In the language of meteorology and climatology, a centre of low pressure is designated a cyclone; a centre of high pressure is an anticyclone. Cyclones and anticyclones may be of the stationary, or semipermanent type, or they may be rapidly moving pressure centres such as create the weather disturbances.
We give the rule that in the northern hemisphere winds move anti clockwise (counterclockwise) about a cyclone; clockwise about an anticyclone. In the southern hemisphere these patterns are exactly reverse; clockwise about a cyclone; anti clockwise about an anticyclone.
For surface winds, which move obliquely across the isobars, the systems for cyclones and anticyclones in both hemispheres are shown in below. Winds in a cyclone in the northern hemisphere show an anti clockwise inspiral; in an anticyclone, a clockwise outspiral. Note the reversal between the labels anti clockwise and clockwise in the southern hemisphere. In both hemispheres the surface winds spiral inward upon the centre of the cyclone, hence the air is converging upon the centre and must also rise to be disposed of. For the anticyclone, by contrast, surface winds spiral out from the centre, which represents a diverging of air flow and must be accompanied by a sinking (subsidence) of air in the center of the anticyclone to replace the out moving air.
Measurement of winds - A description of winds requires measurement of two quantities: direction and speed. Direction is easily determined by a wind vane, one of the commonest of the amateur weather instruments.
Speed of wind is measured by an anemometer. There are several types. The commonest one seen at weather stations is the cup anemometer.
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