Type I and Type II polar stratospheric clouds (PSCs), seen at an altitude of 15,300 m from a NASA DC-8 research aircraft. The dark red/brown band of clouds are the Type I PSCs. These are composed of crystals of nitric acid trihydrate. The white clouds are made of water ice, called Type II PSCs. The PSCs are observed in the circumpolar vortex winds that circle both poles. Type I PSCs are thought to be the site of chemical reactions that are depleting the ozone layer over both the Antarctic and Arctic regions.