A halo occurs when the light from the sun or the moon is refracted and reflected by ice crystals in the atmosphere, usually in a thin layer of high cirrostratus clouds. Under certain circumstances a second, or outer, halo appears, which is fainter than the inner one. At times white or colored luminous arcs are also seen lying somewhat parallel to the horizon and passing through the source of light, called mock suns, parhelia, or sun dogs for the sun, and paraselene for the moon. A single mock sun, the anthelion, directly opposite the sun, may be added.
In general a white halo results from the reflection of light by ice crystals, while one which appears as colored rings results from the refraction of light by ice crystals. Halos are more brilliant and complex near the poles than in other parts of the world. The theory attributing their formation to the presence of ice crystals was first suggested by the 17 th cent. French philosopher Descartes. Similar to a halo and sometimes confused with it is the sun’s corona. In X-ray electron diffraction, the term halos refers to the broad rings that appear on a photographic film as a result of the diffraction of a mono energetic beam of X rays or electrons from a crystalline powder located at the center of the camera.
Sun Dogs One of the optical phenomena are called Sun Dogs. Their scientific name is Parhelia, and they are also called mock suns. They are known to be a small halo (as mentioned above) or rainbow near the horizon just off the Parhelia circle. Classed as atmospheric optical phenomena, they occur often in the North. Sun dogs are among the most commonly seen sky phenomena; usually occur in pairs, one on each side of the halo, each at the same altitude as the sun and appearing most prominently when the sun is low. A sun dog is a rainbow-like spot in a cirrus cloud.
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Light shining through ice crystals in the cloud makes a sun dog, much like light shining through raindrops makes a rainbow. “They are reddish on the side facing the sun and often have bluish-white tails stretching horizontally away from them. Cirrus clouds (those high fleecy white bands or patches in the sky) are mostly tiny particles of ice. Ice can take on many forms and shapes. The cloud ice, however, is shaped like hex bathroom tiles or stubby pencils each no bigger than the tiniest grains of sand. These ice crystals bend light like a prism, disperse its colors, and cause sun dogs When the crystals line up like tiles on a table, the light shining through makes sun dogs The horizontal crystals bend the light 22 degrees.
As the light enters and exits the crystal. Light colors fan out from the bending and display as a sun dog. “They usually appear in pairs two hand breadths on either side of the sun when it rises or sets behind a very thin veil of high cirrus clouds.” Sun dogs are formed by the refraction of the sun’s rays through a very specific type of ice crystal in the air. These crystals, called plates, are hexagonal (they have six sides), are quite flat, and slowly settle to the ground in a similar manner to maple leaves, wobbling back and forth somewhat, with the flat sides horizontal. These crystals grow in temperatures between -9^0 and -20^0 C. (15-25^0 F.
).
Temperature inversions, though, often allow sun dogs to be visible when it’s much colder than that at ground level. Because Sun dogs always appear 22^0 on either side of the sun, it takes a very wide lens to capture them on film. They are multi-colored, but the degree to which the rainbow colors are visible depends on the amount of wobbling the ice crystals are doing… The amount of vertical “stretch” of the sun dogs is caused by the wobble (more wobble = more stretch), and to a lesser degree by the height of the sun above the horizon. The difference between sundogs and halos is the preferential orientation of the ice crystals through which the light passes before reaching our eyes.
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If the hexagonal crystals are oriented with their flat faces horizontal, a sundog is observed. If the hexagonal crystals are randomly oriented, a halo is observed.
