NOCTILUCENT CLOUDSNOCTILUCENT CLOUD SEASON BEGINS: Over the weekend, sky watchers in northern European countries spotted electric-blue tendrils of light reaching out of the western sky at sunset. This signals the beginning of the 2013 season for noctilucent clouds (NLCs).
Noctilucent clouds are a summertime phenomenon. In the upper atmosphere, 80+ km high at the edge of space itself, tiny ice crystals nucleate around meteoroids and other aerosols; when the crystals catch the rays of the setting s*n, 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 thoght the clouds were caused by the eruption, but long after Krakatoa's ash settled, the clouds remained. 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. Could this be a sign of climate change? A NASA spacecraft named "AIM" is in orbit to investigate.