Wegener's anthelic arc

EE 56

Crystal orientation:
main axes horizontal
Path of light:
very rarely
Picture Wegener's anthelic arc
Bright anthelion with a Wegener's anthelic arc.
Photo: © Heino Bardenhagen, Helvesieck/Germany, 04/23/97


Two white arcs extend from the upper tangent arc, intersecting at the anthelion. When the sun in high in the sky, these arcs extend even further than to the anthelion until they curve back to the infralateral arc, forming a kind of loop.

Simulation of a Wegener's anthelic arc at a sun elevation of 20°. There are other halos too, which are caused by column-shaped ice crystals. Grafik: W. Tape; Atmospheric Halos; S.112; Am. Geophys. Union; 1994


Wegener's anthelic arc is caused by column-shaped ice crystals of an almost ideal orientation. The arc becomes diffuse when the crystals diverge just a bit from their horizontal orientation. That is the reason why it can be seen at just very few occasions. The light rays enter a prism face, are reflected from the base face and leave the crystal through the next but one prism face.