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A polar stratospheric cloud ( PSC) is a cloud that forms in the winter polar stratosphere at altitudes from . They are best observed during civil twilight, when the Sun is between angular distance the horizon, as well as in winter and in more northerly latitudes. One main type of PSC is composed of mostly supercooling droplets of water and nitric acid and is implicated in the formation of ozone depletion. The other main type consists only of , which are not harmful. This type of PSC is also called nacreous (; from nacre, or mother of pearl), due to its iridescence.
Due to their high altitude and the curvature of the surface of the Earth, these clouds will receive sunlight from below the horizon and reflect it to the ground, shining brightly well before dawn or after dusk.
PSCs form at very low temperatures, below . These temperatures can occur in the lower stratosphere in polar winter. In the Antarctic, temperatures below frequently cause type II PSCs. Such low temperatures are rarer in the Arctic. In the Northern Hemisphere, the generation of lee waves by mountains may locally cool the lower stratosphere and lead to the formation of lenticular (lens-shaped) PSCs.
of sunlight within the clouds produces a pearly-white appearance. Particles within the Thin-film optics clouds cause colored interference fringes by diffraction. The visibility of the colors may be enhanced with a polarising filter.
Only Type II clouds are necessarily nacreous whereas Type I clouds can be iridescent under certain conditions, just as any other cloud. The World Meteorological Organization no longer uses the alpha-numeric nomenclature seen in this article, and distinguishes only between super-cooled stratiform acid-water PSCs and cirriform-lenticular water ice nacreous PSCs.
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