by Marcin Polkowski in Poland, June '08. ©Marcin Polkowski, shown with permission.
The bright white arc is part of the parhelic
circle at the 'anthelic point' opposite and at the same altitude
as the sun. Two faint arcs form a cross at the anthelic
point - Wegener
arcs. The arcs are formed by column crystals
drifting in cirrus with their long axes horizontal. Wegener arc rays
pass between prism sides inclined at 60° just like the rays that
form the much more common
circumscribed halo and (at lower
sun) the upper tangent arc. The
critical difference is that while inside the crystal, Wegener arc
rays are reflected once from an end face.
All the halos in the all-sky HaloSim ray tracing (with
the exception of the faint 22° halo) were made by horizontal
column crystals. HaloSim's virtual crystals are perfect. In
the real sky, Wegener arcs need relatively short crystals with near
perfect end faces. End faces are most prone to imperfections.