Sundogs: Rainbows Beside the Sun

A sundog (or sun dog) is a bright, rainbow-colored patch of light that occurs on either side of the sun when it is low on the horizon—just after sunrise or before sunset, for instance. Sometimes, a pair of sundogs will appear—one on the sun's left, and another on the sun's right.

Why Are Sundogs Called Sundogs?

It isn't exactly clear where the term "sundog" originated, but the fact that these optical events "sit" beside the sun—like a loyal dog attends its owner—likely has something to do with it. Because sundogs appear as bright-yet-miniature suns in the sky, they are also sometimes called "mock" or "phantom" suns.

Their scientific name is "parhelion" (plural: "parhelia").

Part of the Halo Family

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What Causes Rainbow-Colored Clouds in the Sky?

By Tiffany Means

Sundogs form when sunlight is refracted (bent) by ice crystals that are suspended in the atmosphere. This makes the phenomenon related to atmospheric halos, which are white and colored rings in the sky that form by the same process. 

The shape and orientation of the ice crystals through which the light passes determines the type of halo you'll see. Only ice crystals that are flat and hexagonal—known as plates—can create halos. If the majority of these plate-shaped ice crystals are positioned with their flat sides horizontal to the ground, you will see a sundog. If the crystals are positioned at a mix of angles, your eyes will see a circular halo without the distinct "dogs."

Sundog Formation

Sundogs can and do occur worldwide and during all seasons, but they are most common during winter months when ice crystals are more abundant. All that's needed for a sundog to form are either cirrus clouds or cirrostratus clouds; only these clouds are cold enough to be made of the necessary plate-shaped ice crystals. The size of the sundog will be determined by the size of the crystals.

The sundog occurs when sunlight is refracted off of these plate crystals per the following process:

• As the plate ice crystals drift in the air with their hexagonal faces horizontal to the ground, they wobble back and forth slightly, similarly to how leaves fall.
• Light hits the ice crystals and passes through their side faces.
• The ice crystals act like prisms, and as sunlight passes through them, it bends, separating into its component color wavelengths.
• Still separated into its range of colors, the light continues traveling through the crystal until it bends again—at a 22-degree angle—upon exiting the crystal's other side. This is why sundogs always appear at 22-degree angles from the sun.

Does something about this process sound vaguely familiar? If so, it's because another well-known optical weather phenomenon involves the refraction of light: the rainbow!

Sundogs and Secondary Rainbows

Sundogs may look like bite-sized rainbows, but inspect one closer and you'll notice that its color scheme is actually reversed. Primary rainbows are red on the outside and violet on the inside, while sundogs are red on the side nearest the sun, with colors grading through orange to blue as you travel away from it. In a double rainbow, the colors of the secondary bow are arranged in this same way.

Sundogs are like secondary rainbows in another way too: Their colors are fainter than those of a primary bow. How visible or whitewashed a sundog's colors are depends on how much the ice crystals wobble as they float in the air. The more wobble, the more vibrant the sundog's colors. 

A Sign of Foul Weather

Despite their beauty, sundogs are indicative of foul weather, just like their halo cousins. Since the clouds that cause them (cirrus and cirrostratus) can signify an approaching weather system, sundogs themselves often indicate that rain will fall within the next 24 hours.