Sundogs: Rainbows Beside the Sun

How Weather Creates the Illusion of Multiple Suns

High dynamic range photo of sundogs and a solar halo around the Sun.
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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, for example, just after sunrise or just prior to sunset. Sometimes a pair of sundogs will appear -- one on the Sun's left, and another on the Sun's right.

When these same bright spots occur at night around the moon, they are known as moondogs. Moondogs typically only occur when the bright light of a full moon or nearly-full moon is available. 

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. In fact, because sundogs appear as bright yet smaller mini suns in the sky, they are sometimes called "mock" or "phantom" suns. Their scientific name is "parhelia" ("parhelion" for one).

A Part of the Halo Family

Sundogs form as sunlight is bent (refracted) by ice crystals suspended in the atmosphere. This makes them related to atmospheric halos -- white and colored rings in the sky that form by this 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 (have six sides) -- known as plates -- can create halos. If the majority of these plate-shaped ice crystals are positioned with their flat sides horizontal to you, the observer, you will see a sundog. (If the crystals are positioned at a mix of angles, your eyes will see a circular halo without the "dogs.")  

Sundog Formation

Sundogs can and do occur worldwide and during all seasons, but they are most common during winter months when the sun is low in the sky and ice crystals are more common. All that's needed for a sun dog to form are cirrus or cirrostratus clouds. Only these clouds are cold enough to be made of the plate-shaped ice crystals we mentioned above. The sun dog occurs when sunlight is refracted off of these crystals per the following process:

As the plate ice crystals drift in the air, they wobble back and forth slightly with their flat sides horizontal to the air (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 upon exiting the crystal's other side at a 22-degree angle downward toward your eyes. (This is why sundogs always appear at 22° angles from the sun.)

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

Photography Tip: When photographing sundogs, it is best to use a wide-angle lens. Otherwise, you won't be able to capture the sun, pair of sundogs, and 22° halo ring that co-occurs with them.

A sundog's size depends on how much the plate-shaped ice crystals wobble as they float. Larger plates wobble more and thus produce larger sundogs.

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 opposite. Primary rainbows are red on the outside and violet on the inside. Sundogs are red on the side nearest the Sun, with colors grading through orange to blue as you travel away from it. If you recall, a double rainbow's secondary bow colors are arranged in this same way (red inside, violet outside).

Sundogs are like secondary bows 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 colors. 

A Sign of Foul Weather

Like their halo cousins, sun dogs are indicative of foul weather. Since the clouds that cause them (cirrus and cirrostratus) can signify an approaching weather system, sun dogs themselves often signal that rain will fall within the next 24 hours.