What's that cloud? Pearls in the sky

Mesmerising polar stratospheric clouds were recently spotted across parts of northern Scandinavia.

Polar stratospheric clouds, also known as nacreous, or mother-of-pearl clouds, are clouds that occur during the polar winter generally at high northerly latitudes, when temperatures are very low, lower than -78C.

Most clouds form at altitudes of 10,000 to 15,000 metres high in the troposphere, but these particular clouds develop much higher up, in the stratosphere at 15,000 to 25,000 metres.

Polar stratospheric clouds are an exceptional and rare type of cloud known for their iridescent and luminous aesthetic, best observed during civil twilight. However they also have a darker side to them.

Research has shown that they are destructive to the ozone layer. The ice crystals in the clouds trigger a chemical reaction between the ozone layer and gases such as bromine and chlorine.

According to the US Environmental Protection Agency, just one atom of chlorine in the stratosphere can destroy more than 100,000 ozone molecules.

They are rarely visible in the UK, though some people have spotted them before, such as recently in the Scottish Highlands by a Twitter user above.