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A highly layered cloud with an iridescent appearance. It is akin to high-altitude cirrus clouds. Also called a mother-of-pearl cloud. Clouds of unknown composition, whose form resembles that of cirrus or altocumulus lenticularis. These clouds show very strong irisation, or iridescence, similar to that of the mother-of-pearl, especially when the sun is several degrees below the horizon. They occur at heights of about 12 to 18 miles (20–30 km).