1. 22° halo

    A 22° halo is an optical phenomenon whereby a large ring or halo is formed 22 degrees around the sun. Light is refracted in hexagonal ice crystals in the cirrus clouds. Red light is refracted 21.54° and blue light 22.37°. This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish. The centre of the ring is often darker because no light is refracted beyond the red line.
    22 degree halos are often thought to be a warning of approaching storms because the cirrus clouds in which they form are often present a few days before a weather change.

    Image: 22° halo around the sun in southern Victoria, credit Alex Jaeger

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