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rainbow Nacreous Clouds - Iceland - Polar stratospheric clouds or PSCs, also known as nacreous clouds (from nacre, or mother of pearl, due to its iridescence), are clouds in the winter polar stratosphere at altitudes of 15,000–25,000 meters. They are best observed during civil twilight when the sun is between 1 and 6 degrees below the horizon. They are implicated in the formation of ozone holes.

Wild cirrus sky. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *