What are sun dogs and what causes them to form?

This is a sundog: a spot of rainbow-colored light that forms on one or both sides of the sun.

This atmospheric optic is formed when light from the sun passes through ice crystals shaped like hexagonal plates suspended horizontally in the atmosphere. The light hits the crystal and is refracted at a 60-degree angle.

The light is refracted as it passes through the ice crystals, splitting up the beam of light into its component colors - the colors we associate with the rainbow. Red light is refracted the least of the colors while blue light at the other end of the spectrum is refracted the most, causing the inner edges of the sun dog to look red.

These resulting refractions are what form the bright spots of color at an approximately 22-degree angle on either side of the sun.

Sundogs are also called “mock suns” or parhelia, meaning “with the sun”.