Environmental Engineering Reference
In-Depth Information
Figure 6.8 The changing magnitude of the Coriolis force with
latitude.
equator, where it reaches zero. The reason for this is shown in Figure 6.8. As one
proceeds towards the equator, so Earth's surface eventually becomes parallel to the axis
of rotation. Its effect can be demonstrated by pendulum experiments. Using the Foucault
pendulum, which portrays free motion in space as closely as can be achieved at Earth's
surface, a disc will rotate under the freely swinging pendulum in one day at the poles. At
latitude 30° it will take two days to rotate (sin 30° = 0·5) and at the equator it does not
turn at all.
GEOSTROPHIC WIND
Let us now return to our parcel of air experiencing a pressure gradient force on the
rotating Earth. Initially the parcel will move down the pressure gradient, but as soon as it
begins to move it will start to be affected by the Coriolis force, which pulls at 90° to the
flow, so that it will
Figure 6.9 Balance of forces (a) for the geostrophic wind
when isobars are straight and (b) for the gradient wind when
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