Page 305 - Fundamentals of Air Pollution

III. Laws of Motion 261

Fig. 17-14. Effect of friction on the balance of forces, causing wind to blow toward low
pressure.

Note that this frictional effect will cause pollutants released at two differ-
ent heights to tend to move in different directions.
In the friction layer where the isobars are curved, the effect of frictional
drag is added to the forces discussed under gradient wind. The balance of
the pressure gradient force, the coriolis deviating force, the centrifugal
force, and the frictional drag in the vicinity of the curved isobars results
in wind flow around low pressure and high pressure in the Northern
Hemisphere, as shown in Fig. 17-16.

F. Vertical Motion—Divergence
So far in discussing motion in the atmosphere, we have been emphasizing
only horizontal motions. Although of much smaller magnitude than hori-
zontal motions, vertical motions are important both to daily weather forma-
tion and to the transport and dispersion of pollutants.
Persistent vertical motions are linked to the horizontal motions. If there
is divergence (spreading) of the horizontal flow, there is sinking (downward
vertical motion) of air from above to compensate. Similarly, converging
(negative divergence) horizontal air streams cause upward vertical motions,

Fig. 17-15. Hodograph showing variation of wind speed and direction with height above
ground. SFC = surface wind.