Bright Parhelion Above the San Fernando Valley
January 30, 2011
Parhelia are prismatic refractions through horizontally oriented ice crystals. The ice crystals are like old-fashioned bathroom tiles: six sided with the large hexagonal face oriented horizontally. This happens because aerodynamic drag orients the crystals as they slowly fall through the air, much as a leaf will fall more-or-less horizontally. The crystals are found in cirrus clouds and measure 20-40 micrometers across. Smaller crystals remain randomly oriented and give rise to the circular, sun-centered 22 degree halo. Larger crystals tend to spin and produce other haloes whose shape changes with solar elevation. A parhelion is often called a sun dog.
The parhelion shown here above the San Fernando Valley of California was exceptionally bright – so bright in fact that I had to underexpose the picture by several stops just to capture its color. As a result, the clouds look dull gray and the sky a dark, subdued blue. To the eye, the clouds and sky were brilliant white and blue. The inset shows a “normal” exposure, in which the parhelion saturates the picture. This sundog is slightly extended in the vertical direction because the crystals are wobbling slightly. Also evident is part of the colorless parhelic circle stretching left from the parhelion. It's caused by reflection of sunlight off the vertical rectangular side faces of ice crystals. Be sure to protect your eyes from the Sun when looking at halo phenomena. Photo taken on November 5, 2010.