Skies across England and Scotland looked more colourful than usual this week after onlookers spotted some nacreous clouds.
The rare phenomenon, identifiable by an array of shimmery pastel colours, happens only when the conditions in the air are right.
People from Scotland, north England, and the West Midlands marvelled at the rainbow-like scene they witnessed.
"Still gasping at the beauty of yesterday's rainbow clouds. I doubt I'll ever see the like again," wrote one person on X, formerly Twitter.
Some people were slightly confused by what they saw, with one writing on X: "What’s the craic with this strange rainbow effect in the clouds? Never seen anything like it before !!"
So what exactly are nacreous clouds — and will there be any more opportunities to see more before the end of the year?
What are nacreous clouds?
Nacreous clouds are known for reflective, iridescent colours that appear to shimmer in the sky.
According to the Met Office, their appearance is similar to the way colours reflect oil when it floats on top of the water.
Such clouds are natural but are considered a rare phenomenon that occurs high in the sky when certain temperatures and air conditions exist.
Meteorologists often classify nacreous clouds into three categories based on their chemical conditions. However, to the naked eye, they're best recognised by their unusual coloring.
The last few days have been the best time to spot such clouds across the UK. They were visible from places in the UK such as Edinburgh and Fife in Scotland and Staffordshire, Merseyside, Lincolnshire, and Cheshire in England.
Iridescent nacreous clouds over Leeds!
— Carey Davies (@carey_davies) December 21, 2023
Very rare and usually only seen in polar regions*
*so I understand from 20 seconds of googling pic.twitter.com/AuD79ileof
What causes nacreous clouds?
Nacreous clouds are polar stratospheric clouds that can form only when the sky's temperature drops below a chilly -78 °C.
This means they're normally visible in the UK only in the coldest part of winter.
So how are they formed? With temperatures in the stratosphere so low, water particles turn into tiny ice crystals in the sky.
As the sun dips below the horizon, the light rays hit the crystals and refract, scattering the light in colorful, luminescent displays.
These specific conditions also tend to rely on a polar vortex (or cold air coming from the polar regions) circulating in the stratosphere.
But when the conditions are right, as they were earlier this week in the UK, onlookers can marvel at the 'mother of pearl' clouds lighting up the sky at sunrise and sunset.
What weather is associated with nacreous clouds?
Nacreous clouds rely on conditions created in a polar vortex, meaning these clouds are associated with extremely cold weather.
Nacreous clouds develop up to 25,000 metres in altitude, where such frosty temperatures can be achieved, meaning they're much higher than common clouds.
According to the Met Office, nacreous clouds are best viewed from 1º and 6º below the horizon and are more common in arctic regions in Scandinavia and Canada.
Are nacreous clouds bad?
It's worth noting that while these clouds display pretty colours, there's a darker side to them.
Scientists have found that polar stratospheric clouds damage the planet's ozone layer due to chemical reactions producing chlorine gas within the clouds.
This gas then destroys the ozone layer, which is crucial to protecting our planet from harmful UV rays.
What is the difference between nacreous and noctilucent clouds?
Another type of phenomenon that is occasionally spotted in the sky is called a noctilucent cloud.
Similar to nacreous clouds, noctilucent clouds are also extremely rare and form due to water vapour or dust high up in the sky. Both water vapour and dust aren't commonly found at such high altitudes, adding to the rarity of these clouds.
Noctilucent clouds develop in the mesosphere, which is even higher than the stratosphere. As a result, they're also referred to as polar mesospheric clouds or 'night-shining' clouds.
While these clouds require cold conditions, they're most commonly seen in the summer after sunset.
