Also known as mother-of-pearl clouds due to their soft and pearly look, nacreous clouds are the only clouds to form in the stratosphere at altitudes greater than 30 kilometers (100,000 feet). They are often seen in high latitudes during the winter months at sunrise or sunset, when the sun is able to best illuminate them. They appear to be in either solid or supercooled liquid form but their exact composition is not yet known.

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/

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