Subhelic arc
Encyclopedia
A subhelic arc is a rare halo
, formed by internal reflection through ice crystals, that curves upwards from the horizon and touches the tricker arc above the anthelic point. Subhelic arcs are a result of ray entrance and exit through prism end faces with two intermediate internal reflections.
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The subhelic arc touches the top of the tricker arc, an indication the two have closely related ray paths.
The subhelic arc crosses the parhelic circle
in an acute angle, and at a sun elevation of 27° it passes exactly through the 120° parhelion.
, Finland, in April 1996.) (A halo display including a subhelic arc, observed in Oulu
, Finland, May 2008.)
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...
, formed by internal reflection through ice crystals, that curves upwards from the horizon and touches the tricker arc above the anthelic point. Subhelic arcs are a result of 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 leave the crystal in the exact opposite angle, resulting in a net deviation angle of 120°, the angle for the formation of 120º parhelia120° parhelion
A 120° parhelion is a relatively rare halo, an optical phenomenon occasionally appearing along with very bright sun dogs as ice crystal-saturated cirrus clouds fill the atmosphere...
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The subhelic arc touches the top of the tricker arc, an indication the two have closely related ray paths.
The subhelic arc crosses 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....
in an acute angle, and at a sun elevation of 27° it passes exactly through the 120° parhelion.
External links
(Including a drawing from a halo observation in OulunsaloOulunsalo
Oulunsalo is a municipality in the region of Northern Ostrobothnia, in Finland. The municipality is located in Northern Finland just south-west of the city of Oulu. The municipality has a population of and covers an area of of which is water. The population density is...
, Finland, in April 1996.) (A halo display including a subhelic arc, observed in Oulu
Oulu
Oulu is a city and municipality of inhabitants in the region of Northern Ostrobothnia, in Finland. It is the most populous city in Northern Finland and the sixth most populous city in the country. It is one of the northernmost larger cities in the world....
, Finland, May 2008.)