photos by ann hawthorne in antarctica of polar stratospheric clouds, so named because they form over eighty thousand feet high, which is high enough, given the curvature of the planet, to be illuminated after sunset by light reflected from below the horizon. it is this iridescence that gives the clouds their informal name, mother of pearl clouds.
where the stratosphere is usually much too high for for water molecule to remain stable and form clouds, the temperature in the antarctic winter season drops to such an extent that what sparse water molecules are present in the upper atmosphere condense from the pressure drop, forming wide stretches of thin cloud.
unfortunately, these clouds also contain nitric acid, which reacts violently with chlorine released into the atmosphere by industrial processes elsewhere on the planet. the reaction causes holes to form in the ozone layer, which in turn serves to further cool the upper atmosphere, thus spurring the formation of more stratospheric clouds, and more chlorine radicals, and more holes in the ozone layer.