Polar Stratospheric Clouds

Polar stratospheric clouds appear high in the sky in the polar regions in winter.   

Photos in Iceland. Photos by @ h0rdur c/o Fabulous Weird

They are composed of ice crystals and supercooled water droplets, sometimes mixed with nitric acid.

Wikipedia says: "Particles within the optically thin clouds cause colored interference fringes by diffraction." 

The visibility of the colors may be enhanced with a polarising filter."

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Thanks, Atlas Obscura and Fabulous Weird