22° halo

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A 22 degree halo around the moon in Atherton, CA.

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.[1]

Pathway of light through a hexagonal prism in the optimal angle resulting in minimum deviation.

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.[2] As no light is refracted at angles smaller than 22° the sky is darker inside the halo.[3] A 22° halo may be visible on as many as 100 days per year—much more frequently than rainbows.[1]

22° solar halo and parhelion (sun dog) in Salem, Massachusetts, Oct 27, 2012. Parry arc and Upper tangent arc are also visible.

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.[4] However, the same clouds can also occur without any associated weather change, making a 22° halo unreliable as a sign of bad weather ahead.[5]

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.[6]

See also[edit]

References[edit]

  1. ^ a b Pretor-Pinney, Gavin (2011). The Cloud Collector's Handbook. San Francisco, California: Chronicle Books. p. 120. ISBN 978-0-8118-7542-4. 
  2. ^ Carl R. Nave. "The 22° Halo". Georgia State University. Archived from the original on 1 April 2007. Retrieved 2007-04-18. 
  3. ^ Les Cowley. "22° Halo Formation". Atmospheric Optics. Retrieved 2007-04-15.  (Including excellent illustrations and animations.)
  4. ^ Harrison, Wayne (February 1, 2012). "Nelson: Ring Around Moon Sign Of Approaching Storm". The Denver Channel (Denver). TheDenverChannel.com. Retrieved February 4, 2012. 
  5. ^ http://www.atoptics.co.uk/halo/circular.htm
  6. ^ Les Cowley. "22° Circular halo". Atmospheric Optics. Retrieved 2007-04-15. 

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