One of the most elusive cloud formations in the world, and one I’ve never seen, is called a noctilucent cloud. It forms at extremely high altitude, about 50 miles up, where the air is extremely cold and thin. It is so cold that virtually any water vapor floating around will immediately adhere and freeze onto anything solid. Of course, at 50 miles up, there aren’t very many solids to adhere to. Terrestrial dust and salt particles from ocean evaporation, typical condensation nuclei for raindrops and snowflakes born lower down, are virtually non-existent at 50 miles. It is thought that the ice crystals in noctilucent clouds grow onto space dust such as comet debris. Because these clouds are so thin, they can only be seen on a clear night when the sun is below the horizon where it can illuminate the exceedingly thin clouds at a time when the sky is dark. For this reason, they are also called “night shining clouds.”
