Halo

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Halo, phenomenon of light refraction caused by ice crystals in the atmosphere between the observer and the sun or moon. The commonest form of halo is a circle of colored light surrounding the disk of the sun or moon. Light from the sun or moon is bent by the atmospheric ice crystals at a 22° angle toward the observer. Thus, the halo is a circle with a radius 22° from the center of the disk. Sometimes, a secondary halo caused by the refraction from ice crystals is seen outside the primary halo at a distance of 46° from the center of the sun or moon. Colored images resembling the disk of the sun may also be seen. Called parhelia, or sun dogs, they sometimes can be seen spaced 22° from the sun in a vertical or horizontal direction.

Halos are larger in diameter than the coronas seen around the sun or moon in hazy weather. Coronas are caused by the diffraction of light by water particles in the atmosphere. A corona is similar to a rainbow and a fogbow. Fogbows occur when sunlight strikes a fog bank (see Fog), producing a colored arc about 40° from the center of the disk of the sun. See Meteorology; Optics.

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