Page 304 - Fundamentals of Air Pollution

260 17. The Physics of the Atmosphere

Fig. 17-12. Balance of forces resulting in gradient wind around low pressure.

case of curvature around high pressure, a balance of forces occurs when
the sum of the pressure gradient and centrifugal forces equals the coriolis
force (Fig. 17- 13). To maintain a given gradient wind speed, a greater
pressure gradient force (tighter spacing of the isobars) is required in the
flow around low-pressure systems than in the flow around high-pressure
systems.

E. The Effect of Friction

The frictional effect of the earth's surface on the atmosphere increases
as the earth's surface is approached from aloft. Assuming that we start
with geostrophic balance aloft, consider what happens to the wind as we
move downward toward the earth. The effect of friction is to slow the wind
velocity, which in turn decreases the coriolis force. The wind then turns
toward low pressure until the resultant vector of the frictional force and
the coriolis force balances the pressure gradient force (Fig. 17-14). The
greater the friction, the slower the wind and the greater the amount of
turning toward low pressure. The turning of the wind from the surface
through the friction layer is called the Ekman spiral. A radial plot, or hodo-
graph, of the winds through the friction layer is shown diagrammatically
in Fig. 17-15.

Fig. 17-13. Balance of forces resulting in gradient wind around high pressure. Note that
the wind speed is greater for a given pressure gradient force than that around low pressure.