What are sundogs?

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, like an ice crystal, several things can happen. Some rays are turned back in the direction from which they came, the familiar process of reflection. Other rays are transmitted into the crystal. Some of the transmitted rays change direction, a process known as refraction.

Sundogs appear because ice crystals in the shape of 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 causes blue light to be bent more than red light, and so sundogs can show a spectrum of colors with red nearest the Sun.

Sundogs are usually 22 degrees away from the sun, or about a hand width from the center of the Sun when your arm is fully extended. Sundogs are often accompanied by a halo around the sun. A halo is a white ring that encircles but does not touch the sun. It is an optical phenomenon that also owes its existence to refraction of light 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 high, thin cirrostratus clouds.

Category: Phenomena
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