June 03, 2003
This wonderful photo shows a portion of a 22 degree halo (top) and a brilliant circumhorizontal arc (bottom). It was taken on May 20, 2003 near Richmond, VA. Unlike circumzenithal arcs (see yesterday's EPOD), circumhorizontal arcs can only be seen when the Sun is high in the sky. Therefore, compared to circumzenithal arcs, circumhorizontal arcs are seen less often. They're usually only observed during mid-day in the late spring and summer months (in the mid latitudes) when the Sun attains an elevation greater than about 58 degrees above the horizon. Thus, they can never be seen in polar regions. In the case of circumhorizontal arcs, sunlight enters through a vertical face of hexagonal plate crystal in cirrus clouds and leaves through the lower horizontal basal face -- opposite of circumzenithal arcs.