The dark side of iridescent beauty

Cloud watchers in Scotland and northern England have been treated to some stunning clouds this month. Rare nacreous (mother-of-pearl) cloud formations – usually only spotted in polar regions – have been shimmering at sunrise and sunset. The high-level clouds occur when the upper atmosphere is very cold and dry, as it was during early February. But unfortunately these delightful iridescent clouds have a darker side, and may aid the formation of a large ozone hole over the Arctic this spring.

This year the upper atmosphere (around 20km altitude) in the northern hemisphere has been colder than usual, with temperatures regularly dipping below –78°C – the temperature at which nacreous clouds start to form. The particles (nitric acid trihydrate) that makes up these clouds, readily react with chlorine- containing compounds and produce chlorine gas. Once spring arrives, this chlorine, with the help of sunlight, can be split into reactive chlorine atoms, which smash up the protective ozone existing at this altitude. “Just one Cl atom is estimated to destroy up to 100,000 ozone molecules over its lifetime,” writes Michaela Hegglin, on the University of Reading’s Weather and Climate blog

Right now the sun is inching higher over the Arctic horizon every day; spring is on the way, but as yet no-one is sure how much ozone will be destroyed. However, the nacreous clouds are a sign that conditions are ripe for ozone loss. If a strong polar vortex persists, keeping Arctic air isolated, then there is concern that the Arctic ozone hole could surpass the record hole observed in spring 2011. Keep your eyes on the sky.