A subhelic 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'' (franch ...
, formed by
internal reflection through ice crystals, that curves upwards from the horizon and touches the
tricker arc above the
anthelic point
The antisolar point is the abstract point on the celestial sphere directly opposite the Sun from an observer's perspective. This means that the antisolar point lies above the horizon when the Sun is below it, and vice versa. On a sunny day, t ...
. Subhelic arcs result from ray entrance and exit through prism end faces with two intermediate internal reflections.
Formation
A subhelic arc is formed when sun rays enter one end face of an ice crystal in singly oriented columns and Parry columns, reflect off two of the crystals side faces, and exits the crystal through the opposite end face. The ray leaves the crystal at the exact opposite angle, resulting in a net deviation angle of 120°, the angle for the formation of
120° parhelia.
[Cowley, ''South Pole Halos - Zenith View'']
The subhelic arc touches the top of the tricker arc, an indication the two have closely related ray paths.
[Cowley, ''South Pole Halos - Anthelic View'']
The subhelic arc crosses 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. ...
at an acute angle, and at a sun elevation of 27° it passes exactly through the 120° parhelion.
[Vornhusen, ''First Report on the Danzig Halo Display'']
See also
*
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° ha ...
*
Wegener arc
Wegener may refer to:
Astronomy
* 29227 Wegener, a main-belt asteroid
* Wegener (lunar crater)
* Wegener (Martian crater)
Places
* Wegener Range, an Antarctic mountain range
* Mount Wegener, an Antarctic mountain in the Read Mountains in t ...
Notes
References
*
*
*
External links
* (Including a drawing from a halo observation in
Oulunsalo
Oulunsalo ( sv, Oulunsalo, also formerly ) is former municipality in the region of Northern Ostrobothnia, in Finland. Along with Haukipudas, Kiiminki and Yli-Ii it lost its municipal status and was merged with the city of Oulu on 1 January 2013. ...
, Finland, in April 1996.)
* (A halo display including a subhelic arc, observed in
Oulu
Oulu ( , ; sv, Uleåborg ) is a city, municipality and a seaside resort of about 210,000 inhabitants in the region of North Ostrobothnia, Finland. It is the most populous city in northern Finland and the fifth most populous in the country after: ...
, Finland, May 2008.)
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Atmospheric optical phenomena