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.