Polar Stratospheric Cloud, seen from the NASA DC-8 on 14 January 2003 (photograph by Paul Newman, GSFC). These are naturally occurring clouds composed of small nitric acid and water particles. The clouds have a colorful appearance because they contain similar sized, small particles each of which defracts sunlight in a similar manner. This image is notable for illustrating the high altitude (approximately 70,000 ft) of the polar stratospheric clouds, well above the cirrus cloud deck that lies below the DC-8 airplane. Reactions that occur on the surface of these clouds convert chlorine, most of which is due to human sources, from relatively inert forms to compounds that lead to rapid ozone destruction. Click here for more information on Polar Stratospheric Clouds