WIND ENERGY SYSTEMS - Cd3wd

Chapter 2—Wind Characteristics 2–5
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Figure 2: Coriolis force
it on faith.
Another statement we shall accept on faith is that the deflection of the parcel of air must
cease when the wind direction becomes parallel to the isobars. Otherwise the wind would
be blowing in the direction of increasing pressure, which would be like water running uphill.
Since the Coriolis force acts in a direction 90 degrees to the right of the wind, it must act
in a direction opposite to the pressure gradient at the time of maximum deflection. If there
are no other forces present, this Coriolis force will exactly balance the pressure gradient force
and the wind will flow parallel to the isobars, with higher pressure to the right of the wind
direction. For straight or slightly curved isobars this resultant wind is called the geostrophic
wind.
When strongly curved isobars are found, a centrifugal force must also be considered. Fig. 3
shows one isobar around a cyclone, which is a low pressure area rotating counterclockwise
(Northern Hemisphere). Fig. 4 shows an isobar around a high pressure area which is rotating
clockwise (Northern Hemisphere). This region is called an anticyclone. As mentioned earlier,
the low pressure area is usually associated with bad weather, but does not imply anything
about the magnitude of the wind speeds. A cyclone normally covers a major part of a state or
several states and has rather gentle winds. It should not be confused with a tornado, which
covers a very small region and has very destructive winds.
The wind moving counterclockwise in the cyclone experiences a pressure gradient force f p
inward, a Coriolis force f c outward, and a centrifugal force f g outward. For wind to continue
moving in a counterclockwise direction parallel to the isobars, the forces must be balanced,
so the pressure gradient force for a cyclone is
f p = f c + f g (7)
The pressure force inward is balanced by the sum of the Coriolis and centrifugal forces.
The wind that flows in such a system is called the gradient wind. The term “geostrophic
Wind Energy Systems by Dr. Gary L. Johnson November 20, 2001