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Graphic: Symbol MemoPad.


120° Parhelia

120° parhelia are relatively rare. If the normal parhelia are strong, chances are good that there will also be 120° parhelia and a parhelic circle. All these halos are formed by plate crystals. 120° parhelia occur as roughly circular patches at an angular distance of 120° from the Sun.

They are white because the angle of incidence at which rays enter a crystal face is equal to that of the leaving ray. The dispersion produced when the ray leaves the crystal after internal reflections divert it through 120° is exactly opposite to that produced when the ray enters. There is therefore no colour.

Right hand side: 120° parhelion on April 4th, 2006. 

: parhelic circle with parhelia, 120° parhelia and a circumzenithal arc. Simulation with HaloSim by © Les Cowley and Mike Schroeder shown with permission.

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