Circumhorizontal Arc Over Gunnison, Colorado
October 17, 2004
The above photo shows a lovely circumhorizontal arc taken near Gunnison, Colorado on July 21, 2004. This phenomenon is caused by refraction in ice crystals rather than from raindrops, as would be the case if the colorful band resulted from a rainbow. Cirrus clouds, behind the lower cumuloform clouds in the foreground, are composed of ice crystals, which here are similarly oriented. Sunlight enters though a vertical side face of a hexagonal ice crystal and exits through the lower horizontal basal face. In order for circumhorizontal arcs to be observed, the Sun must be at least 57.8 degrees above the horizon. Since the picture was taken at about 2:30 p.m. local time, the Sun was sufficiently high at this latitude, and for this time of year, to view such an arc. See also the Earth Science Picture of the Day for August 12, 2004.