FIRST NLCs of 2009: The first noctilucent clouds (NLCs) of 2009 have been sighted over northern Europe. Last night, May 29th, photographers recorded wispy electric-blue tendrils spreading across the twilight skies of Denmark, Northern Ireland and Scotland. This follows a similar display over Russia on May 27th. These sightings signal the beginning of the 2009 NLC season, which is expected to last until late July. Early-season NLCs are usually feeble, but these were fairly bright and vibrant, suggesting that even better displays are in the offing. Check today's edition of http://spaceweather.com for photos.
Noctilucent clouds are an unsolved puzzle. They float 83 km above Earth's surface at the edge of space itself. People first noticed NLCs in the late 19th century. In those days you had to travel to high northern latitudes to see them. In recent years, however, the clouds have been sighted in the United States as far south as Oregon, Washington and even Colorado. Climate change, space dust, and rocket launches have all been cited as possible explanations for the phenomenon. Interestingly, low solar activity seems to promote the clouds, so the ongoing deep solar minimum could set the stage for a good season in 2009.
The best time to look for NLCs is just after sunset or just before sunrise when the sun is between 6 and 16 degrees below the horizon. That's when the geometry is just right for sunlight to illuminate the tiny ice crystals that make up the clouds. Observing tips and sample photos may be found in the 2008 Noctilucent Cloud Gallery: