An unusual temperature drop in the polar stratosphere.
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NASA (2008) said:
Scientists recently discovered that polar stratospheric clouds, long known to play an important role in Antarctic ozone destruction, are occurring with increasing frequency in the Arctic. These high altitude clouds [which] form only at very low temperatures help destroy ozone in two ways.
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NASA Ozone Watch (Images, data, and information for the Northern Hemisphere).
Early start for seasonal polar stratospheric clouds
Posted: December 21, 2023 by oldbrew in atmosphere, Clouds, ozoneTags: Antarctic, arctic
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Tallbloke's Talkshop 
In pictures: Rare ‘rainbow clouds’ seen in north of England
21 December 2023
https://news.sky.com/story/storm-pia-latest-ba-grounds-dozens-of-flights-as-winds-of-up-to-80mph-expected-commuters-told-do-not-travel-13035524
This year the polar vortex will move over Siberia and Europe.
How do we know what these clouds are composed of? Maybe CO2 – unlikely, at 400 ppm – but not impossible. What is the water concentration at that elevation? Maybe even H2CO3 .. so many possibilities.
PSCs are classified into two main types each of which consists of several sub-types
https://en.m.wikipedia.org/wiki/Polar_stratospheric_cloud#Types
BEST DISPLAY OF PSCs IN DECADES:
Polar stratospheric clouds are a sign of extreme cold. They form when the temperature in the Arctic stratosphere drops to a staggeringly-low -85 C. Then, and only then, can widely-spaced water molecules in the dry stratosphere coalesce into tiny ice crystals. High-altitude sunlight shining through the crystals creates intense iridescent colors that rival auroras.
“I have never seen them so large and vividly colorful before,” reports Ronny Tertnes, who sends this sunrise photo from Bergen, Norway [below].
“The photos don’t really justify the wonderful sight,” he says.
https://spaceweather.com/archive.php?view=1&day=23&month=12&year=2023
Spaceweather.com says…
RECORD-COLD WEATHER IN THE STRATOSPHERE: Now we know why polar stratospheric clouds (PSCs) have suddenly exploded. According to NASA’s MERRA-2 climate model, temperatures in the Arctic stratosphere just hit a 40-year record low for the month of December: [bold added]
https://spaceweather.com/archive.php?view=1&day=24&month=12&year=2023
All change…
SUDDEN STRATOSPHERIC WARMING: During the week before Christmas, a near-record cold wave hit the Arctic stratosphere. Temperatures as low as -75C created aurora-like polar stratospheric clouds (PSCs) seen as far south as Italy. Now the opposite is happening. Suddenly, the Arctic stratosphere is warming:
. . .
Cold air bottled up in the Arctic can suddenly break free and spill down to lower latitudes.
https://spaceweather.com/archive.php?view=1&day=31&month=12&year=2023