large, aspherical particles, consisting of nitric acid trihydrate (NAT). Type Ib clouds contain small, spherical particles (non-depolarising), of a liquid supercooled ternary solution (STS) of sulfuric acid, nitric acid and water. Type Ic clouds consist of metastable water-rich nitric acid in a solid phase. Type II clouds, which are very rarely observed in the Arctic, have cirriform and lenticular sub-types and consist of water ice only. 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. See also Aurora Circumhorizontal arc Cloud iridescence Noctilucent clouds References ETH PSC page meteoros.de External links Research Nacreous Clouds at atoptics.co.uk Polar Stratospheric Clouds Above Spitsbergen at Alfred Wegener Institute News reports https://news.yahoo.com/s/ap/20060801/ap_on_sc/antarctica_clouds_4 http://www.cnn.com/2006/TECH/science/08/01/antarctica.clouds.ap/index.html Cloud types Ozone depletion