From spaceweather.com An apparition of polar stratospheric clouds is underway around the Arctic Circle. “It is almost as good as the aurora borealis,” says Göran Strand, who took this picture last night from Östersund, Sweden:

Eric Schandall of Oslo, Norway, adds this report: “We have seen them for three evenings over Oslo, with the ones on Jan. 13th being the most dramatic and beautiful so far.”

Also known as “nacreous” or “mother of pearl” clouds, these icy clouds 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.

“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.”