This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

Dear Tom,
What are mother-of-pearl clouds?
—Oscar Johns, Wilmette
Dear Oscar,
They are polar stratospheric clouds, also known as nacreous clouds or simply pearl clouds. They are rare, high altitude clouds that are seen only in far northern or far southern areas of the world. In the Northern Hemisphere, they are visible across Arctic areas of Alaska, northern Canada and northern Siberia. Chicago is just too far south to view them. Pearl clouds form at altitudes of 9 to 16 miles and are visible within two hours before sunrise/after sunset when the sun is one to six degrees below the horizon because at those heights they are still sunlit. They appear nearly stationary because of their great height. Pearl clouds display vivid colors of red, green, orange and purple and shine brightly in the semi-dark sky.