22 Degree Halo

March 28, 2002

Halooleary

Provided by: Mike O'Leary
Summary authors & editors: Jim Foster

The photo above is a fine example of a 22 degree halo (the ring of colored light is 22 degrees from the Sun). Halos occur more often than rainbows, but many people fail to notice them since, unlike rainbows, in order to see them you have to look toward the Sun. Halos can occur if hexagonal plate or column crystals in cirrus-type clouds are randomly oriented. Sunlight enters one side of a crystal, is refracted, and then exits the opposite side, where it is again refracted by the same amount and in the same direction as the original refraction. Notice that the inner edge of the halo on the photo is somewhat more distinct than the outer edge, and whereas the inner edge has a reddish hue, the outer rim is tinged with blue.

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