June 02, 2003
The vivid circumzenithal arc shown above was visible in the sky over southern Ohio during a complex halo display on March 11, 2003. Circumzenithal arcs form when sunlight passes through the uppermost basal faces of oriented, hexagonal plate crystals and then exits through vertical side faces. The angle between these two faces is 90 degrees, which disperses the colours widely. Because sunlight is very nearly parallel, there is little overlap between the colours, and they thus have very pure hues, purer than those of a rainbow. In order to observe these graceful, lovely arcs, the Sun must be lower than about 32 degrees above the horizon.