Circumzenithal Arc Above Ft. Collins, Colorado

October 03, 2008


Provided and copyright by: George Janson, Larimer County Search and Rescue
Summary Author: George Janson, Jim Foster

The morning sky was brilliant azure on this March day at Lory State Park, northwest of Fort Collins, Colorado. Larimer County Search and Rescue, an all-volunteer donation-supported organization of professionals, had just arranged the veteran members and new recruits into teams for their basic map and compass training. I had pointed out sundogs a bit earlier in the morning, when I looked up and saw this brilliant circumzenithal arc. I announced over the radio system for everyone to immediately look directly overhead -- an auspicious beginning to an event where training is to improve our ability to 'see' in the backcountry. Circumzenithal arcs form when similarly oriented hexagonal ice crystals are suspended high in the sky -- sunlight enters through the upper basal face and exits through one of the 6 vertical side faces. Photo taken on March 9, 2008.