Recent data from NASA's AIM spacecraft show that noctilucent clouds (NLCs) are like a great "geophysical light bulb." They turn on every year in late spring, reaching almost full intensity over a period of no more than 5 to 10 days. News flash: The bulb is beginning to glow. Jesper Grønne photographed a bank of NLCs rippling over Silkeborg, Denmark, on June 1st:
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© Jesper Grønne

"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."
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© Lars ZielkeJun. 1, 2010. Tvis, Denmark

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 60 to 160 below the horizon. If you see luminous blue-white tendrils spreading across the sky, you may have spotted a noctilucent cloud.