Polar stratospheric clouds (PSCs) are clouds in the winter polar stratosphere at altitudes of 15-25 km. They are implicated in the formation of ozone holes.
The stratosphere is very dry and clouds, as commonly seen in the troposphere, rarely form. In the extreme cold of the polar winter, PSCs of two types may form: various forms of type I of nitric and sulfuric acid; and type II (rarer) of water ice which form below -90°C.
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