What are pearl clouds and when do we see them?

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Dear Tom,
What are pearl clouds and when do we see them?
— Hellevi Miller
Dear Hellevi,
Polar stratospheric clouds — also called nacreous clouds, mother-of-pearl clouds or simply pearl clouds — are rare, high-altitude clouds that are seen only in far northern or far southern areas of the world. In the Northern Hemisphere, they can occasionally be seen across Arctic areas, including Alaska and northern Canada. Chicago is just too far south to view them.
Pearl clouds form at altitudes of 9 to 16 miles and are mainly visible within an hour or two before sunrise or after sunset (when the sun is about one to six degrees below the horizon), because at those heights they are still sunlit. They appear nearly stationary due to their great height. Pearl clouds display vivid, iridescent colors of red, green, orange and purple and shine brightly in the semi-dark sky.

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