For the second time in a week, nacreous clouds have made an appearance over Scandinavia. On Dec. 17th they were sighted over Trondheim, Norway. Yesterday, the clouds came to Kittila, Finland.
Nacreous clouds
© Sauli KoskiNacreous clouds

"These were our first nacreous clouds of the season," says photographer Sauli Koski. "I could watch them all day!"

Nacreous clouds are located in the stratosphere some 9 to 16 miles high. Their iridescent "mother of pearl" colors come from sunlight striking tiny ice crystals inside the clouds. Very low temperatures near -85o C are required to form the crystals, which is why nacreous clouds are seen mainly during winter over places like Alaska, Iceland and Scandinavia.

These clouds are supposed to be rare, yet earlier this year Scandinavians witnessed a veritable "nacreous storm." For more than a week in January 2008, hardly a night went by without someone spotting mother-of-pearl colors in the sky. No one knows what caused the storm or if it could happen again. One thing is sure: northern sky watchers should be alert for more. The best time to look is during the twilight hours before dawn or after sunset.