Nacreous clouds take their name from nacre, or mother of pearl, and are less romantically known as Polar Stratospheric Clouds, or PSCs. They form at extremely low temperatures high in the stratosphere, ten miles and more above the Earth, whereas normal clouds might reach six miles at most into the troposphere (the inner layer of the atmosphere).
This picture was taken at McMurdo Station, the US Antarctic research centre, in late August, the end of the dark Antarctic winter, when the sun was starting to return. At such times the sunlight comes from below and is diffracted through the ice crystals which make up the clouds. The photographer, Chad Carpenter, says he had to stop himself shaking with excitement when he saw this sight, and took photographs until his camera battery died.
The silhouetted shape is the shrine of Our Lady of the Snows, a memorial to a construction worker who died here in 1956 when his tractor fell through the ice. There is danger written in these pearly clouds, too: their beautiful iridescent surfaces catalyse the reactions that create the hole in Antarctica’s ozone layer.