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Forum News Service, Published February 09 2014

Bitter cold brings shining sun dogs

RIVER FALLS, Wis. – The sun wasn’t alone when it rose on a recent morning. It was accompanied by two bright patches of light, one on each side, and a faint, glowing halo of light.

These “sun dogs” – so called because they faithfully “follow” the sun like well-trained dogs – have sent many college students clicking photos on their phones and bringing them to University of Wisconsin River Falls’ physics professor Eileen Korenic.

“There were a lot of people who came in with pictures on their phones and wanted to know what causes that,” Korenic said.

Korenic said sun dogs form much the same way as rainbows, which are created by raindrops acting as prisms.

A prism is something that can take light and separate it into the different colors of the light spectrum.

In cold weather, water droplets freeze and form snowflakes. But if it’s cold enough, Korenic said, instead of just snowflakes, those water droplets freeze into hexagonal ice crystals.

“Now the crystals can behave just like prisms do,” Korenic said. “So when light passes through those hexagonal crystals, the light gets spread out into its different colors.”

The separation of colors is not as pronounced as it is with a rainbow, Korenic said. But the light closest to the sun is red, and the light farthest from the sun is blue.