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arises when air moves in circular orbits, indicated in
gray, around an extreme value of pressure. In a
northern-hemisphere low pressure system or a
southern-hemisphere high, the motion is counter clockwise, as
shown. For a northern-hemisphere high or a southern-hemisphere
low, it is in the opposite sense. Here, the
inward decrease of pressure (blue arrow) accelerates air
parcels toward the center of circulation and balances the
sum of the centripetal and Coriolis accelerations (green
arrow) required by the curved path. The speed of
motion (black arrow) is determined by the strength of
the pressure gradient, radius, and the Coriolis parameter
(twice the component of the Earth's rotation vector
about the local vertical). |