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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).