These are clouds found in the stratospheric region of the atmosphere. Unlike the moisture laden troposphere, the stratosphere is almost moisture free. A lack of moisture means that cloud formation in the stratosphere is a relatively uncommon occurrence. Because of the very low temperatures, nacreous clouds, a type of stratospheric cloud, occur during winter conditions at the higher latitudes. A second type of stratospheric cloud, called a polar stratospheric cloud (PSC), occurs in the polar regions during winter and early spring. Polar stratospheric clouds are of particular interest to scientists studying the Antarctic Ozone Hole. Polar stratospheric clouds form under extremely cold conditions, whereby nitric acid, water vapor and other trace chemicals freeze to form ice crystals. PSCs provide a solid surface on which chlorine-containing reservoir compounds can gather, and on this surface the chemical reactions involved in the depletion of ozone are greatly increased.
[Science; v264; 527-8; 1994.] [Nature; v355; 534-7; 1992.]
Source: Atmospheric Chemistry Glossary