"Sun Dogs" appear on frigid winter days when the sun appears to have two smaller companions on either side.
These dogs appear with a rainbow of colors as ice crystals in the sky refract sunlight like a prism.
What's in a name
The scientific name for this optical phenomenon is parhelion (singular) and parhelia (plural). Parhelion is derived from the Ancient Greek word for "beside the sun".
The nickname, sun dog, may have originated from Norse mythology and the Danish word "solhunde."
Science of halos and sun dogs
Sun dogs branch off from a more general term called sun halos.
Ice crystals will refract incoming sunlight and form an illuminated circle, or halo, around the sun. These can appear high in the sky when ice-filled cirrus clouds are present.
When there are enough suspended ice crystals in the lower atmosphere, these halos can form near the horizon and dusk and dawn. During these low-horizon halos is when sun dogs are possible.
Among the thousands of tiny ice crystals in the sky, the flat hexagonal-shaped crystals (plate shape) will orient themselves horizontally as they fall. Picture dropping a piece of paper. It will float to the ground, favoring the horizontal position.
With the majority of plate-shaped crystals oriented in the same direction, this creates a higher concentration of light at the horizontal points of a halo. And the dogs appear!
Depending on your location and atmospheric conditions, one or both dogs can be seen. The color pattern is different from a rainbow, with red on the inside and blue/violet on the outside.
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Another cold-weather atmospheric light display is light pillars. Those same plate-shaped crystals create columns of light from the ground up at night.