flash spectrum ...photography by Robert B Slobins
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Astronomy Photos: Noctilucent Clouds

Noctilucent clouds are the highest clouds in the earth's atmosphere, occurring between 76-85 km altitude: the mesosphere. At that height, the atmosphere is extremely thin, dry and cold. Noctilucent clouds are composed of water ice crystals. A fraction of these crystals, 100 nanometers in size, form without any nucleus. This indicates temperatures no higher than -120 C.

Noctilucent clouds are too faint to be seen except when the sun is at a low altitude, lighting them from below. From the surface, the sun is below the horizon, so the dark sky provides the contrast. We can best see these clouds from latitudes between 50 and 65 degrees around the time of the summer solstice because of the prolonged twilight, when the sun is between 6 and 16 degrees below the surface horizon.

Therefore, Prague, at 50 degrees North, is at the southern margin for the best displays, although people have reported them from locations up to ten degrees further south. Even so, large displays such as what I present in this gallery are extremely rare in Prague. The 13-14 July 2009 display was one of the biggest, with observations from Paris and further south.

We still understand very little about noctilucent clouds. Solar activity minima seem to provide the best conditions for these displays. Polar lights or aurorae end where these clouds occur, so perhaps high geomagnetic activity can disturb and disperse the clouds. 2009 was in the middle of the longest and deepest solar minimum in a century, so it was only a matter of time that noctilucent clouds would act as a backdrop to some of Prague's most famous sights.

Robert B Slobins 1970-2011   |   About   |   Sitemap   |   Contact