Photographs of spectacular rainbows are a dime a dozen, but a similar event seldom witnessed is the occurrence of sun dogs. While rainbows are caused by beams of sunlight refracting through water droplets in the atmosphere, sun dogs, also called mock suns, are created by sunlight refracting through ice crystals high in the sky. However, this is where the similarity to rainbows ends. These bright patches of light, known to scientists as parhelia, are a far less familiar phenomenon, appearing only at particular times and limited by atmospheric requirements.

A distinct combination of optical and meteorological conditions is required in order for a sun dog to form. For one, there must be tiny six-sided ice crystals suspended in the air, usually in thin, high cirrus clouds. Moreover, these flat crystals must drift downward with their faces roughly level to the ground so that they bend the light at a precise angle. Finally, the sun must be low on the horizon. Under these conditions, a viewer may see one or two glowing patches of light positioned about 22 degrees to the left and right of the sun. The lower the sun sits, the more clearly the pair of mock suns stands out against the sky.