An infralateral arc (or lower lateral tangent arc) is a rare
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), ...
, an
optical phenomenon appearing similar to a
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 ...
under a white
parhelic circle. Together with the
supralateral arc
A supralateral arc is a comparatively rare member of the halo 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 familia ...
they are always located outside the seldom observable
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 ...
, 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 observed Sun reaches about 50° over Earth's horizon, two infralateral arcs are located on either side (e.g.
lateral) of the 46° halo, their convex apexes lying
tangent
In geometry, the tangent line (or simply tangent) to a plane curve at a given point is the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points on the curve. Mo ...
to the 46° halo. As the observed Sun reaches above 68° the two arcs unite to a single concave arc tangent to the 46° halo vertically under the Sun.
[ (Including a photo from January 1996 and a 1980 computer simulation of infra- and supralateral arcs relative to a 46° halo.)]
Infralateral arcs form when sunlight 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 ...
al
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 halos and
upper tangent arcs.
See also
*
Circumzenithal arc
*
Tangent arc
*
Parry arc
A Parry arc is a rare halo, an optical phenomenon which occasionally appears over a 22° halo together with an upper tangent arc.
Discovery
The halo was first described by Sir William Edward Parry (1790–1855) in 1820 during one of his Arct ...
References
External links
Atmospheric Optics - Supralateral & infralateral arcs- including
HaloSim computer simulations and crystal illustrations.
Atmospheric optical phenomena
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