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 delighted some River Falls residents this winter.
They have sent many university students clicking photos on their phones and bringing them to 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, who specializes in optics, or “the science of lights,” as she puts it, said she was inundated with photos from students.
Korenic, also UWRF’s planetarium director, said sundogs 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.
Sun dogs can form at any time of the year, as long as the upper atmosphere is cold enough and has enough water vapor in it to form ice crystals.
Because it has been extremely cold for long stretches this winter, Korenic said the ice crystals are able to last longer in the atmosphere. This means the sun dogs have been lasting further into the day than is typical.
Sun dogs are typically more easily seen around sunup.
“But how many people are up then to see that?” Korenic said. “Now they’re lasting until 8:30-9 in the morning, more of us are up and observing.”
Korenic said she thinks sun dogs are a nice compensation for the bitter cold.
“I think they’re really beautiful, and they’re also really surprising,” Korenic said. “Because your first thought is that the light is someplace up there close to the sun, but in fact, it’s actually quite close to you.”