Spaceweather.com- 5/27/13
Over the weekend, sky watchers in northern Europe and Canada 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). Photographer Silvar Mehik sends this picture from the island of Saaremaa in Estonia:
NLCs are Earth's highest clouds. In the upper atmosphere, 80+ km high, tiny ice crystals nucleate around meteoroids and other aerosols. When the crystals catch the rays of the setting sun, they glow electric blue. For reasons that are not fully understood, these highest and coldest of clouds form during the warmest months on the ground--late spring and summer.
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 days, NLCs were a polar phenomenon confined mainly ro 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.
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