polar stratospheric cloud (PSC) A cloud, consisting of ice crystals, which occurs in the stratosphere late in winter over Antarctica and, less commonly, over the Arctic. The water vapour that freezes may be derived from the dissociation of methane and the cloud forms in the very still, very cold air of a vortex over the pole (the less common occurrence of such clouds in the northern hemisphere is owing to the generally higher temperature of Arctic stratospheric air and the briefer duration of the vortex). The reactions that deplete polar stratospheric ozone in the late winter and early spring take place on the surface of the ice crystals in polar stratospheric clouds.

polar stratospheric cloud (PSC) A cloud, consisting of ice crystals, which occurs in the stratosphere late in winter over Antarctica and, less commonly, over the Arctic. The water vapour that freezes may be derived from the dissociation of methane and the cloud forms in the very still, very cold air of a vortex over the pole (the less common occurrence of such clouds in the northern hemisphere is owing to the generally higher temperature of Arctic stratospheric air and the briefer duration of the vortex). The reactions that deplete polar stratospheric ozone in the late winter and early spring take place on the surface of the ice crystals in polar stratospheric clouds.