As January comes to an end, sky watchers in Scandinavia are recovering from a veritable storm of nacreous clouds. After mid-month, hardly a night went by without someone spotting the phenomenon. "It was incredible! They were all over the sky," says Morton Ross of Oslo, Norway. This picture, taken by Ross on Jan. 25th, shows a typical apparition:

©Morten Ross

Also known as "Mother of Pearl" clouds, nacreous clouds are peppered with tiny ice crystals that blaze with iridescent color when struck by light from the setting sun. It is these crystals that make nacreous clouds so rare: they require exceptionally low temperatures of minus 85 Celsius (-120 F) to form. Icy nacreous clouds float 9 to 16 miles high, curling and uncurling hypnotically as they are modulated by atmospheric gravity waves.

For much of January, these clouds rolled across the Arctic circle with puzzling regularity. Why the sudden abundance? Is the show over? No one knows. Stay tuned for February!