The 2011 season for noctilucent clouds (NLCs) is underway, and it is intensifying. Observers are now reporting electric-blue waves and filaments in the sunset skies of both Europe and North America. Last night, Bob Conzemius photographed a vivid display over Trout Lake, Minnesota, and--bonus--he caught some Northern Lights, too. Click on the image to set the scene in motion:


NLCs are a summertime phenomenon. In the upper atmosphere, 80+ km high at the edge of space itself, tiny ice crystals nucleate around microscopic meteoroids and other aerosols; when the crystals catch the rays of the setting sun, they glow electric blue. Ironically, these highest and coldest of clouds form during the warmest months on the ground.

Noctilucent clouds first appeared in the 19th century after the eruption of super-volcano Krakatoa. At the time, people thought the clouds were caused by the eruption, but the clouds persisted long after Krakatoa's ash settled. In those early days, NLCs were a polar phenomenon, mainly seen in far-northern places such as Scandinavia or Alaska. In recent years they have intensified and spread with sightings as far south as Utah and Colorado. A NASA spacecraft named AIM is in orbit to investigate.

Readers, especially you at high latitudes, be alert for NLCs in the evenings ahead. Observing tips may be found in our 2009 Noctilucent Cloud Gallery.

more images: from Mikael Johannesen of Hvidovre, Denmark; from Rance Ball of La Ronge Sk Canada; from Emil Harritz of Kjeldbjerg, Denmark; from Jesper Grønne of Silkeborg Denmark; from Sallie Carlson of Lutsen, Minnesota; from Martin McKenna of Maghera, Co. Derry, N. Ireland;