Forming 10–20 miles up, in the stratosphere, at –85˚C (-121˚F), nacreous clouds show beautiful iridescent pastel hues as they scatter the light from the Sun when it is just below the horizon.
Sometimes called ‘mother-of-pearl clouds’, their tiny, uniform ice crystals are very good at diffracting sunlight. This separates the light into bands of colour, to create a much more dramatic version of the iridescence sometimes seen in lower clouds.
Also known as ‘polar stratospheric clouds’ since they tend to appear over higher-latitude regions of the world, nacreous clouds are like a stratospheric version of the lenticularis species of wave cloud. They form when the atmosphere is so stable that waves produced as air flows over mountains down at ground level are transferred up through the atmosphere, and push moisture into the lower stratosphere. The best time of year to spot them is in winter, when temperatures are lowest.
Sadly, these most mesmerising of clouds are also the most destructive for our environment. Their tiny ice crystals act as catalysts that speed up the destruction of the protective ozone layer by the CFC gases we’ve released into the atmosphere. For clouds to have such otherworldly beauty, there always had to be a catch.