Many years ago, the nature-essayist Hal Borland wrote in his book Sundial of the Seasons that "Sun dogs and moon dogs are beautiful accents to a winter day or night as the rainbow is to a showery Summer day." Sun dogs usually appear in pairs, and are loyal to the sun (as moon dogs are loyal to the moon), sitting on each side of the solar orb along a horizontal line through the solar disk. Sun dogs appear in January, April, August, and October, 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 and ice crystals in the atmosphere are more common, but we can see them in any of the other seasons whenever cirrus clouds fuzz the sky above.
Sun dogs on 22o 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 22o 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.
Sun dogs emerge when sunlight passing through the ice plate's thin sidefaces is refracted. The more perfectly aligned the falling crystals are to the horizontal, the more compact the resulting sun dog. Crystal misalignment from true horizontal will spread the sun dog vertically its angular height being approximately four times the maximum crystal angular tilt.
Sun dogs frequently display a reddish tint on the side facing the sun and may sport bluish-white tails which stretch horizontally away from it. The degree to which colours are visible depends on the amount of wobble in the ice crystal's fall: the more wobble, the more colour. The sun dog's tail is formed by light passing through the crystal at angles other than the optimal deviation angle.
Sun dogs typically appear when the sun is low to the horizon, usually just prior to sunset or after sunrise, or during winter months at mid-latitudes. If the sun is low (horizon to about 15o above it), each sun dog is separated from the sun by 22o (or about two handsbreadth on extended arms), and both will lie on the circle of the 22o halo if one is present.
Sun dogs form tightest to the sun at lowest solar altitudes, but they are never less than 22o from it. As the Sun climbs in the sky, the sun dogs slowly move away from the 22o separation, although they remain on the line through the sun parallel with the horizon. When the sun has climbed to more than 45o altitude, sun dogs are fainter and noticeably off the 22o circle, and they vanish altogether above 61o solar altitude.
Over two millennia ago, the Greeks recognized that haloes and sun dogs foretold rain. Today we known this is often a valid prediction, because haloes and sun dogs are produced by ice crystals that form the cirroform clouds which make up the typical cloud sequence preceding a precipitating warm front.
One last word. There are also moon dogs that appear alongside the moon and are formed by lunar light passing through ice crystals. Moon dogs (or paraselenae) are less commonly seen because the moon can only produce them when bright and because they appear during the night when most of us are asleep.
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