Circumhorizontal Arc Over Viterbo, Italy

July 13, 2006

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Provided and copyright by: Paolo Candy, Cimini Astronomical Observatory
Summary authors & editors: Jim Foster; Paolo Candy

The above is a photo collage of a lovely circumhorizontal arc, which graced the skies above Viterbo, Italy on May 31, 2006 (12:30 p.m. local time). It extended approximately 100 degrees across the early afternoon sky. When sunlight enters though the vertical side faces of the hexagonal ice crystals that ccompose cirrus clouds and exits through the lower horizontal basal faces, these beautiful arcs can sometimes be detected. However, circumhorizontal arcs can only be observed when the Sun is quite high in the sky (at least 57 degrees above the horizon). Therefore, in the mid latitudes, they're a late spring and summertime phenomenon and are typically seen around solar noon.

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