While many other halos are the result of one particular type of ice crystal or another, the parhelic circle is far more pluralistic than that. Anyone can come to the party as long as they participate.

The parhelic circle appears as a white circle in the sky that rests parallel to the horizon at the same elevation as the sun. Though it's circular like the halos, it is actually a collection of arcs. When it is diffuse it can look like a line of bright spots in cirrus clouds. It rarely extends all the way around the sky, but when present, often extends beyond sundogs. It is called the parhelic circle because sundogs (parelia) rest on it, as does the anthelion. If the light source of the parhelic circle is the moon, it is called the paraselinic circle.

Even though the light from a parhelic circle can come from multiple refractions, these tend to neutralize each other, so the light tends to be a very pure white.

Near the sun, the halo's light is almost all externally reflected off of any vertically-oriented faces of ice crystals. Away from the sun, the light comes more and more from reflections off of one internal face. Directly opposite the sun and for several degrees on either side, the light can come from two or more internal reflections. The diagrams for the ray paths of 5 or more reflections off of internal faces looks like some instruction manual for hyper-raquetball. Yet all these varying ray paths contribute equally to a single, nearly-perfect circle.

  • http://www.meteoros.de/arten/ee13e.htm
  • http://www.sundog.clara.co.uk/halo/parcirc.htm
  • http://amsglossary.allenpress.com/glossary/search?id=paranthelion1