Weather 101: Sundogs

Weather 101

Learn About This Interesting Optical Phenomenon!

What are They and do They Have Anything to do with Actual Canines?

No sundogs do not have any ties to puppy dogs. They are tied to ice crystals though. The official scientific name for this phenomenon is parhelia. They also have the nickname of “mock suns”. Sundogs are typically associated with the 22 degree halo. This halo, out of all the ice halos that occur is the most frequent. In the space of a year, these 22 degree halos are seen on average about 100 of those days.

Dual Sundogs Over Northwest Arkansas Credit: Scott Roberts

Formation

These halos are created by the presence of ice crystals. In the case of sundogs, the ice crystals are shaped like hexagonal plates. These types of ice crystals are most commonly found in cirrus clouds although in colder climates, they can appear in diamond dust closer to the ground.

Diagram of Hexagonal Ice Crystals

In most cases the plate crystals will drift and float downward within the clouds. As they fall, the hexagonal faces will sometimes become horizontally tilted.

The light rays then enter into the side face of the ice crystal and refract (bend) as they enter. As the rays travel through the crystal, they begin to separate and exit at around a face at a 60 degree angle from the original entrance point. As the rays exit, they bend once again further separating the colors.

By the time the rays completely exit the crystal, they have been deviated by at least 22 degrees from the double refraction.

Sundog Formation Credit: atoptics.co.uk

Visual Appearance

Sundog Close Up. Credit: Mitsy Marx

Although all sundogs have slightly different color hues, they generally follow the ROY-G-BIV color spectrum. Red light tends to be deviated the least amount in refraction creating the inner (towards the sun) red glow of the sundogs. You can always tell where the sun is located by simply looking at the color distribution. The red colors are going to point towards the sun everytime. In the sundog pictured above, the sun is located to the right of the photo.

Sundogs can range from being blindingly bright to just a mere colored smudge in the sky. They are brightest when the sun angle is low such as during the winter months. Solar angles above 40 degrees produce very faint sun dogs that are difficult to see. A true 22 degree sundog is only possible at sunset or sunrise.

Of course the atmospheric flow in reality is chaotic, and the plate crystals rarely float perfectly horizontal. They wobble as they tumble through the air. The more they wobble, the taller the sundog tends to become. This happens especially with the larger plate crystals.

Sundogs are visible all across the globe and at any time of the year. In North America and Europe they can be seen at least twice a week if searched for.

If you happen to catch a sundog while outside, snap a horizontal photo of it and email it to us at weather@knwa.com. Be sure to check the rest of our digital weather content (Weather 101 and Weather Word of the Week) under the weather tab on our website!

Copyright 2021 Nexstar Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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