CHICAGO (CBS) — The Polar Vortex disturbance that has caused extreme weather in Chicago created a halo effect at sunrise over Lake Michigan on Thursday. It’s called a “sun dog.”

Ice crystals in the atmosphere refract sunlight, creating the halo and the appearance of smaller suns. With Thursday’s bitter temperatures, there were plenty of crystals in the air.

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The effect is most noticeable when the sun is low over the horizon.

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The temperatures this morning at 31st Street and Lake Michigan in Chicago were around minus 15. The record-setting conditions were brutal but not as bad as Wednesday morning, when minus 50 degree wind chills made it nearly impossible to be outside for more than a minute or two.