Parhelic circle

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.cite web
url = http://www.atoptics.co.uk/halo/parcirc.htm
title = Parhelic Circle | publisher = Atmospheric Optics
accessdate = 2007-04-15 | author = Les Cowley (?)]

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.cite web
url = http://www.engl.paraselene.de/html/parhelic_circle.html
title = Parhelic Circle | publisher = www.paraselene.de
accessdate = 2007-04-15 | author = (including an excellent HaloSim simulation of a parhelic circle.)]

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.cite web
url = http://www.atoptics.co.uk/halo/pcpaths.htm
title = Parhelic Circle Formation | publisher = Atmospheric Optics
accessdate = 2007-04-15 | author = Les Cowley (?)]

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

References

See also

* Tangent arc
* Circumzenithal arc

External links

* [http://www.atoptics.co.uk/halosim.htm Atmospheric Optics - Ice Halos]