Polar stratospheric clouds. A polar winter vortex also forms in the Northern Hemisphere. The particles grow from the con-densation of water and nitric acid (HNO3). Also known as nacreous clouds, these are the clouds that form in the winter polar stratosphere. Polar stratospheric clouds can form at temperatures below about 195 K. Chemical reactions on the surfaces of the particles that form these clouds convert chlorine compounds from inert forms into highly reactive species. Lower average winter temperatures in the upper atmosphere might explain why nacreous clouds seem to be appearing with increased frequency and distribution. The polar vortex and polar stratospheric clouds exist only over Antarctica, which is why the ozone "hole" exists only there. During the continuously dark polar winter, the air inside the polar vortices becomes extremely cold, a necessary condition for polar stratospheric cloud formation. 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. However, in general, it is neither as strong nor as cold as the one that forms in the Antarctic. These beautiful images of polar stratospheric clouds were taken over Tromso City in Norway. While the clouds look amazing, they can be potentially quite dangerous to our planet. Polar stratospheric clouds (PSCs) are common in … These are best observed during winters in more northerly latitudes. Polar stratospheric clouds create the conditions for drastic ozone destruction, providing … Although polar stratospheric clouds can form in the Arctic, they rarely last long enough for extensive decreases in ozone. Below: Dangerous beauty. The clouds Figure Q10-2. The nitric acid in polar stratospheric clouds reacts with chlorofluorocarbons to form chlorine, which catalyzes the photochemical destruction of ozone [3, 34]. Arctic ozone decreases of as much as 40 percent have been measured. Polar mesospheric clouds (PMCs) are a diffuse scattering layer of water ice crystals near the summer polar mesopause. Polar stratospheric clouds (PSCs) are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 m (49,000–82,000 ft). PSCs appear in the lower stratosphere near the poles in winter. Meanwhile, chlorine concentrations build up during the polar winter, and consequent ozone destruction is greatest when the sunlight returns in polar winter . Polar stratospheric clouds (PSCs) are the exception. But the bigger the hole gets, the thinner the ozone layer will become over the rest of the earth. This photo-graph of an Arctic polar stratospheric cloud (PSC) was taken from the ground at Kiruna, Sweden (67°N), on 27 January 2000. Description. This means more dangerous UV radiation will reach the Earth's surface. "A 'warm' polar stratosphere is typically in the temperature range -73 o to -63 o C. 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