form chlorine, which catalyzes the photochemical destruction of ozone. Chlorine concentrations build up during the polar winter, and the consequent ozone destruction is greatest when the sunlight returns in spring. These clouds can only form at temperatures below about . Since there is greater air exchange between the Arctic and the mid-latitudes, ozone depletion at the north pole is much less severe than at the south. Accordingly, the seasonal reduction of ozone levels over the Arctic is usually characterized as an "ozone dent", whereas the more severe ozone depletion over the Antarctic is considered an "ozone hole". That said, chemical ozone destruction in the 2011 Arctic polar vortex attained, for the first time, a level clearly identifiable as an Arctic "ozone hole". Outside Earth Other astronomical bodies are also known to have polar vortices, including Venus (double vortex – that is, two polar vortices at a pole), Mars, Jupiter, Saturn, and Saturn's moon Titan. Saturn's south pole is the only known hot polar vortex in the solar system. See also Polar amplification Saturn's hexagon – a persisting hexagonal cloud pattern around the north pole of Saturn Windward Performance Perlan II – will be used to study the northern polar vortex Polar front Cut-off low References Further reading External links Atmospheric dynamics Regional climate effects Polar regions of the Earth Vortices