greenhouse gases. 2. 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. Pure nacreous clouds, a type of polar stratospheric cloud, are composed of ice crystals. a gas composed of molecules that absorb and radiate infrared radiation from the sun. As temperatures in the lower stratosphere cools below -80'C, Polar Stratospheric Clouds (PSC's) start to form. But nacreous clouds are different. CALIPSO = Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations; PSC = polar stratospheric cloud. ə s /, from nacre, or mother of pearl, due to its iridescence), are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters (49,000–82,000 ft). Polar Stratospheric Cloud Background These clouds exist at very high altitude (~70,000 ft) within Earth's stratosphere.Clouds do not normally form in the stratosphere due to its extreme dryness. For comparison, some of the highest clouds in the troposphere have a ceiling height of about 40,000 feet. Polar Stratospheric Clouds Tropical Cyclones One explanation for the decrease in the equator to pole temperature difference (EPTD) during equable climates examines the influence of polar stratospheric clouds (PSCs) on longwave radiation leaving the Earth in the high-latitudes and, as a result, on temperatures near the poles. Bottom line: Polar stratospheric clouds – also called nacreous clouds or mother-of-pearl clouds – have been putting on a show for those at latitudes near the Arctic Circle. Nacreous clouds develop at very high altitudes, within the lower stratosphere at 70,000 feet or above. In spite of nearly three decades of research, significant gaps in our understanding of processes in Polar Stratospheric Clouds (PSC) still exist. Clouds generally do not form in the stratosphere because there is not enough moisture. Polar stratospheric clouds occur only in high-latitude regions during the winter, or near winter, when temperatures in the lower and middle stratosphere fall below about −78 °C for nitric acid trihydrate, −81 °C for supercooled ternary solution polar stratospheric clouds and −85 °C for ice polar stratospheric clouds. The chlorine activation and subsequent ozone loss of the northern winter lower stratosphere have been modelled using different schemes for type I polar stratospheric clouds (PSCs) and sulphate aerosols. a cloud that forms at altitudes of about 21,000 m during the Arctic and Antarctic winter or early spring, when air temperatures drop below -80C. 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