Parhelic Circle

 
An unusually complete parhelic circle seen in Gotland, Sweden by M�ns Hagberg. The image is a composite of two.
Images ©M�ns Hagberg.




The parhelic circle ice halo is centred on the zenith like the circumzenithal and circumhorizon arcs. Unlike them it has, almost, no colour.

Bright spots, extra suns, stud it. M�ns� image has five, the sun itself, two 22� parhelia and two 120� parhelia. An earlier (1535!) Swedish display shows six suns.

Plate crystals made the extra suns.

Horizontal column crystals made the circumscribed halo above and around the sun. Purely by coincidence it passes through the sundogs. As the sun elevation increases, sundogs move further from the sun and the circumscribed halo contracts to eventually form a circle. The combination of effects gives an overlap with the parhelia at a sun elevation of ~41�.

Both plate crystals and icy columns contributed to the parhelic circle. The parhelic circle is formed by more ray paths and crystal orientations than any other halo.
Atmospheric
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At left is an unfiltered HaloSim ray tracing. Adjoining it the simulation shows only externally reflected rays - their parhelic circle is weak. Most PC brightness comes from rays reflected once inside the plate crystals.

A few of the many ray paths that might contribute to a parhelic circle PC.

At top (A) is the simplest and most oft quoted - external reflection from a plate side face. As the filtered HaloSim simulation shows, this path only contributes near to the sun.

The most efficient PC forming rays are (B) and (C) where a reflection occurs inside the crystal.

More exotic paths with two (D) three (E) or even five (F) internal reflections light the PC in the direction away from the sun and also form the 120° parhelia.

Horizontal column crystals also form a PC and contribute yet more ray paths!