Circumscribed Halo Over Viterbo, Italy
June 26, 2006
This pronounced circumscribed halo was observed over Viterbo, Italy during the morning of April 21, 2006. It was very bright indeed and accompanied the Sun for several hours. Since early in the morning, diffuse cirrus clouds partially covered the sky, but it wasn't until approximately 10:40 a.m. (local time) that a halo began to take shape, reaching maximum brilliance about one half hour later. Circumscribed halos form in column-shaped ice crystals. Sunlight enters through one of the side faces of these hexagonal columnar crystals and exits through an alternate side face. The "c" axes (flat ends) of the crystals must be more or less horizontally oriented, and the "a" axes (side faces) assume random orientations. In order to observe circumscribed halos, the Sun has to be higher than approximately 30 degrees above the horizon.
Photo details: Canon EOS D30 camra, Pentax Takumar Fish-Eye 17 mm lens, f/3.5 stop, at F/8, and 1/2000 sec. exposure.Related Links: