Nacreous clouds are unique clouds that form in the stratosphere at extremely low temperatures of around negative 85 degrees Celsius. These clouds occur when temperatures fall below the frost point in the stratosphere, creating ice crystals. They are typically found in higher latitudes and are characterized by their small particle size, which allows them to scatter light in a distinctive way, resulting in their iridescent or rainbow appearance.

According to the Meteorological Office of the UK, nacreous clouds usually form at elevations between 68,500 and 100,000 feet. During the day, they may appear similar to pale cirrus clouds, but they become truly spectacular during sunrise or sunset when the sun is 1-6 degrees below the horizon.

Nacreous clouds are a type of polar stratospheric cloud and are associated with freezing and dry weather. Scientists classify them into three types, la, lb, and ll, based on their chemical composition, ambient temperature, and elevation.