A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon (also called a moon ring or winter halo). It forms as sunlight 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 arms length.
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. As no light is refracted at angles smaller than 22° the sky is darker inside the halo. A 22° halo may be visible on as many as 100 days per year.
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 clouds that often come a few days before a large storm front.
- Pretor-Pinney, Gavin (2011). The Cloud Collector's Handbook. San Francisco, California: Chronicle Books. p. 120. ISBN 978-0-8118-7542-4.
- Carl R. Nave. "The 22° Halo". Georgia State University. Archived from the original on 1 April 2007. Retrieved 2007-04-18.
- 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.
- Les Cowley. "22° Circular halo". Atmospheric Optics. Retrieved 2007-04-15.
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