photo #5420281

While visiting an ice hotel at Jukkasjärvi, up in the Lapland region of northern Sweden, Judy Wagner (Member 61,795) spotted clouds also made of ice up in the stratosphere. They were nacreous clouds, which form at altitudes of around 10 to 15 miles (15 to 25 km), which is considerably higher than most clouds, in the troposphere below. The extreme-altitude clouds take their name from nacre, the mother-of-pearl lining in the shells of some aquatic molluscs such as oysters, due to their shimmering iridescent appearance. The pastel hues of the clouds are caused by their tiny ice crystals diffracting the sunlight. As they are so high, they catch sunlight shining from over the horizon and so shine against the darkened sky in the hours just before sunrise or after sunset. Forming over higher latitudes in the winter months, nacreous clouds are for many a rare sight. But Judy told us that the pearly skies accompanied her for three of the days in her trip. ‘Our local photography guide was very excited,’ she said. ‘He’d never seen these clouds hang around for so long.’