Morse Code of Weather: Explaining sun dogs and other atmospheric optical phenomena

BISMARCK, N.D. (KFYR) - Sun dogs are always a bright spot on a bitterly cold winter day. But how do they actually form?

Let’s first remember the difference between refracted light and reflected light.

Refraction is the change in direction of light due to a change in its speed. This is most commonly observed when light passes from one medium to another at any angle other than 90° or 0°. A great example of refraction is when you stick a pencil in a glass of water, the pencil appears to be bent due to the refraction of light within the water. So, when light is refracted inside an ice crystal or water droplet in the air, it is broken into its component colors (like a prism). This creates rainbows with water droplets and sun dogs and halos in the winter with ice crystals in the atmosphere.

On the other hand, reflection occurs when the light encounters a surface or object that does not absorb the energy of the light and bounces the light away from the surface. The light is not separated into its component colors because it is not being “bent” (or refracted).

So what is required for sun dogs to form? On a bitterly cold winter day, ice crystals can be suspended in the air (sometimes from blowing snow, other times just due to water vapor molecules in the atmosphere freezing into tiny ice crystals). On clear days with an Arctic area of high pressure overhead, these ice crystals can be slowly moving vertically down in the atmosphere thanks to the gradual sinking motion of air under regions of high pressure. Having a thin veil of cirrus clouds in the atmosphere can also cause sun dogs to be present since cirrus clouds are made of tiny ice crystals.

Some of these ice crystals are flat, hexagonal plates that can refract the sunlight when the sun is lower on the horizon. The reason sun dogs are more common at sunrise or sunset is because this is when the sun is lower, allowing for the sunlight to pass through more of Earth’s atmosphere and its light to be refracted by the ice crystals in the atmosphere.

The hexagonal ice crystals floating around in the atmosphere act like prisms, refracting the sunlight and breaking it up into its component colors. When the sunlight is refracted, two bright spots occur at 22° on either side of the sun and we call them sun dogs. A halo, or a ring of brighter light around the sun, can also form at this 22° angle in a similar way due to the refraction of sunlight within ice crystals.

Here is an example of what the hexagonal ice crystals can look like when they’re floating around in the atmosphere:

The shape of these hexagonal ice crystals and our perspective of observing the sun dogs and halos is why the 22° angle comes into play in determining where the bright spots (or sun dogs) are on either side of the sun.

A lot of different and cool atmospheric optical phenomena can occur with ice crystals in the atmosphere, as shown in the image below. The orientation and exact shape and size of the hexagonal ice crystals in the atmosphere dictate how each of the optical phenomena forms (for example, randomly oriented ice crystals will create halos while horizontally oriented ice crystals will produce sun dogs).

Notice in the image below that we have a halo, a faint upper tangent arc, a parhelic circle, and sun dogs!

And as a part of the sun dogs, a rainbow of colors can be seen going from red closest to the sun out to violet farther away from the sun (as shown in the image above). This is because as the sunlight passes through the ice crystals and is refracted, it is broken up into its component colors. The red light is refracted, or bent, the least (due to its longer wavelength) causing it to appear closer to the sun, while the violet light is refracted the most (due to its shorter wavelength) causing it to appear farther from the sun.

If you have a very keen eye, you’ll notice that the order of these colors is revered from what we typically see with a rainbow. Primary rainbows are red on the outside and violet on the inside, while sundogs are red on the side nearest the sun, with colors transitioning to violet as you travel away from the sun. In a double rainbow (as seen below), the secondary rainbow has this same order of colors as sun dogs do from red on the inside to violet on the outside.

The reasoning behind this order of colors is due to the number of times the light is being refracted and reflected.

With sun dogs, the light is refracted twice: once when it enters the ice crystal and then again when it exits the ice crystal (as shown on the left side of the image below).

With primary rainbows, light is also refracted twice (once when it enters the raindrop and then again when it exits the raindrop). But the light is also reflected within the raindrop, flipping the order of colors so that red is on the outside and violet on the inside of the rainbow from the perspective of the observer.

With secondary rainbows, light is also refracted twice (once when entering the raindrop and then again when it exits the raindrop) and first reflected once. But the light does not escape the raindrop after the first reflection, and a second reflection within the raindrop flips the colors again resulting in the colors going from violet on the outside and red on the inside of the rainbow.

Additionally, when you look at a rainbow, the sun is behind you (this is why the light needs to be reflected within the water droplet to reverse its direction) versus when you look at a sun dog, you are looking directly towards the sun.

We even saw moon dogs this winter! The same principles hold true for moonlight as they do for sunlight, you just need a full enough moon for there to be enough light to be refracted by the ice crystals in the atmosphere at night.

Sun pillars form in a similar way to sun dogs and halos, requiring ice crystals to be slowly falling through the air or a thin veil of cirrus clouds in the atmosphere, however in this case the sunlight reflects off the surfaces of the ice crystals (instead of the refraction process that occurs for sun dogs and halos). Sun pillars are seen only at sunrise or sunset when the sun is really low on the horizon, and the ice crystals reflect some of the sunlight vertically creating an enhanced area of brightness above the sun.

A couple more fun facts about sun dogs and their name origin:

• The scientific name for them is parhelion, which comes from Ancient Greek meaning “beside the sun” or “with the sun.” Sun dogs are also sometimes called “mock suns.”
• Also relating to Greek mythology, it was believed that Zeus walked his dogs across the sky and that the bright “mock suns” on either side of the actual sun were his dogs.

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