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Also known as mother-of-pearl clouds due to their soft and pearly look, nacreous clouds are the only clouds to form in the stratosphere at altitudes greater than 30 kilometers (100,000 feet). They are often seen in high latitudes during the winter months at sunrise or sunset, when the sun is able to best illuminate them. They appear to be in either solid or supercooled liquid form but their exact composition is not yet known.