Polar Stratospheric Clouds

Dec. 9, 2016: The stratosphere above the Arctic Circle is getting cold … very cold.  That’s the only way to explain these colorful clouds that materialized over Kiruna, Sweden, on Dec. 9th:

“Polar stratospheric clouds are back in the subarctic,” reports photographer Mia Stålnacke. “They were brilliantly beautiful today.”

Icy polar stratospheric clouds (PSCs) form in the lower stratosphere when temperatures drop to around -85ºC. That’s how cold it has to be for ice crystals to form in the very dry stratosphere. High-altitude sunlight shining through tiny ice particles ~10µm across produce the characteristic bright iridescent colors.

“Once seen they are never forgotten,” says atmospheric optics expert Les Cowley. “Polar stratospheric clouds have much more vivid colors than ordinary iridescent clouds, which are very much poor relations and seen frequently all over the world.”

Once thought to be mere curiosities, some PSCs are now known to be associated with the destruction of ozone. Indeed, an ozone hole formed over the UK in Feb. 2016 following an outbreak of ozone-destroying Type 1 PSCs.

“Last winter we had these clouds almost daily for long periods of time,” says Stålnacke. Arctic sky watchers are encouraged to be alert for more in the days ahead. The best time to look is just before sunrise or after sunset.

Ozone Hole over the UK

Feb. 5, 2016: For the past week, sky watchers in the UK have witnessed a rare apparition of polar stratospheric clouds (PSCs). Normally restricted to the Arctic Circle, the fantastically colorful clouds have appeared over the British Isles almost every day since Jan. 31st. Colin Fraser photographed the display over Edinburgh, Scotland, on Feb. 2nd:

PSCs form in the lower stratosphere when temperatures drop to a staggeringly-cold -85ºC. High-altitude sunlight shining through tiny ice particles ~10µm wide produce bright iridescent colors by diffraction and interference.

But there is more to PSCs than ice. Some polar stratospheric clouds contain very small droplets of naturally occurring nitric and sulphuric acids. These droplets destroy ozone. Indeed, atmospheric optics expert Les Cowley points out that a temporary ozone hole has formed over Ireland and the UK. It is the blue patch in this Feb. 1st ozone map from NASA’s Arctic Ozone Watch:

“The acid droplets destroy the stratospheric ozone layer that protects us from harmful solar ultra-violet rays,” says Cowley. “They catalyse unreactive forms of man-made chlorine into active free radicals (for example ClO, chlorine monoxide). The radicals destroy many ozone molecules in a series of chain reactions..”

This outbreak of PSCs is truly unusual. “Prior to this outbreak I have seen PSCs over the UK only twice in the last 20 years!” says Cowley. “This episode is exceptional at such low latitudes.  If it goes on any longer my camera will be worn out.”

UPDATE: On Feb. 5th, the outbreak of PSCs has subsided, and so has the UK ozone hole. Click here for updates.