Weird Things In The Sky

Polar Stratospheric Clouds (PSCs) are clouds that live - as the name suggests - in the stratosphere, which is around 10 to 25km above the ground. All the other clouds we see are in the troposphere and that's the same part of the atmosphere that we're in. PSCs are generally seen just before sunrise or after sunset and are made up of water-ice or water-acid compunds, forming when stratospheric temperatures drop below -90ºC.

There's chemical reactions ascociated with PSCs involving chlorine and bromine that, when mixed with sunlight, are understood to deplete the ozone in the upper atmosphere. Also, pretty much all the chlorine and most of the bromine comes from human activity - go us!

Actually, Wikipedia has a pretty good description of them and the page can be found here.

A slightly out of focus photo of a PSC. As one of the expeditioners here put it, they essentially look like "an oil slick in the sky".