22 Degree Halo and Circumhorizon Arc

November 03, 2009

22 Degree Halo and Circumhorizontal Arc
Photographer:
Lisa Gonnelli
Summary Author: Lisa Gonnelli; Jim Foster

The photo above showing a classic 22 degree ice halo and a splendid circumhorizon arc (CHA) was taken from Pilesgrove, New Jersey on May 24, 2009, at approximately 11:25 a.m. Halos are formed by randomly oriented hexagonal ice crystals typically found in cirrus type clouds whereas, CHAs result from crystals having preferred orientations. Sunlight enters through the vertical side faces and exits the lower horizontal basal faces. For CHAs, the blue side (not easily visible) is nearest to the horizon and the red side nearest the zenith. These gorgeous arcs can only be observed when the Sun is high in the sky, higher than 58 degrees above the horizon. In the mid-latitudes, look for them near noon in late spring and early summer. Note that infralateral arcs, which can be confused with CHAs, seem to curve slightly upwards from the horizon.