Double Dog

August 15, 2005


Provided by: John A. Adam, Old Dominion University
Summary authors & editors: John A. Adam

This photo of what appears to be a double sundog was taken in Norfolk, Virginia, one evening early this past June. Clearly, the lower coloration, at the same altitude as the setting Sun, is a "standard" sundog or perihelion, and the upper coloration is presumably part of a 22 degree halo. However, if this is the case, I would have expected it to be displaced more to the north or to the "right," given that sundogs appear on or outside the horizontal diameter of the halo. Instead, it's almost vertically above the sundog, indeed, if anything, it is slightly to the "left." Sundogs are formed when light is refracted through the side faces of hexagonally-shaped ice crystals high in the atmosphere, almost always in thin cirrus clouds. For the commonly observed 22º halo, sunlight passes through poorly aligned hexagonal prism crystals -- through faces inclined at 60° to each other. The poor orientations result in a circular halo.

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