22° halo
A 22° halo is a halo, one type of optical phenomenon, forming a circle with a radius of approximately 22° around the Sun or Moon (in which case it is also called a moon ring or winter halo). It forms as the sun- or moonlight is refracted in millions of hexagonal ice crystals suspended in the atmosphere.[1] The halo is large; the radius is roughly the size of an outstretched hand at arm's length.[2] A 22° halo may be visible on as many as 100 days per year—much more frequently than rainbows.[3]
Formation
Even though it is one of the most common types of halo, the exact shape and orientation of the ice crystals responsible for the 22° halo are still the topic of debate. Hexagonal, randomly oriented columns are usually put forward as the most likely candidate, but this explanation posits problems, such as the fact that the aerodynamic properties of such crystals leads them to be orientated horizontally rather than randomly. Alternative explanations include the involvement of clusters of bullet-shaped ice columns.[4][5]
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.[6]
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.[2]
Weather relation
In folklore, moon rings are said to warn of approaching storms[citation needed]. 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.[7] However, the same clouds can also occur without any associated weather change, making a 22° halo unreliable as a sign of bad weather.
Gallery
-
22° solar halo and parhelion (sun dog) in Salem, Massachusetts, Oct 27, 2012. Parry arc and upper tangent arc are also visible.
-
Full-circle solar halo with parhelia and lower tangent arc, South Pole, 12 Jan 2009.jpg
22° solar halo with parhelia and rare lower tangent arc at South Pole, 12 January 2009.
See also
References
 |
Wikimedia Commons has media related to Lua error in package.lua at line 80: module 'strict' not found.. |
- ↑ http://www.atoptics.co.uk/halo/circ2.htm
- ↑ 2.0 2.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found. (Including excellent illustrations and animations.)
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
