Also known as nacreous clouds or mother-of-pearl clouds, as their name suggests they are special types of clouds that form in the stratosphere over the poles during the long polar winter. During these times the temperature, especially over the South Pole, can drop as low as -80° C, and chemicals present in the statosphere can freeze up. Their precise composition is presently unknown, but they are believed to be of two types: Type 1 clouds are believed to be made of nitric acid and ice crystals, and normally form at -78° C or so, while Type 2 clouds are less common and form at lower temperatures (-85° C or lower) and at higher altitudes, 25 km. above sea level or so, and are generally made of ice. These clouds generally form in the polar vortex that appears on the poles every winter.
Polar stratospheric clouds or PSC's are important in meteorology and environmental research as it seems that along with chloroflourocarbons (CFC's) they are a link in the cycle of ozone depletion. CFC's are relatively chemically inert (which is why they were for a time widely used as refrigirants and solvents) while they remain in the troposphere. But once they drift into the stratosphere, conditions become highly conducive for their reactions that destroy the ozone layer. During austral winter large numbers of polar stratospheric clouds form over the South Pole, and these clouds later form surfaces on which the CFC's ozone-destroying reactions can take place much more rapidly. Once the sun rises on winter's end, the circle is complete, solar radiation provides the missing energy needed to complete the reactions, and vast quantities of ozone are destroyed in the southern latitudes come spring.