Supralateral arc
Encyclopedia
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.
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.
Halo (optical phenomenon)
A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...
, an optical phenomenon
Optical phenomenon
An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....
often confused with the indeed infrequently appearing 46° halo
46° halo
A 46° halo is a rare optical halo centred on the sun. At sun elevations between 15-27°, it is often confused with the more colourful and frequently observed supralateral and infralateral arcs. It is named for crossing the parhelic circle 46° from the sun....
. 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
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. 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
Circumzenithal arc
The circumzenithal arc or circumzenith arc , also called the Bravais' arc, is an optical phenomenon similar in appearance to a rainbow; but it arises from refraction of sunlight through horizontally-oriented ice crystals, generally in cirrus clouds, rather than from raindrops...
centred around zenith
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
(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
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....
(below is the infralateral arc
Infralateral arc
An infralateral arc is a rare halo, an optical phenomenon appearing similar to a rainbow under a white parhelic circle...
) and is never perfectly circular.
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
22° halo
A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere...
and generally white with a possible red inner edge. The supralateral arc, in contrast, can even be confused with the rainbow
Rainbow
A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...
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
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
sides. Supralateral arcs occur about once a year.
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