A synoptic-scale weather system with low pressure near the surface is called a “cyclone” (Fig. 13.1). Horizontal winds turn cyclonically around it (clockwise/counterclockwise in the Southern/ Northern Hemisphere). Near the surface these turning winds also spiral towards the low center. Ascending air in the cyclone can create clouds and precipitation.
Tropical cyclones such as hurricanes are covered separately in a later chapter. Extratropical cyclones (cyclones outside of the tropics) are covered here, and include transient mid-latitude cyclones and polar cyclones. Other names for extratropical cyclones are lows or low-pressure centers (see Table 13-1). Low-altitude convergence draws together airmasses to form fronts, along which the bad weather is often concentrated. These lows have a short life cycle (a few days to a week) as they are blown from west to east and poleward by the polar jet stream.