Nacreous Clouds

Rare, brilliantly colored clouds that form in the stratosphere, far higher than ordinary weather clouds, typically over polar regions during the depths of winter when stratospheric temperatures drop low enough for ice or nitric acid crystals to form despite the extreme dryness at that altitude. Visible only when the sun is just below the horizon, they display an intense, shifting iridescence often compared to mother-of-pearl, and also play a direct role in stratospheric ozone depletion.