Nacreous clouds

Clouds that have a vivid iridescent appearance. They are formed of small droplets which scatter light in such a way that creates the distinctive luminescent appearance. Nacreous clouds form below -78 °C temperatures at altitudes 68,500 - 100,000 ft above the Earth's surface (in the lower stratosphere) over polar regions.They are also known as Polar Stratospheric Clouds (PSC) or 'mother-of-pearl clouds'.