photo #8990800

Finland’s Iso Pielpajärvi lake is in the Arctic Circle, where the Sun does not rise above the horizon for weeks at a time during winter. Towards the end of this long period of polar night, and a few hours before the Sun made a brief appearance, Kees Neve (Cloud Appreciation Society Member 57,271) watched from here as the sunrise drenched the horizon in a deep orange and blended into iridescent hues in the clouds above, with a faint dark shadow in between.The pearlescent formations are nacreous clouds. Known officially as Type II polar stratospheric clouds, they’re rare high-altitude formations that are up in the stratosphere, at about 10-15 miles (15-25 km) above Earth’s surface. Since the minuscule ice crystals in these cloud are of a consistent size, they cause the light waves that pass around them to diffract and interfere and become separated into different wavelengths that appear as these prismatic mother-or-pearl hues.The dark band between the sunrise and the nacreous is a region of a different form of polar stratospheric clouds, known as Type I. Also up in the stratosphere, these are made not of water ice but of nitric acid and sulfuric acid.