Ever seen a rainbow-colored patch of light on the side of the Sun? This is called a “sundog” (or “sun dog”). Discover this optical phenomenon, how sundogs form, and even how they predict the weather!
What Are Sundogs?
A sundog is similar to a rainbow, and more common than rainbows. Sometimes they look like bright rainbows on either side of the Sun. Other times they are brighter and actually look like two extra Suns.
Sundogs are also known as mock suns or parhelia, which means “with the Sun”. The most common name, however, for these bright lights that faithfully follow the Sun is “sundogs.”
Both rainbows and sundogs are formed by moisture filtering the sunlight. Rainbows form when drops of rain act as prisms, breaking sunlight into a multitude of colors. Sundogs appear when sunlight hits clouds of ice crystals and the ice acts as prisms.
A sundog is seen about 22° to the left or right of the Sun. Sundogs often form in pairs on either side of the Sun. Often they appear white but sometimes they are quite colorful, looking like patches of rainbow. The colors usually go from red closest to the Sun, out to blue on the outside of the sundog.
Like rainbows, sundogs are created when sunlight is filtered by moisture in the sky. Used by permission HyperPhysics, C.R. Nave Georgia State University.
What are Sun Halos?
You may have heard of a related phenomenon called a “sun halo.” In this case, it’s an entire circle of light 22° wide around the Sun. Similar to sundogs, sunlight is refracts through ice crystals; these hexagonal ice crystals are suspended in cirrostratus clouds.
Viewing a Sundog
There are some differences between rainbows and sundogs, however.
- You see rainbows when you look away from the Sun.
- You see sundogs when you look toward the Sun. If the ice crystals are falling flat, then you see a bright point of light on either side of the Sun.
Look for sundogs when the Sun is rising or setting, so near the horizon. (Sun halos can appear anywhere, even when the Sun is high in the sky.)
Sundogs and Weather Prediction
Probably the biggest difference between the two is that a rainbow usually signals an end to the rain, while a sundog often means that rain, or snow is on the way. Next time you see a sundog, look out for wet weather!
Good luck? In medieval times, the three bright lights were sometimes interpreted as the sign of the trinity, a sign of great fortune. Nowadays, they are a sign that you were lucky to be looking at the sky at just the right time. You get to see those faithful companions of our sun—sundogs.