by Thomas Kuhn
The iridescent colours of these mother-of-pearl clouds became vivid about half an hour after sunset. Despite the very cold temperatures, they made me leave my lab at the space campus in Kiruna to enjoy their shifting colours. These clouds are a special type of polar stratospheric clouds (PSC) that is quite rare and occurs only when temperatures in the stratosphere are very cold in the polar winter months. The tiny ice crystals that form in these clouds have to be quite uniform in size so that, when the clouds are close to the sun, diffraction can create their shining colours. They are best seen at dawn or dusk when they are still illuminated by the sun, which is below the horizon so that normal clouds in the troposphere are not illuminated any more.
Thomas Kuhn (distributed via imaggeo.egu.eu)