A parhelic circle is a halo, an optical phenomenon appearing as a horizontal white line on the same altitude as the sun, or occasionally the Moon. If complete, it stretches all around the sky, but more commonly it only appears in sections.
Even fractions of parhelic circles are less common than sun dogs and 22° halos. While parhelic circles are generally white in colour because they are produced by reflection, they can however show a bluish or greenish tone near the 120° parhelia and be reddish or deep violet along the fringes.
Parhelic circles form as beams of sunlight are reflected by vertical or almost vertical hexagonal ice crystals. The reflection can be either external (e.g. without the light passing through the crystal) which contributes to the parhelic circle near the sun, or internal (one or more reflections inside the crystal) which creates much of the circle away from the sun. Because an increasing number of reflections makes refraction asymmetric some colour separation occurs away from the sun. A complete parhelic circle was observed in the sky in perfect conditions above northern Belgium at around midday on May 29, 2010, and lasted about an hour before the crystals dispersed. Sun dogs are always aligned to the parhelic circle (but not always to the 22° halo).
- "A magnificent halo". NOAA. 1980-12-21. Archived from the original on 2006-12-13. Retrieved 2007-04-14.
- Koby Harati. "Parhelic Circle". Atmospheric Optics. Retrieved 2007-04-15.
- "Parhelic Circle". www.paraselene.de. Retrieved 2009-02-01. (including an excellent HaloSim simulation of a parhelic circle.)
- Les Cowley. "Parhelic Circle Formation". Atmospheric Optics. Retrieved 2007-04-15.
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