What are sun dogs?

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What are sun dogs?

On a day with high ice clouds, you are likely to see shiny, colored regions at either side of the sun. These are sundogs, an optical effect caused by refraction and dispersion of the Sun’s light through ice crystals. When the light rays strike the boundary between the air and water, as at an ice crystal, several things can happen. Some rays are returned in the direction ,of their source, the familiar process of reflection.

Other rays are transmitted into the crystal and change direction, a process ,of refraction. Refraction also occurs in eyeglasses and contact lenses.

Sundogs appear because ice crystals shaped like hexagonal dinner plates tend to drift downward with their flat bases parallel to the ground. The sunlight passes through the crystal and refracts sideways. If the Sun is low enough in the sky, you see spots of bright light on one or both sides of the sun, depending on where the clouds are. Refraction bends blue light more than red light, and so sundogs can show a spectrum of colors. Red will be nearest the Sun.

Sundogs are usually 22 degrees away from the sun, or about a hand width from the center of the Sun at arm’s length. Sundogs are often accompanied by a halo around the sun. A halo — a white ring that encircles the sun — is an optical phenomenon that also owes its existence to refraction by ice crystals. Because the light must shine through a fairly uniform layer of ice crystals that are thin enough to let light through, halos are usually associated with thin, cirrostratus clouds.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.