Sundogs spotted in Waterloo Region: an explainer

Waterloo Region residents were treated to an uncommon sight on Friday morning — sundogs, or what residents variously called "winter rainbows," "frostbows," or halos with bright spots that surround the sun.

The following is an explanation of what causes this curious phenomenon.

1. What are sundogs?

Sundogs occur when sunlight is refracted through hexagonal ice crystals that occur in cirrus clouds. When ice crystals in the Earth's atmosphere are arranged in nearly horizontal positions, light is refracted in so we can see two bands or spots of light on either side of the sun, according to Les Cowley, an expert on atmospheric optics. That phenomenon is commonly known as a sundog. Blue light is refracted more strongly than red light, which means the inner part of the sundog closer to the sun appears red. This effect makes the sundogs look like rainbows. While the halo can appear all around the sun, the sundogs themselves usually appear strongest at around 22 degrees to the left and right of the sun.

2. Do sundogs have a scientific name? 

The scientific name for the spot on the halo is parhelion - or parhelia pluralized. Sundogs are also called "mock suns."

3. When do they happen?

Sundogs can happen at any time of year. However, they're more likely to be seen in the winter because of the low angle of the sun in the sky. This is when sun dogs are brightest.

4. Where am I most likely to see sundogs?

They can happen anywhere in the world. There is a report of sundog occurring in July in Hong Kong for example.

5. Can you see these halos around the moon?

Yes, though they're rarer to spot than sundogs. Moondogs are also called paraselene. The plural is paraselenae. The best time to see moondogs is on a night with a full or nearly full moon, when the light being reflected off the moon is brightest, according to Cowley.