A possible outbreak of polar stratospheric clouds (PSCs) is underway around the Arctic Circle. Unlike normal grey-white clouds, which hug Earth's surface at altitudes of only 5 to 10 km, PSCs float through the stratosphere (25 km) and they are fantastically colorful. Ivar Marthinusen sends this picture of the phenonenon from Skedsmokorset, Norway:
"Right after sunset on Dec. 22nd, the clouds were so bright they were uncomfortable to look at directly," says Marthinusen.

Also known as "nacreous" or "mother of pearl" clouds, these icy structures form in the lower stratosphere when temperatures drop to around minus 85ºC. Sunlight shining through tiny ice particles ~10µm across produce the characteristic bright iridescent colors by diffraction and interference. Once thought to be mere curiosities, some PSCs are now known to be associated with the destruction of ozone.

"Nacreous clouds far outshine and have much more vivid colours than ordinary iridescent clouds, which are very much poor relations and seen frequently all over the world," writes atmospheric optics expert Les Cowley. "Once seen they are never forgotten."