbed flow consists of a cavity

Fig. 17-22. Urban plume (moderate regional wind).