A synoptic-scale area of low atmospheric pressure with winds spiralling about a central low. As air near ground level flows into a cyclone, its absolute vorticity increases, and it is therefore subject to horizontal convergence, causing air to ascend. This rising causes cooling, which often leads to condensation, so that precipitation is associated with cyclones. See Kristjánsson and McInnes (1999) Q’ly J. Royal Met. Soc. 25, 560 on Greenland and North Atlantic cyclones; see McTaggart-Cowan et al. (2007) Month. Weather Rev. 135, 12 on Hurricane Katrina. See also Löptien et al. (2008) Clim. Dynamic., online.