Page 309 - Fundamentals of Air Pollution

IV. Local Wind Systems 265

TABLE 17-4
Generalized Mesoscale Windflow Patterns Associated with Different Combinations of
Wind Direction and Ridgeline Orientation

Wind direction Ridgeline orientation
relative 1C) i irrie 01
ridgeline day East-west North-south
Parallel Day 1" South-facing slope is 2 Upslope flow on both
heated — single helix heated slopes — double
helix
Night 3 Downslope flow on both 4 Downslope flow on both
slopes-double helix slopes — double helix
Perpendicular Day South-facing slope is 6 Upslope flow on both
heated. heated slopes —
5a North wind — stationary stationary eddy on one
eddy fills valley side of valley
5b South wind — eddy
suppressed, flow
without separation
Night 7 Indefinite flow — 8 Indefinite flow — extreme
extreme stagnation stagnation in valley
in valley bottom bottom
" Numbers refer to Fig. 17-20.

facing slope may be near normal with midday sunshine. A north-south
valley will have both slopes heated at midday. The effect of flow in relation
to valley orientation is such that flows perpendicular to valleys tend to
form circular eddies and encourage local flows; flows parallel to valleys
tend to discourage local flows and to sweep clean the valley, especially
with stronger wind speeds.
Keep in mind that the flows occurring result from the combination of
the general and local flows; the lighter the general flow, the greater the
opportunity for generation of local flows.
Complicated terrain such as a major canyon with numerous side canyons
will produce complicated and unique flows, especially when side canyon
drainage flows reinforce the drainage flow in the main valley.

C. Urban-Rural Circulations

Urban areas have roughness and thermal characteristics different from
those of their rural surroundings. Although the increased roughness affects
both the vertical wind profile and the vertical temperature profile, the
effects due to the thermal features are dominant. The asphalt, concrete,