January 10, 2003
The variety of clouds -- the classic cumulus, stratus, and so on -- seems near endless. In the simplest classification, however, there are only two kinds, water vapor and ice (plus the inevitable mix). The difference is quickly noted in their interaction with sunlight. Here, as seen from a high-flying aircraft, floating ice crystals in the cloud deck below mirror the Sun to produce a "subsun" so bright that the eye turns away. It's really the inverse of the sunpillar seen so commonly from the ground. Instead of the cloud deck being above the Sun, as in the case of the sunpillar, here it's below it. When the ice disappears, so does the subsun. See also the Earth Science Picture of the Day (Brilliant Sunpillar) for December 14, 2002.