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 with a radius of approximately 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 arm's length.[1] A 22° halo may be visible on as many as 100 days per year—much more frequently than rainbows.[1]

Formation[edit]

Pathway of light through a hexagonal prism in the optimal angle resulting in minimum deviation.
Path of the light from the clouds to the observer.

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]

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.[3]

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.[4]

Weather relation[edit]

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.[5] However, the same clouds can also occur without any associated weather change, making a 22° halo unreliable as a sign of bad weather ahead.[6]

Gallery[edit]

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. ^ Les Cowley. "22° Circular halo". Atmospheric Optics. Retrieved 2007-04-15. 
  5. ^ Harrison, Wayne (February 1, 2012). "Nelson: Ring Around Moon Sign Of Approaching Storm". The Denver Channel (Denver). TheDenverChannel.com. Retrieved February 4, 2012. 
  6. ^ http://www.atoptics.co.uk/halo/circular.htm

External links[edit]