A 22° halo is a halo, one type of optical phenomenon, forming a circle with a radius of approximately 22° around the Sun, or occasionally the Moon (also called a moon ring or winter halo). It forms as the sun- or moonlight is refracted in millions of randomly oriented hexagonal ice crystals suspended in the atmosphere. The halo is large; the radius is roughly the size of an outstretched hand at arm's length. A 22° halo may be visible on as many as 100 days per year—much more frequently than rainbows.
This description ignores that the crystal may have a different geometry other than hexagonal. See Walter Tape's book Atmospheric Halos for situations where various geometries result in a 22 degree halo.
As light passes through the 60° apex angle of the hexagonal ice prisms it is deflected twice resulting in deviation angles ranging from 22° to 50°. The angle of minimum deviation is almost 22° (or more specifically 21.84° on average; 21.54° for red light and 22.37° for blue light). This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish.
The ice crystals in the clouds all deviate the light similarly, but only the ones from the specific ring at 22 degrees contribute to the effect for an observer at a set distance. As no light is refracted at angles smaller than 22°, the sky is darker inside the halo.
Another phenomenon resulting in a "ring around the Sun/Moon"—and therefore sometimes confused with the 22° halo—is the corona. Unlike the 22° halo, however, it is produced by water droplets instead of ice crystals and it is much smaller and more colorful.
In folklore, moon rings are said to warn of approaching storms. Like other ice halos, 22° halos appear when the sky is covered by thin cirrus or cirrostratus clouds that often come a few days before a large storm front. However, the same clouds can also occur without any associated weather change, making a 22° halo unreliable as a sign of bad weather.
- Les Cowley. "22° Circular halo". Atmospheric Optics. Retrieved 2007-04-15.
- Pretor-Pinney, Gavin (2011). The Cloud Collector's Handbook. San Francisco, California: Chronicle Books. p. 120. ISBN 978-0-8118-7542-4.
- Les Cowley. "22° Halo Formation". Atmospheric Optics. Retrieved 2007-04-15. (Including excellent illustrations and animations.)
- Harrison, Wayne (February 1, 2012). "Nelson: Ring Around Moon Sign Of Approaching Storm". The Denver Channel (Denver). TheDenverChannel.com. Retrieved February 4, 2012.
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