Parhelic circle

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A halo phenomenon observed over the South Pole. Featured in the photo are several distinct phenomena: A parhelic circle (horizontal line), a 22° halo (circle) with two sundogs (bright spots), and an upper tangent arc.
Photo: Cindy McFee, NOAA, December 1980.^[1]

A crisp parhelic circle (horizontal line) over South Pole Station.
Photo: John Bortniak, NOAA, January 1979.

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.^[2]

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.^[3]

Parhelic circles form as beams of sunlight is 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 contribute to the parhelic circle near the sun, or internal (one or more reflections inside the crystal) which create 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.^[4]

Sun dogs are always aligned to the parhelic circle (but not always to the 22° halo).

[edit] References

^ A magnificent halo. NOAA (1980-12-21). Retrieved on 2007-04-14.
^ Les Cowley (?). Parhelic Circle. Atmospheric Optics. Retrieved on 2007-04-15.
^ Parhelic Circle. www.paraselene.de. Retrieved on 2007-04-15. (including an excellent HaloSim simulation of a parhelic circle.)
^ Les Cowley (?). Parhelic Circle Formation. Atmospheric Optics. Retrieved on 2007-04-15.