In the depths of winter, when conditions are just right, glittering clouds turn the polar skies into a spectacle of colour.
Rainbow clouds – also known as ‘nacreous’ or ‘mother-of-pearl’ clouds after their resemblance to the iridescent shells of some molluscs – are an extremely rare atmospheric phenomenon.
Known scientifically as ‘polar stratospheric clouds’, they’re found much higher in the atmosphere than clouds normally form. Most clouds are found less than 10km (6 miles) above Earth’s surface.
But rainbow clouds form at altitudes between 15 and 25km (9 and 15 miles), in an atmospheric layer known as the lower stratosphere.
Usually, the stratosphere is far too dry for clouds to form, but when the air temperature drops below about -80°C (-112°F), the sparse water molecules group together to form tiny ice crystals, which aggregate into clouds.
The tiny ice crystals scatter sunlight, separating it into different colours and creating the dazzling rainbow effect these clouds are named after.
They also scatter light forward, resulting in a pearlescent appearance. Because they’re found at such high altitudes, the curvature of Earth means that rays of sunlight from below the horizon hit the underside of these high-altitude clouds and are reflected, illuminating the clouds brightly around dawn and dusk.
The thickest, most vivid rainbow clouds are formed of pure water crystals. But atmospheric pollutants, such as nitric acid often from the production of fertilisers, can form high-altitude ice crystals at warmer air temperatures, producing thin clouds that are less vibrant. This type of polar stratospheric cloud is linked to degradation of the Antarctic ozone layer.
If you’d like to witness a rainbow cloud for yourself, your best chance is to visit Antarctica during winter and look skyward around sunrise or sunset. They can also occur in parts of Norway, Sweden, Finland and Alaska, however, and have even been observed as far south as Scotland.
This article is an answer to the question (asked by Ollie Peterson, via email) 'What causes rainbow clouds?'
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