Iridaceae clouds, also known as nacreous clouds, are a stunning and rare type of cloud that forms at very high altitudes.
Iridaceae clouds in the Dornoch Firth area, Scotland. (Photo: Michael Traill).
Iridaceae clouds are formed by ice crystals and are typically found in extreme cold conditions above the polar regions. However, on the evening of January 29 and the following morning, a team of BBC weather experts spotted such clouds in the Aberdeenshire, Highlands, and Moray regions. Also known as polar stratospheric clouds, they are among the most beautiful clouds in the twilight sky. Their name comes from the array of colors produced when sunlight refracts around tiny ice crystals.
Almost all clouds form in the troposphere at altitudes ranging from approximately 30,000 to 50,000 feet above the Earth’s surface. However, iridaceae clouds typically occur in the stratosphere, at altitudes of 68,000 to 100,000 feet. Because the stratosphere is so dry, temperatures must be extremely cold, below -78°C, for moisture to freeze into ice crystals.
Iridaceae clouds are more common in winter in Scandinavia and Canada, but are very rare in the UK and regions located at lower latitudes. These clouds can appear in the UK when cold polar air moves south due to changes in the polar vortex. Iridaceae clouds are also most visible at twilight, a brief period just before sunrise and after sunset, when the Sun is below the horizon by 6 degrees. Due to their high altitude, they remain illuminated by sunlight, with their vibrant colors standing out against the darker sky surrounding them.
“In the UK, it is very rare for atmospheric temperatures to drop low enough to create ice crystals. Currently, the polar vortex is slightly shifting toward the UK, allowing the cold stratospheric air to create conditions for iridaceae clouds to form,” said Tom Tobler, a meteorologist at the weather forecasting company MetDesk.
