It’s a cold and frosty morning in late January. Wispy white clouds decorate the sky. The sun is just rising above the horizon at the optical latitude of the barn. As it rises, Bryan reports that it is accompanied by a pair of sun dogs.
Sun dogs are one of several optical phenomena known as haloes created by the sun shining through ice crystals in cirrus clouds.
Cirrus clouds are the thin, wispy clouds riding high in the atmosphere (above 20,000 feet) also known as mares’ tails. They are composed of ice crystals that form directly into ice around mineral or metal particles at temperatures well below zero. Several shapes of crystals form in these clouds, including solid columns, hollow columns, plates, , rosettes, and conglomerations. The crystals that are responsible for optical phenomena such as sun dogs are shaped like flattened hexagonal plates. When these plate-like crystals are lying in a nearly horizontal plain, light from the sun refracts through the crystals. The ice crystals act like prisms, bending the separate wave lengths of light into the colors of the rainbow. They are usually most visible when the sun is at or very near the horizon.
Sun dogs or false suns are bright spots of light that appear to flank the sun on either side. They exhibit blurry rainbows which start off red closest to the sun and move through overlapping colors of the rainbow to the blue on the side farthest from the sun.
Etymology of the English term sun dogs is questionable, but might refer to the spots of light following the true sun as dogs follow their master. In the 1882 edition of Abram Palmer’s delightfully titled “Folk-etymology: A Dictionary of Verbal Corruptions Or Words Perverted in Form Or Meaning, by False Derivation Or Mistaken Analogy,” he defines sun dogs as “false suns which sometimes attend or dog the true [sun] when seen through the mist”
Two other phenomena created by these plate-shaped ice crystals include sun pillars and diamond dust.
Sun pillars look like shafts of bright light rising or descending from the sun. They usually appear when the sun is near the horizon. Sun pillars occur when Sunlight is shining through the ice crystals as they fall to earth. Unlike with sun dogs, the ice crystals are not aligned in any particular way. They are all jumbled up. so instead of refracting the light, they reflect it, and we see white light instead of rainbows.
The third phenomenon is called diamond dust. It generally occurs when surface temperatures are very cold and the plate-shaped ice crystals maintain their shape all the way to the ground. They reflect points of light in a sparkly haze.
Photo by Author. Alt text: Two brilliant stars rise next to the barn on a frigid Iowa morning. Only one, of course; the one on the left is a sun dog.