Polar stratospheric cloud
Polar stratospheric clouds (PSCs) are clouds in the winter polar stratosphere at altitudes of . They are best observed during civil twilight, when the Sun is between 1 and 6 degrees below the horizon, as well as in winter and in more northerly latitudes. One main type of PSC is made up mostly of supercooled droplets of water and nitric acid and is implicated in the formation of ozone holes. The other main type consists only of ice crystals which are not harmful. This type of PSC is also referred to as nacreous (, from "nacre", or mother of  pearl, due to its iridescence). Formation. The stratosphere is very dry; unlike the troposphere, it rarely allows clouds to form. In the extreme cold of the polar winter, however, stratospheric clouds of different types may form, which are classified according to their physical state (super-cooled liquid or ice) and chemical composition. 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. Forward scattering of sunlight within the clouds produces a pearly-white appearance. Particles within the optically thin clouds cause colored interference fringes by diffraction. The visibility of the colors may be enhanced with a polarising filter. Types. PSCs are classified into two main types each of which consists  of several sub-types 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.