This is a polar stratospheric cloud, and it's very, very pretty. It forms high enough to catch the right angle to the evening sun and fill the entire sky with rainbows. It also makes chemical war on the ozone layer... and is attempting to burn us alive.
Generally, clouds don't form that high in the atmosphere, which makes this a polar stratospheric cloud. There's too little water at extremely high altitudes for molecules to clump together and form clouds; the few intrepid water molecules that do cling to each other are torn apart by the slightest amount of heat. Near the poles, in the winter season, the temperature drops so much that the sparse water molecules can cling to each other and form wide stretches of thin cloud.
But water isn't the only thing in those clouds. There's also nitric acid lacing the cloud. This combination of chemicals interacts with chlorine atoms pumped into the atmosphere due to industrial processes. Chlorine comes in many forms, from relatively sedate to extremely reactive. At the surface of polar stratospheric clouds, chlorine molecules are converted from their more inert forms to extremely active molecules ready to rip apart anything in their path. The first thing that comes across their path is generally ozone, and the chlorine tears it apart. Enough clouds and the ozone layer is chewed up and spat out. These clouds are carrying out a chemical war.
But scientists think it's not an unprovoked chemical war. Ozone locks in heat, and while the ozone layer exists, it heats up the upper atmosphere. This heat keeps polar stratospheric clouds from forming, and once the ozone layer started getting depleted, the upper atmosphere cooled slightly. This allows polar stratospheric clouds — and their reactive form of chlorine — to form, which, in turn, depletes the ozone layer more. The cycle of chemical violence never ends.
Image: Mathias Midbøe