Wednesday, October 24, 2012

Sun dogs

Sun dogs formed by high level ice crystal clouds.

Sun dogs are also called mock suns or perihelia, Greek for ``beside the Sun.'' They consist of one or two patches of light that appear on either or both sides of the Sun. Sun dogs make it appear that there are two or three suns in the sky. When two sun dogs occur, one may be brighter than the other, or higher than the other. They may appear white or colored. Sun dogs often appear just outside the circumference of a 22° halo.

Occasionally these patches of light are seen around a very bright, full moon. In that case, they are called moon dogs.

Sun dogs are produced by the refraction of sunlight that shines through plate-like ice crystals with diameters around 50 micrometers (.0019 inch) or larger. Aerodynamic drag causes the plate-like ice crystals to fall slowly through the air much like leaves falling from a tree. When the ice crystals are positioned horizontally, with large, flat ends parallel to the ground, they will refract sunlight at an angle of 22° and produce the sun dogs. When the ice crystals are randomly oriented, a 22° halo is produced. It takes millions of falling ice crystals, all oriented so that they refract sunlight at 22°, to produce sun dogs.

Where these falling ice crystals are relatively large and plentiful, the sun dogs will be colorful. This color is produced by the selective refraction of light, also called dispersion. In the process of dispersion, each ice crystal acts like a tiny prism, separating sunlight into the spectrum of colors.

The amount by which each color is refracted by an ice crystal varies slightly. Red light has the longest wavelength and is slowed the least as it passes through the ice crystal. Hence, red is bent the least. On the other extreme, violet light has the shortest wavelength and is slowed the most as it passes through the ice crystal. Hence, violet is bent the most.

The result is that red light appears on the edge of the sun dog closest to the Sun and blue appears on the edge farthest from the Sun. The reason why blue, and not violet, appears is that the human eye is better able to perceive blue than violet.

Occasionally a halo will also be colorful, rather than its characteristic white. This dispersion of sunlight into bands of color, by the process just described, occurs when the ice crystals are relatively large and of uniform size and shape.

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