From the
upper contact arc, two whitish arcs run that intersect at the
anthelion. When the sun is high, the arcs extend beyond the anthelion back to the
infralateral arc, creating a kind of loop.
Wegener's counter-sun arc is caused by nearly ideally
horizontally oriented column crystals. The arc becomes indistinct as soon as the crystals deviate slightly from the horizontal position. Therefore, it is very rarely seen in cirrus. In halo displays in the ice fog of snow cannons in ski resorts it occurs much more often. The light rays enter a prism face, are reflected at the base face, and exit the crystal at the next but one prism face. The same light path in Parry oriented column crystals produces
Hastings' anthelic arc. The arc is named after Alfred Wegener, who described it theoretically.
