From the past one week UK Sky watchers are witnessing a very rare apparition of polar stratospheric clouds (PSC). These PSC are commonly seen in Arctic Circle. But these colourful clouds have appearing over British Isles almost every day since 31st January. The below photo graph of PCS is taken on 2nd Feb by Colin Fraser which were formed over Edinburgh.
PSC’s usually forms only in lower stratosphere when the temperature falls to over -85 degree Celsius. High-altitude sunlight shining through tiny ice particles ~10µm wide produces 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. They catalyze 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…” Cowley said.
“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! This episode is exceptional at such low latitudes. If it goes on any longer my camera will be worn out,” He added.