Clouds that form in winter in the stratosphere over Antarctica and less commonly over the Arctic at altitudes of 15–25 km. The clouds form in the very still air in the polar night vortex produced by the strong geostrophic wind that circulates in winter. There are two principal types of PSCs. Type 1 consist of liquid droplets, about 0.001–0.1 mm in diameter, composed of a mixture of sulphuric (H2SO4) and nitric (HNO3) acids dissolved in water. These clouds form at about 15km where the temperature of approximately 195K (−78°C) is above the frost point. Type 2 PSCs are made from ice crystals and form at about 25km where the temperature is approximately 188K (−85°C). Type 2 PSCs are seldom seen over the Arctic, where air temperatures are usually too high for them to form. The reactions that deplete stratospheric ozone (see ozone ‘hole’) take place on the surface of PSC ice crystals.
Subjects: Meteorology and Climatology — Ecology and Conservation.