Page 307 - Fundamentals of Air Pollution

IV. Local Wind Systems 263

Fig. 17-17. Sea breeze due to surface heah'ng over land, resulting in thermals, and subsi-
dence over water.

surface out over the water, completing a circular or cellular flow (Fig. 17-
17). Any general flow due to large-scale pressure systems will be superim-
posed on the sea breeze and may either reinforce or inhibit it. Ignoring the
larger-scale influences, the strength of the sea breeze will generally be a
function of the temperature excess of the air above the land surface over
that above the water surface.
Just as heating in the daytime occurs more quickly over land than over
water, at night radiational cooling occurs more quickly over land. The
pressure pattern tends to be the reverse of that in the daytime. The warmer
air tends to rise over the water, which is replaced by the land breeze from
land to water, with the reverse flow (water to land) completing the circular
flow at altitudes somewhat aloft. Frequently at night, the temperature
differences between between land and water are smaller than those during
the daytime, and therefore the land breeze has a lower speed.

B. Mountain and Valley Winds
Solar heating and radiational cooling influence local flows in terrain situa-
tions. Consider midday heating of a south-facing moutainside. As the slope
heats, the air adjacent to the slope warms, its density is decreased, and
the air attempts to ascend (Fig. 17-18). Near the top of the slope, the air
tends to rise vertically. Along each portion of the slope farther down the
mountain, it is easier for each rising parcel of air to move upslope, replacing
the parcel ahead of it rather than rising vertically. This upslope flow is the
valley wind.
At night when radiational cooling occurs on slopes, the cool dense air
near the surface descends along the slope (Fig. 17-19). This is the downslope
wind. To compensate for this descending air, air farther from the slope
that is cooled very little is warmer relative to the descending air and rises,
frequently resulting in a closed circular path. Where the downslope winds