Noctilucent Clouds.

During the summer months in Scandinavia, a very peculiar phenomenon can be observed at times in the night sky. If the conditions are right, the twilight heavens are illuminated by light, shining clouds. Such was the occasion this past week. As I was getting ready to turn in for the night, closing the living room window, a sight of pearlescence in the sky - resembling platinum glowing waves - greeted me.

Called Noctilucent Clouds, these consist of ice crystals and are the highest known clouds in the atmosphere. Positioned in the mesosphere at altitudes of around 76 to 85 kilometers - literally at the edge of space - they are not fully understood and are a recently discovered meteorological phenomenon; there is no evidence that they were observed before 1885.

Noctilucent clouds can be seen by observers at a latitude of 50 to 65 degrees and seldom at lower latitudes. Occurring during summer, from mid-May to mid-August in the northern hemisphere and between mid-November and mid-February in the southern hemisphere, they are very faint and tenuous, and can only be spotted in twilight at sunrise or sunset in the north cardinal direction. At this time the clouds of the lower atmosphere are in shadow and the noctilucent clouds are illuminated by the sun.

This was my second sighting, much better then the one I observed last June, particularly as the position of the clouds occurred in my immediate view to the north-northwest, on the still sunset coloured night sky.

To those who found this subject intriguing; please visit this audio slideshow on BBC page.

(Please click the image below for a larger view)