Supralateral arc

A supralateral arc is a rare halo, an optical phenomenon often confused with the indeed infrequently appearing 46° halo. Distinguishing between the two is furthermore difficult as supralateral arcs typically only appears in fragments while the 46° halo is very faint.

In contrast to the static 46° halo, the shape of a supralateral arc varies with the elevation of the sun. Before the sun reaches 15°, the bases of the arc touches the lateral (oriented sidewise) sides of the 46° halo. As the sun rises from 15° to 27°, the supralateral arc almost overlap the 46° halo, why most reported observations of the latter most likely are observations of the former. As the sun goes from 27° to 32°, the apex of the arc touches the circumzenithal arc centred around zenith (as do the 46° halo when the sun is located between 15° and 27°). In addition, the supralateral arc is always located above the parhelic circle (below is the infralateral arc) and is never perfectly circular.cite web
url = http://www.meteoros.de/arten/ee21e.htm
title = Supralateral arc | publisher = Arbeitskreis Meteore e.V.
accessdate = 07-04-16 | language = English
(Including a photo from January 1996, a 1980 computer simulation of a supralateral arc relative to a 46° halo, and a table pinning down differences between 46° halos and supralateral arcs.)] cite web
url = http://www.engl.paraselene.de/html/supralateral_arcs.html
title = Supralateral Arcs | publisher = www.paraselene.de
accessdate = 2007-04-16 | language = English
]

Arguably the best way of distinguishing the halo from the arc is to carefully study the difference in colour and brightness. The 46° halo is six times fainter than the 22° halo and generally white with a possible red inner edge. The supralateral arc, in contrast, can even be confused with the rainbow with clear blue and green strokes.

Supralateral arcs form when sun light enters horizontally oriented, rod-shaped hexagonal ice crystals through a hexagonal base and exits through one of the prism sides. Supralateral arcs occur about once a year.

See also

* Parry arc
* Lowitz arc

References

External links

* [http://www.atoptics.co.uk/halo/supinf.htm Atmospheric Optics - Supralateral & infralateral arcs] - including HaloSim computer simulations and crystal illustrations.
* [http://www.engl.paraselene.de/html/supralateral_arc_mar_2002.html paraselene.de - Gallery of images from March 2002]
* [http://www.engl.paraselene.de/html/supralateral_arc_dec_26__2004_.html paraselene.de - Gallery of images from December 2004]


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