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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.]
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