Solar halos with foreground, Salem, MA, Oct 27, 2012

A supralateral arc is a comparatively rare member of the

halo Halo, halos or haloes usually refer to: * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head HALO, halo, halos or haloes may also refer to: Arts and entertainment Video games * Halo (franchise), ...

family which in its complete form appears as a large, faintly rainbow-colored band in a wide arc above the sun and appearing to encircle it, at about twice the distance as the familiar

22° halo A 22° halo is an atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 22° around the Sun or Moon. When visible around the Moon, it is also known as a moon ring or winter halo. It forms as sunligh ...

. In reality, however, the supralateral arc does not form a circle and never reaches below the sun. When present, the supralateral arc touches the (much more common)

circumzenithal arc The circumzenithal arc, also called the circumzenith arc (CZA), upside-down rainbow, and the Bravais arc,"Mémoire sur les halos et les phénomènes optiques qui les accompagnent", J. de l' École Royale Polytechnique 31(18), 1-270, A. Bravais, ...

from below. As in all colored halos, the arc has its red side directed towards the sun, its blue part away from it.

Formation

Supralateral arcs form when sun light enters horizontally oriented, rod-shaped

hexagon In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A ''regular hexagon'' h ...

ice crystal Ice crystals are solid ice exhibiting atomic ordering on various length scales and include hexagonal columns, hexagonal plates, dendritic crystals, and diamond dust. Formation The hugely symmetric shapes are due to depositional growth, na ...

s through a hexagonal base and exits through one of the

prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...

sides. Supralateral arcs occur about once a year. Icearcs1

Confusion with the 46° halo

Due to its apparent circular shape and nearly identical location in the sky, the supralateral arc is often mistaken for the

46° halo A 46° halo is a rare atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 46° around the Sun. At solar elevations of 15–27°, 46° halos are often confused with the less rare and more colourful ...

, which does form a complete circle around the sun at approximately the same distance, but which is much rarer and fainter. Distinguishing between the two phenomena can be difficult, requiring the combination of several subtle indicators for proper identification. In contrast to the static 46° halo, the shape of a supralateral arc varies with the elevation of the

sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared rad ...

. Before the sun reaches 15°, the bases of the arc touch the

lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

(oriented sidewise) sides of the 46° halo. As the sun rises from 15° to 27°, the supralateral arc almost overlaps the upper half of the 46° halo, which is why many 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

centered on

zenith The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction ( plumb line) opposite to the gravity direction at that location ( nadir). The zenith is the "high ...

(as does the 46° halo when the sun is located between 15° and 27°). In addition, the supralateral arc is always located above the

parhelic circle A parhelic circle is a type of 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. ...

(the arc located below it is the

infralateral arc An infralateral arc (or lower lateral tangent arc) is a rare halo, an optical phenomenon appearing similar to a rainbow under a white parhelic circle. Together with the supralateral arc they are always located outside the seldom observable 46° h ...

), and is never perfectly circular. (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.) 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

and generally white with a possible red inner edge. The supralateral arc, in contrast, can even be confused with the

rainbow A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows ...

with clear blue and green strokes.

Gallery

Image:Supralateral_20jun09.jpg, Solar Halo, Upper Tangent Arc, Supralateral Arc : 20 June 2009 : Khonkaen, Thailand

References

External links

Atmospheric Optics - Supralateral & infralateral arcs
- including HaloSim computer simulations and crystal illustrations.
Paraselene.de - Gallery of images from March 2002

Paraselene.de - Gallery of images from December 2007
{{DEFAULTSORT:Supralateral Arc Atmospheric optical phenomena

Formation

Confusion with the 46° halo

Gallery

See also

References

External links