Polar Stratospheric Clouds
Polar Stratospheric Clouds (PSC) form in the winter polar stratosphere at altitudes around 15-30 km at sufficiently low temperatures.
The cloud particles provide the surfaces on which the ozone destroying reactions occur, because HCl and ClNO3 are here converted into reactive forms of Cl while denitrifying the air. Generally there are two types of PSCs: Type I PSCs, with diameters of ∼1 μm, are composed primarily of nitric acid trihydrates (NAT), while Type II PSCs, with larger diameters (>5 μm) are determined to be ice clouds. Type I PSCs form at temperatures higher (195 K) than those needed to condense ice on Type II PSCs (∼188 K).
PSCs are thus strongly associated with increased ozone depletion in the polar regions.
Read more about PSCs here (Danish only for now):