Circumzenithal Arc Above Chesapeake, Virginia

Photographer: Rob. Bruner
Summary Author: Rob Bruner; Jim Foster

This photo of a snappy circumzenithal arc (CZA) was taken near Chesapeake, Virginia on August 16, 2009. These lovely, crescent shaped swatches appear 46-50 degrees above the Sun, centered on the zenith. They can only be observed when the Sun is rather low in the sky -- no higher than 32 degrees above the horizon. CZAs form when sunlight refracts through horizontally oriented, plate shaped ice crystals in cirrus type clouds. Light enters the upper, horizontal basal face of these crystals and exits through one of their vertical side faces. Though not shown here, this CZA was accompanied by a pair of sundogs positioned 22 degrees on either side of the Sun. Photo taken approximately 6:00 p.m. local time.