A sun dog or sundog (scientific name parhelion) is a relatively common atmospheric optical phenomenon associated with the refraction of sunlight by small ice crystals that make up cirrus or cirrostratus clouds. Sundogs typically appear as a bright and colorful patch in the sky at a position 22 degrees or more to the left and/or right of the sun. They are a halo. Other common associated phenomena, collectively called “ice halos,” are the circumzenithal arc, upper tangent arc, parhelic circle, and lower tangent arc. There are many other named ice halo phenomena that can be seen given optimal conditions.
The ice crystals responsible are hexagonal plate shapes 0.05 - >1mm in size. These ice crystals refract the sunlight in many directions but with a minimum deviation angle of about 158Â°, resulting in the appearance of sundogs about 22Â° from the sun. The refraction depends on wavelength and so sundogs have a red inner edge and more muted colours further from the sun as colours increasingly overlap. Solar altitude is important and sundogs draw away from the sun at increasing solar altitudes.
Sundogs are seen in short arcs always at the same altitude as the sun because the plate crystals are preferentially aligned by aerodynamic drag effects with their large basal faces approximately horizontal.
Although often less vivid and more diffuse than the ones depicted in the photographs, sundogs are actually rather common; they are often overlooked because one must look in the general direction of the bright sun in order to spot them.
In remote stretches of Western Texas, sundog refers colloquially to a segment of a common rainbow
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