Cyclones and anticyclones are regions of relatively low and high pressure, respectively. They occur over most of Earth’s surface in a variety of sizes ranging from the very large semipermanent examples described above to smaller, highly mobile systems. The latter are the focus of discussion in this section. Common to both cyclones and anticyclones are the characteristic circulation patterns. The geostrophic-wind and gradient-wind models dictate that, in the Northern Hemisphere, flow around a cyclone—cyclonic circulation—is counterclockwise, and flow around an anticyclone—anticyclonic circulation—is clockwise. Circulation directions are reversed in the Southern Hemisphere (see above the diagrams of mean sea-level pressure). In ...(100 of 37038 words)
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