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