Infralateral arc
Encyclopedia
An infralateral 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° halo
, but in contrast to supralateral arcs, infralateral arcs are always located below the parhelic circle.
The shape of an infralateral arc varies with the elevation of the sun
. Between sunrise and before the sun reaches about 50° over the horizon, two infralateral arcs are located on either side (e.g. lateral) of the 46° halo, their convex apexes lying tangent
to the 46° halo. As the sun reaches above 68° the two arcs unite to a single concave arc tangent to the 46° halo vertically under the sun.
Infralateral 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. Infralateral arcs occur about once a year. They are often observed together with circumscribed halo
s and upper tangent arc
s.
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....
appearing similar to a 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...
under a white 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....
. Together with the supralateral arc
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°...
they are always located outside the seldom observable 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....
, but in contrast to supralateral arcs, infralateral arcs are always located below the parhelic circle.
The shape of an infralateral 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...
. Between sunrise and before the sun reaches about 50° over the horizon, two infralateral arcs are located on either side (e.g. lateral) of the 46° halo, their convex apexes lying tangent
Tangent
In geometry, the tangent line to a plane curve at a given point is the straight line that "just touches" the curve at that point. More precisely, a straight line is said to be a tangent of a curve at a point on the curve if the line passes through the point on the curve and has slope where f...
to the 46° halo. As the sun reaches above 68° the two arcs unite to a single concave arc tangent to the 46° halo vertically under the sun.
Infralateral 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. Infralateral arcs occur about once a year. They are often observed together with circumscribed halo
Circumscribed halo
A circumscribed halo is a type of halo, an optical phenomenon that circumscribes a related phenomenon, the 22° halo centred on the sun.When observed, a circumscribed halo is normally oval in shape and, at the points directly below and above the sun, lies tangential to the 22° halo...
s and upper tangent arc
Upper tangent arc
An upper tangent arc is a halo, an atmospheric optical phenomenon which appears over and tangent to the 22° halo around the sun.The shape of an upper tangent arc varies with the elevation of the sun; while the sun is low it appears as an arc over the sun forming a sharp angle...
s.
External links
- Atmospheric Optics - Supralateral & infralateral arcs - including HaloSim computer simulations and crystal illustrations.