The Boulder, Colorado, Concentric Halo Display of 21 July 1986

Abstract

An exceptional concentric halo display was observed in Boulder, Colorado, on 21 July 1986. As many as six halos were observed and photographed simultaneously between 1530 and 1945 UTC. Extensive photographic documentation (>100 photographs) captured the evolution of the display. The halos were produced by solid, pyramidal ice crystals in the upper troposphere. The pyramidal crystals possessed six different prism angles, each of which produced a halo with a different angular radius (9°, 18°, 20°, 22°, 24°, and 35° halos). The crystals may have been formed on the previous day by thunderstorms over the southwestern United States.