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Sundogs and Rainbows
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One question we are asked frequently here at The Weather Notebook is what are sundogs. Hi, I'm Dave Thurlow.

Sundogs are something you may have never even heard of, let alone actually seen. They look like tiny section of rainbows which, on the other hand, are seen often, if not in real life then at least in pictures and paintings. The irony of this is that sundogs are more common than rainbows because they can occur any time of year, whereas rainbows are limited pretty much to the warmer seasons. The scientific difference is that rainbows are formed by sunlight striking liquid raindrops and sundogs come from sunlight striking clouds made of ice crystals which form in winter and in summer way up in the atmosphere.

When visible, sundogs are always seen horizontally just to the right or left of the sun. They look like a shiny, iridescent patch of cloud, about the size of the sun. In fact they're sometimes called mock suns because the real sun may be hidden behind a cloud. When sunlight, which is made up of all the colors of the spectrum, hits the ice crystals in the cloud, it bends a little. The ice crystal acts as a prism, separating the sunlight into different colors and forming a sundog. Rainbows are basically the same type of thing, except that in rainbows it is raindrops that serve as prisms instead of ice crystals.

Another difference between the two, rainbows are seen as you face away from the sun as opposed to sundogs that are seen only when facing toward the sun. And probably the biggest difference between the two is that a rainbow usually signals an end to the rain, while a sundog often means that rain, or snow is on the way.

Our show is underwritten by Subaru with major funding by the National Science Foundation.