In spite of nearly three decades of research, significant gaps in our understanding of processes in Polar Stratospheric Clouds (PSC) still exist. The Type 1 PSCs form when the stratospheric temperature drops below -108 F. They are primarily composed of nitric acid, water, and sulfuric acid. greenhouse gases. 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. As temperatures in the lower stratosphere cools below -80'C, Polar Stratospheric Clouds (PSC's) start to form. For comparison, some of the highest clouds in the troposphere have a ceiling height of about 40,000 feet. 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. polar stratospheric clouds. During winter at high latitudes, however, stratospheric temperature sometimes becomes low enough to promote formation of clouds. 2. The second problem with modeled denitrification shown in Figure 10 is that the HNO 3 at 63°S is about 1–3 ppbv smaller than the observed abundance from mid‐July until the end of the season. But nacreous clouds are different. 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. Polar stratospheric clouds (PSCs) are clouds that form in the polar regions during the winter. 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. Polar stratospheric clouds or PSCs, also known as nacreous clouds (/ ˈ n eɪ k r iː. 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. 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). Clouds generally do not form in the stratosphere because there is not enough moisture. Nacreous clouds develop at very high altitudes, within the lower stratosphere at 70,000 feet or above. 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. CALIPSO = Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations; PSC = polar stratospheric cloud. 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