The Polar Vortex Is Causing Striking Solar Phenomena Called ‘Sundogs.’ Here’s What to Know

The polar vortex that has brought deadly cold and record-breaking low temperatures to the Midwest has also resulted in beautiful solar phenomena – sundogs.

According to the National Weather Service, sundogs form when sunlight refracts off ice crystals in the atmosphere and result in colored spots of light approximately 22 degrees either left, right, or both, from the sun.

The origin of the name “sundog” is not entirely clear, but according to Chicago meteorologist Tom Skilling, the name originates from the Greek myth that Zeus walked his dogs across the sky and that the bright “false suns” in the sky on either side of the sun were Zeus’ dogs.

According to the NWS, sundogs are also known as mock suns or parhelia, which means “with the sun.”

This weather phenomenon generally appears in only extreme cold temperatures needed to form ice crystals, Sioux Falls National Weather Service meteorologist Peter Roger tells TIME. That’s why they’ve been increasingly common in the subzero chill of the polar vortex.

Many Midwesterners, and those elsewhere, have noticed the sundogs that have appeared in this week’s cold and shared photos on social media.

Here is what to know about sundogs:

What causes sundogs to form?

In order for sundogs to form two factors are necessary: ice crystals and the sun being low in the sky.

Ice crystals form when the atmosphere is cold enough, so instead of having little water droplets in the sky, which help form clouds, you actually have ice crystals, Rogers says.

He says when sunlight hits the crystals, it gets bent in a way that refracts the light into what we see as a sundog.

Are sundogs rare?

While you probably won’t see a sundog every day, the phenomenon is not exactly rare. According to Rogers, it’s just a matter of the sun being in the correct orientation with relation to ice crystals in the air.

Rogers says sundogs are to be expected every winter, especially in more northern latitudes, like the Dakotas.

“It’s kind of like a rainbow — you need the exact right conditions for it to show up,” he says.

When is the best time to see a sundog?

Sundogs are most likely to appear when the sun is lowest in the sky, meaning around sunrise or sunset. But the most important factor is that ice crystals are in the correct orientation to the light that is coming in.

“It’s most often when the sun is closest to the ground,” Rogers says. “So whether that be when the sun is coming up in the morning or setting in the evening. As the sun gets higher in the sky [sundogs] are less likely to be seen because of the angle between the sunlight and those ice crystals, so that’s why it’s normally when the sun is close to the ground.”