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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