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Photo by K. Sassen Noctilucent CloudsNoctilucent clouds (NLC) are clouds that literally shine at night. We who live in Alaska are lucky to have the chance to see them, because they only occur in high latitudes where the summer upper atmosphere gets unusually cold. They form at incredible heights, at about 80 km (50 miles) above the Earths surface, which is ten times higher than the usual wispy cirrus clouds we regularly see. The place they form is called the mesopause, a very frigid layer where what little water vapor that is up there can freeze, maybe on meteoritic dust, to form tiny ice crystals. These pictures of NLC were taken at 230 in the morning on 5 August 2004, from near Cleary Summit to get a good view of the northern horizon. If you want to see a rare noctilucent cloud, you need to keep in mind that in Fairbanks there is only a brief period when you can see them, and that is mainly in August when the mesopause is cold enough! You can only see NLC when the sun is below the horizon and the sky is turning dark. Only then can you see the dim sunlight that is reflected off the tiny ice crystals. If you look carefully at the pictures, you can see some stars shining! So, look to the northern horizon a few hours after sunset and you may be lucky enough to see the elusive NLC. Source: http://www.gi.alaska.edu/AtmosSci/ Noctilucentnoc?ti?lu?cent [ n?tə l?ss?nt ] adjective luminous at night: used to describe high clouds that are visible at night [Late 19th century. Coined from nocti- + Latin lucere ?to shine.?] More Information
Created: 11 Jan 2005 19:17:50 -0800
Changed: 01 Apr 2005 12:29:01 -0800 |
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