Source - Space Weather News for June 3, 2010: http://spaceweather.com
Every summer since the late 19th century, Earth's polar skies have lit up with gossamer, electric-blue clouds, twisting and rippling in the twilight sky. They're called noctilucent ("night-shining") because they can be seen after dark. The origin of the clouds, which hover at the very top of Earth's atmosphere in close proximity to space itself, is uncertain. They have been linked to causes as varied as meteoroids, climate change, and the icy exhaust of the space shuttle.
News flash: The 2010 noctilucent cloud season has just begun in the northern hemisphere, with sightings over Russia, Scotland and Denmark. Although noctilucent clouds were once a phenomenon of high latitudes only, in recent years they have been sighted in the United States as far south as Colorado and Utah. Visit http://spaceweather.com for photos and observing tips.