NACREOUS CLOUDS:
In the stratosphere above Scandinavia, the winter air temperature has dropped below -85 C. That's a magic value. At such temperatures, tiny crystals of ice begin to form in the desert-dry air 9 to 16 miles high. And when that happens, voilà--nacreous clouds.
Nacreous clouds are a polar phenomenon. They get their intense colors from sunlight striking ice crystals ~10 microns in diameter. Diffraction produces iridescence, a phenomenon familiar to daytime sky watchers around the world. Unlike the relatively pale iridescent clouds of mid-latitudes, however, polar nacreous clouds are blow-your-Christmas-socks-off bright.
Northern winter is just beginning and the polar stratosphere will grow even icier in the weeks ahead. Sky watchers in cold places like Scandinavia, Iceland and Alaska should be alert for nacreous colors; the best time to look is during the twilight hours after sunset or before sunrise.
Posted: 4:56:06 PM
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