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What’s a sun dog? A Nobel Laureate explains the science behind the cold snap light show


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One effect of the record-breaking cold snap in Western Canada, which is set to continue through this week, is a remarkable series of photographs on social media of “sun dogs,” an atmospheric optical illusion in which two new little suns appear to the left and right of the real sun, connected overhead by a bright halo, often with rainbow effects.

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From North Dakota to Vancouver, people have been posting photos of these “mock suns” as the ancient Greek philosopher Aristotle called the phenomenon when he first observed it in the fourth century BCE. They have been piquing people’s interest ever since.

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For insight into why cold air bends light, the National Post spoke with Donna Strickland, a Canadian optical physicist who won the 2018 Nobel Prize in physics, the third woman ever to do so after Marie Curie, who discovered radioactivity, and Maria Goeppert Mayer, who described the structure of the atomic nucleus.

“It’s the opposite of a mirage on a hot day,” said Strickland, whose primary research at the University of Waterloo is on lasers, a kind of amplified light.

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When you see an imaginary puddle up ahead on the highway, she said, that is because hot air warmed by the road surface is bending light close to the ground. It is a loosely structured optical effect, less visually striking than a solar halo on a winter’s day, but it reflects the same physics, and the same behaviour of light.

Light travels at a constant speed in a vacuum, but when it passes through a medium, such as air or water or ice, it can be slowed down to varying degrees. This bends it.

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Denser air slows it down more than less dense air, so for example when superheated air overtop highway asphalt meets the cooler, denser air above, it creates a rough boundary of densities.

“Anytime there’s that boundary, there’s bending,” Strickland said.

Light also bends in other ways. It can be bent by gravity, for example, but only over vast cosmological distances as it travels through the vacuum of space at its constant speed of approximately 300,000 kilometres per second.

You know it’s cold when the Sun dogs are out. So happy my hubs drove me to do my cake delivery today! What a spectacular sight! #ittybittykitchy #staywarm #fiercewinter #sundog #sundogs #yyc #calgary #cold

Posted by Itty Bitty Kitchy on Monday, December 27, 2021

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Closer to home in Earth’s atmosphere, it is a different story. Light bends as it passes through different media because it is slowed to different degrees. It is the density that makes it travel slower, not the temperature. Physically, this is also why a straight stick looks bent when part of it is underwater, because water is more dense than air.

Strickland has an exercise she likes to do with elementary school classes she visits to illustrate this effect. Imagine a line of eight kids standing horizontally on the squares of a chessboard, holding a skipping rope taut. This rope is the crest of a wave of light, and each kid is photon, advancing together in straight lines. Imagine a diagonal line across the chessboard marking the boundary between air and water. The kids know that in air, they can travel one step at a time, but in water only half a step each time. So as they advance, the kid at the end reaches the water boundary first, and from then on slows down. The next kid in goes a step farther, but slows down also after crossing the boundary.

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Then Strickland can stand back and tell the kids to behold the skipping rope.

“It’s still a line, only the line now is bent,” she said. Kids tend to grasp it that way, she said.

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Light interacts with all media it comes in contact with, and can sometimes be thought of not as passing through, but of being absorbed and re-emitted. Strickland made a comparison to rainbows, formed when white light is separated into constituent colours by a prism. She said it is as if blue light, with its higher energy and shorter wavelength, has a similar energy to the electrons it encounters on its journey, and therefore hangs with them around a bit, whereas red photons, with their long wavelength and lower energy, fail to interact and simply “keep on going.” Red is therefore slowed less and bends less than blue. Across the whole light spectrum, the effect is a rainbow.

At the microscopic level, in ice crystals suspended in the cold Western air for example, the same physical dynamics are at play.

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Ice crystals are hexagonal, six-sided, and their geometry means the light that passes through them is bent twice, on the way in and the way out. The effect is a refraction of at least 22 degrees, which determines the size of the halo for an observer at a given distance.

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This is the most common halo to see in cold weather. There can be wider halos, but not smaller ones, because light does not refract any less than 22 degrees through a hexagonal ice crystal.

Technically, a mock sun is called a parhelion, from the Greek for “beside the sun.” The etymology of “sun dog” is less clear, and might not refer to an actual dog at all.

The bending of light in cold air can have other curious effects. It can distort distant objects and create mirages, even with light from below the horizon. One famous example is the so-called “Croker Mountains,” which the Arctic explorer John Ross thought he saw in Lancaster Sound in 1818, but which are now thought to have been a mirage. Had he not turned back, he might have discovered Lancaster Sound is in fact an entrance to a Northwest Passage.

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