Page 306 - Fundamentals of Air Pollution

262 17. The Physics of the Atmosphere

Fig. 17-16. Effect of friction upon gradient wind around low (left) and high (right) pres-
sures.

producing condensation and perhaps precipitation in moist air masses, as
well as transport of air and its pollutants from near the surface to higher
altitudes.

IV. LOCAL WIND SYSTEMS

Frequently, local wind systems are superimposed on the larger-scale
wind systems just discussed. These local flows are especially important
and may dominate when the larger-scale flow becomes light and indefinite.
Local wind systems are usually quite significant in terms of the transport
and dispersion of air pollutants.

A. Sea and Land Breezes
The sea breeze is a result of the differential heating of land and water
surfaces by incoming solar radiation. Since solar radiation penetrates several
meters of a body of water, it warms very slowly. In contrast, only the upper
few centimeters of land are heated, and warming occurs rapidly in response
to solar heating. Therefore, especially on clear summer days, the land
surface heats rapidly, warming the air near the surface and decreasing its
density. This causes the air to rise over the land, decreasing the atmospheric
pressure near the surface relative to the pressure at the same altitude over
the water surface. The rising air increases the pressure over the land relative
to that above the water at altitudes of approximately 100-200 m. The air
that rises over the land surface is replaced by cooler air from over the water
surface. This air, in turn, is replaced by subsiding air from somewhat higher
layers of the atmosphere over the water. Air from the higher-pressure zone
several hundred meters above the surface then flows from over the land