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Sun Dog Days

© Keith C. Heidorn

August is generally known around North America as the "Dog Days of Summer." The origin of this term has been attributed to the month's extremely hot weather causing dogs to lie about panting as well as to an ancient Egyptian belief that the bright star Sirius, the Dog Star, prominent in this month, added to the heat of August days.

My topic for this essay is, however, another weather dog: the sun dog, though I should perhaps use the plural as sun dogs usually come in pairs. They are the loyal to the sun, sitting on each side of the solar orb along a horizontal line through the solar disk. Sun dogs appear in August (and October and January and April, the month does not really matter), but they are most regularly seen close to their solar master during winter months when the sun is low in the sky.

Sun dogs on 22^o halo flank South Pole sun
(photo courtesy of NOAA/US Dept of Commerce).

Sun dogs, or mock suns, are technically called solar parhelia (parhelia meaning "with the sun") and appear as bright bursts of light formed when sunlight passes through ice crystals at the proper angle. Usually, cirrus clouds in front of the sun produce sun dogs, but other ice clouds, such as ice fog and diamond dust, may also generate them. Sun dogs are sometimes so brilliant that dazzled observers mistake them for the sun. They are often bright white but may show a partial spectrum of color with the red wavelengths on the edge nearest the sun. Sun dogs often have comet-like appearance with a bluish-white tail facing away from the sun.

Sun dogs are the second most frequent halo phenomena behind the 22^o halo and often accompany that halo. The difference between sun dog and halo formation is the orientation of the ice crystals through which sunlight passes before reaching our eyes. Halo formation requires a mixture of random ice crystal orientations in the sky. But if the sky has only horizontally oriented, flat ice crystals, we just see a sun dog.

Ice crystals in the atmosphere are hexagonally shaped. Crystals forming most optical phenomena in the air are typically hexagonal rods, shaped like pencils, or flat, hexagonal plate patterns, like microscopic stop signs or dinner plates. When plate-shaped ice crystals fall unimpaired, drag forces automatically orient them horizontally so that their larger, flat surface parallels the earth like a large maple leaf drifting down from a tree.