Nacreous clouds are also known as
Mother of Pearl Clouds or
Polar Stratospheric Clouds (PSC). Nacreous clouds form incredibly high in the dry, cold
stratosphere at 15-25 km. (For comparison, 99% of common
clouds occur in the
troposphere, at a maximum of 12 km.) They often form as a result of
turbulence produced by fast-flowing
winter winds off of high mountains in
Scandinavia,
Alaska, and Northern
Canada.
Nacreous clouds catch or hold sunlight long before lower clouds and are visible hours before sunrise or hours after sunset. Because of the incredibly small size of their composite ice crystals (10 micrometers in diameter, about the size of a human cell) they produce strikingly brilliant irridescent colors.
There are two types of PSC clouds, uncreatively called Type I and Type II.
Type II are colder, around -85ºC, and made of pure ice. Type I are a little warmer at around -78ºC, more diffuse, and produce less intense colors. Sometimes PSCs contain Type II clouds. Type I are subdivided by their chemical compounds.
Type Is are actually dangerous as well as beautiful. The surface of Type I PSCs act a
catalyst that convert benign manmade
chlorine in the atmosphere into active
free radicals. Spring sunlight striking Type I PSCs can cause
chain reactions that destroy ozone. As you marvel at their beautiful colors recall that they may be slowly boring a hole in our major
solar radiation defence shield.
Sources:
- http://www.meteoros.de/indexe.htm
- http://www.sundog.clara.co.uk/atoptics/phenom.htm
- http://chemistry.about.com/library/weekly/aa121001a.htm
- Terrific images of these and other atmoshpheric effects at
http://www.polarimage.fi/