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Halo

Carlo Forte (March)



This is a good example of a 22 degree halo.
This is a rather frequently appearing optical phenomenon forming a circle 22 degrees around the sun, or occasionally the Moon. It forms as sunlight is refracted in hexagonal ice crystals suspended high in the atmosphere. Normally when sunlight strikes ice crystals most of the light is reflected and forms a completely white halo.
However, if incoming light strikes the crystals at a certain angle some light will be refracted. In such cases the halo will be faintly colored with red appearing nearest the center and blue on the outside. As the light beam passes through two sides of the prism forming a 60 degree angle, the angle of minimum deviation is almost 22 degrees.

In folklore haloes have long been associated with approaching rain and there is some truth to this. The cirrus cloud that can produce a halo may well indicate an approaching frontal system.






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