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Nacreous clouds

High up in the stratosphere, when temperatures drop to around -85 degrees C, micro-sized ice particles form nacreous clouds, also sometimes known as” mother-of-pearl clouds”, due to their bright iridescence. Seen during the hours before dawn or after dusk, the clouds are generally only visible in areas of Scandinavia, Northern Canada, and other extreme latitudes, in the Northern and Southern Hemisphere. The rare moisture that works its way into the stratosphere (some 10-50 km above the Earth’s surface) is pushed there by chemical breakdown of methane gas, and tall storm waves.

These clouds, though beautiful, have been recently studied as catalysts in reactions that accelerate ozone depletion. By providing a surface on which chlorine and bromine compounds can become active, the formation of the polar stratospheric clouds (PSCs) has become a source of interest for atmospheric and climate scientists alike. 

-BN

Photo Credit: Alan Light, via http://www.publicdomainpictures.net/view-image.php?image=10892&picture=nacreous-clouds&large=1

Further Resources:
http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1421
http://www.atoptics.co.uk/highsky/psc1.htm
http://www.atm.ch.cam.ac.uk/tour/psc.html
http://remus.jpl.nasa.gov/info.htm