Water drops and in the atmosphere can act as a prism, splitting visible light into the colors that make it up. That is what produces , sundogs and sun pillars. A sundog looks like an accident of the clouds, yet it turns up at a fixed distance from the sun every time.
Geometry decides it. Sundogs are colored spots of light produced by through ice crystals. They sit approximately 22 degrees to the left, to the right, or on both sides of the sun, depending on where the crystals are. The colors run red nearest the sun and blue on the outside.
The atmosphere does more than produce daily weather. Under the right conditions, water drops and ice crystals can act as a prism, breaking visible light apart into its , which is where halos, sundogs and sun pillars come from.
The distinction that organizes all of them is refraction against reflection. Refraction is a change in direction caused by a change in speed, and it splits light into colors; reflection bounces all wavelengths away at equal angles without bending them, so no colors separate.
A halo is the refracting case at its simplest. It is a ring of light around the sun or moon, formed as light refracts off ice crystals in a thin veil of cirrus cloud. It usually appears as a bright white ring, and sometimes carries color.
Sundogs are the same physics with a fixed address. They develop through refraction in ice crystals and appear approximately 22 degrees to the left, the right, or both sides of the sun. The colors run from red closest to the sun out to blue on the outside.
wat. Hold your arm out: a sundog sits about two hand-widths from the sun.
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A repeatable angle is the signature of a mechanism rather than a coincidence.
How do we know?
The National Weather Service office in La Crosse explains the optics, separating refraction from reflection and setting out what each effect needs.
What is still unknown
An educational page for the public, which describes the effects without deriving the angle.
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Where to next
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