Type II polar stratospheric clouds (PSC II), seen from a NASA DC-8 research aircraft over the Arctic. The bright PCS IIs are at an altitude of 15,300m, and are composed of crystals of water ice. The clouds exhibit a 'wave' structure, caused by the effect of mountains on the winds that move the clouds. PSCs are found in the circumpolar vortex winds that circle both poles. Type I PSCs (made of nitric acid crystals) have been linked with the chemical processes that are depleting the ozone layers over the Arctic and Antarctic regions.