Page 308 - Fundamentals of Air Pollution

264 17. The Physics of the Atmosphere

Fig. 17-18. Upslope wind (daytime) due to greater solar heating on the valley's side than
in its center.

occur on opposite slopes of a valley, the cold air can accumulate on the
valley floor. If there is any slope to the valley floor, this pool of cold air
can move down the valley, resulting in a drainage or canyon wind.
Different combinations of valley and mountain slope, especially with
some slopes nearly perpendicular to the incoming radiation and others in
deep shadow, lead to many combinations of wind patterns, many nearly
unique. Also, each local flow can be modified by the regional wind at the
time which results from the current pressure patterns. Table 17-4 gives
characteristics of eight different situations depending on the orientation of
the ridgeline and valley with respect to the sun, wind direction perpendicu-
lar or parallel to the ridgeline, and time of day. Figure 17-20 shows examples
of some of the mountain and valley winds listed in Table 17-4. These are
rather idealized circulations compared to observed flows at any one time.
The effect of solar radiation is different with valley orientation. An
east-west valley has only one slope that is significantly heated—the south-

Fig. 17-19. Downslope wind (night) due to more rapid radiational cooling on the valley's
slope than in its center.