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Published: 2012-07-18 07:07:38
Updated: 2012-07-18 07:07:38
Posted July 18, 2012
They aren't visible from North Carolina for the most part, but noctilucent clouds (NLCs) can be a delicate and beautiful site, composed of very tiny ice crystals and forming at altitudes of around 50 miles above the earth's surface. This compares to about 5-7 miles for most of the "high" cirrus clouds and contrails we routinely see in our skies, and about 10-11 miles for the tops of really strong thunderstorms.
NLC are too faint to see during the day even in areas where they do form, but become visible once the sun has been down for 30 minutes to an hour or so, when the sky as a whole has darkened considerably and any lower are well within the earth's shadow, leaving only the NLCs illuminated by direct sunlight. Most of the images I've seen of these clouds have been stills, in many cases really nice photos, but a photographer from Sweden recently captured some telephoto time-lapse images of these clouds, and as with time-lapse imagery of many other cloud types, the dynamics involved in their movements and transformations makes for an even more compelling view.
His movie, which strings together several short segments of an evolving NLC, was featured this week on Spaceweather.com, and I've included a link to that day's archive, where you can download the movie for playing on your computer. Enjoy, and keep an eye on the sky after sunset whenever you travel north of about 48 degrees latitude (they are seen on rare occasion farther south). There's also a link to the NLC section of the Atmospheric Optics web site, an excellent reference for many of the eye-catching phenomena (rainbows, glories, halos, etc) seen in our skies, and to a NASA Science story on NLCs that includes some nice illustrations regarding the geometry of how they are seen.