T/G Layout 1

figure 15.

In sharply curved flow, the geostrophic assumption is no longer completely valid. It is

observed that air flow around curved ridges and troughs is still geostrophic in direction

(parallel to lines of equal pressure). But the observed wind speeds are not equal to that

predicted by the geostrophic assumption. Wind speed around a low pressure trough is

slower than predicted by the geostrophic assumption and winds around a high pres-

sure ridge are stronger than predicted by the geostrophic assumption. To explain this

difference in speed but not direction, we must consider centrifugal force. Centrifugal

force is, like the Coriolis effect, an apparent force that is used so that Newton’s laws can

be applied in a rotating frame of reference. An example is shown in figure 16. A block

of wood is tied to the center of a rotating platform. To an observer outside the rotating

platform, the block moves in a circle with force provided by the tension on the line

(T in the figure). However, to an observer on the platform, the block is at rest. To account

for the tension on the string, an apparent outward force— called the centrifugal force

(Ce)—must be introduced.

LL

AIR HH

CF

PGF

PGF

P 1

P 2

P 3

Coriolis deflection

to the right of the

motion

wind parallel

to isobars

Geostrophic Wind

HH

LL

winds at

1 kilometer

or higher

P 1

P 2

P 3

P 1 < P 2 < P 3

P 1 < P 2 < P 3

15a

15b