Nacreous Clouds

Nacreous Clouds
Also called polar stratospheric clouds or mother of pearl clouds, nacreous clouds are mostly visible within two hours after sunset or before dawn. They blaze unbelievably bright with vivid, iridescent colors. These clouds are rare and occur in the polar stratosphere at altitudes of 15,000–25,000 meters. They are so bright because at those heights, they are still sunlit.

Although incredibly beautiful, they have a negative impact on our atmosphere. They create ozone holes by supporting chemical reactions that produce active chlorine which catalyzes ozone destruction.

References:
http://en.wikipedia.org/wiki/Polar_stratospheric_cloud
http://www.atoptics.co.uk/highsky/nacr1.htm
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