From 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 (NLC) season has just begun in the northern hemisphere. Jesper Grønne photographed a bank of NLCs rippling over Silkeborg, Denmark, on June 1st:
“I made a 3-hour movie of the phenomenon,” says Grønne. “There was a lot of activity.” Lars Zielke of Tvis, Denmark, witnessed the same display. “They were visible due north near the horizon. The clouds were not spectacular compared to others I’ve seen, but it’s a start.”
There is a well-known correlation between noctilucent clouds and the solar cycle.
NLC activity tends to peak during (and just after) years of solar minimum, possibly because low solar activity allows the upper atmosphere to cool, promoting the growth of ice crystals that make up the clouds. With the sun slowly emerging from a century-class minimum, the stage could be set for a good season of NLC watching.
Typically, the first NLCs of spring are wan and pale, followed by better displays as summer unfolds. Browse the galleries from previous years to see what may be in the offing: 2009, 2008, 2007, 2006, 2005, 2004, 2003.
Observing tips: Look west 30 to 60 minutes after sunset when the Sun has dipped 6o to 16o below the horizon. If you see luminous blue-white tendrils spreading across the sky, you may have spotted a noctilucent cloud.