Page 303 - Fundamentals of Air Pollution

III. Laws of Motion 259

Here O is the angular speed of the earth's rotation, 2rr/
1
5
(24 x 60 x 60) = 7.27 x 10~ s" , and 0 is the latitude. It is seen that/
is maximal at the poles and zero at the equator. The deflecting force is to
the right of the wind vector in the Northern Hemisphere and to the left in
the Southern Hemisphere. For the present example/ the deflecting force is
3 2
1,45 x 10~ m s~ , and the amount of deflection after the 36-km movement
in 1 hr is 9.43 km.

C. Geostrophk Wind
Friction between the atmosphere and the earth's surface may generally
be neglected at altitudes of about 700 m and higher. Therefore, large-scale
air currents closely represent a balance between the pressure gradient force
and the coriolis force. Since the coriolis force is at a right angle to the
wind vector, when the coriolis force is equal in magnitude and opposite
in direction to the pressure gradient force, a wind vector perpendicular to
both of these forces occurs, with its direction along the lines of constant
pressure (Fig. 17-11). In the Northern Hemisphere, the low pressure is to
the left of the wind vector (Buys Ballot's law); in the Southern Hemisphere,
low pressure is to the right. The geostrophic velocity is

When the isobars are essentially straight, the balance between the pressure
gradient force and the coriolis force results in a geostrophic wind parallel
to the isobars.

D. Gradient Wind

When the isobars are curved, an additional force, a centrifugal force
outward from the center of curvature, enters into the balance of forces. In
the case of curvature around low pressure, a balance of forces occurs when
the pressure gradient force equals the sum of the coriolis and centrifugal
forces (Fig. 17-12) and the wind continues parallel to the isobars. In the

Fig. 17-11. Balance of forces resulting in geostrophic wind.