Page 312 - Fundamentals of Air Pollution

268 17. The Physics of the Atmosphere

Fig. 17-23. Aerodynamic flow around a cube: (a) side view, (b) plan view. Source: After
Halitsky (5).

with strong turbulence and mixing, a wake extending downwind from
the cavity a distance equivalent to a number of structure side lengths, a
displacement zone where flow is initially displaced before entering the
wake, and a region of flow that is displaced away from the structure but
does not get caught in the wake. Wind tunnels, water channels, and/or
towing tanks are extremely useful in studying building wake effects.

V. GENERAL CIRCULATION

Atmospheric motions are driven by the heat from incoming solar radia-
tion and the redistribution and dissipation of this heat to maintain constant
temperatures on the average. The atmosphere is inefficient, because only
about 2% of the received incoming solar radiation is converted to kinetic
energy, that is, air motion; even this amount of energy is tremendous
compared to that which humans are able to produce. As was shown in
Section I, a surplus of radiant energy is received in the equatorial regions
and a net outflux of energy occurs in the polar regions. Many large-scale
motions serve to transport heat poleward or cooler air toward the equator.
If the earth did not rotate or if it rotated much more slowly than it
does, a meridional (along meridians) circulation would take place in the
troposphere (Fig. 17-24). Air would rise over the tropics, move poleward,
sink over the poles forming a subsidence inversion, and then stream equa-