During winter, the polar skies can ignite with mother-of-pearl-like iridescence. Polar stratospheric clouds – also known as nacreous clouds – are a rare, beautiful, and destructive type of cloud found only in high latitudes at altitudes of 15 - 25 km. They are formed from tiny crystals of ice and nitric acid, and they shine brightest a few hours before sunrise or after sunset, when sunlight shines on them but not the surface. Their destructive side is connected with ozone depletion; they serve as reaction sites for chlorofluorocarbons in the atmosphere to react and produce ozone-destroying molecules. The clouds may have cultural significance as well; at least one study suggests they were part of Munch’s inspiration for “The Scream”. (Image credit: A. Light; via Gizmodo)