A Parry arc is a rare

halo HALO, halo, halos or haloes may refer to: Most common meanings * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head * ''Halo'' (franchise), a sci-fi video game series (2001–2021) Arts and en ...

, an

optical phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the Sun or Moon with ...

which occasionally appears over a 22° halo together with an upper tangent arc.

Discovery

The halo was first described by Sir

William Edward Parry Sir William Edward Parry (19 December 1790 – 8 July 1855) was a Royal Navy officer and explorer best known for his 1819–1820 expedition through the Parry Channel, probably the most successful in the long quest for the Northwest Passa ...

(1790–1855) in 1820 during one of his Arctic expeditions in search for the

Northwest Passage The Northwest Passage (NWP) is the sea lane between the Atlantic and Pacific oceans through the Arctic Ocean, near the northern coast of North America via waterways through the Arctic Archipelago of Canada. The eastern route along the Arctic ...

. On April 8, under harsh conditions while his two ships were trapped by ice forcing him to winter over at Melville Island in the northern

Canadian Arctic Archipelago The Arctic Archipelago, also known as the Canadian Arctic Archipelago, is an archipelago lying to the north of the Canadian continental mainland, excluding Greenland (an autonomous territory of the Danish Realm, which is, by itself, much larger ...

, he made a drawing of the phenomenon. The drawing accurately renders the parhelic circle, a 22° halo, a pair of

sun dog A sun dog (or sundog) or mock sun, also called a parhelion (plural parhelia) in atmospheric science, is an atmospheric optical phenomenon that consists of a bright spot to one or both sides of the Sun. Two sun dogs often flank the Sun within ...

s, a lower tangent arc, a 46° halo, and a circumzenithal arc. He did, however, get the upper tangent arc slightly wrong. On the other hand, he added two arcs extending laterally from the bases of the 46° halo, for long interpreted as incorrectly drawn infralateral arcs, but were probably correctly drawn subhelic arcs (both produced by the same crystal orientation but with light passing through different faces of the crystals).Cowley, ''Parry 1820''

Formation

Parry arcs are generated by double-oriented

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 is de ...

al column crystals, i.e. a so-called Parry orientation, where both the central main axis of the prism and the top and bottom prism side faces are oriented horizontally. This orientation is responsible for several rare haloes. Parry arcs are the result of light passing through two side faces forming a 60° angle. The shape of Parry arcs changes with the elevation of the sun and are subsequently called upper or lower arcs to indicate they are found above or under the sun, and sunvex or suncave depending on their orientation.Cowley, ''Parry Arcs''Arbeitskreis Meteore, ''Parry Arc'' The mechanism by which column crystals adopt this special Parry orientation has been subject to much speculation – recent laboratory experiments have shown that it is the presence of crystals with a scalene hexagonal cross-section which are likely to be the cause. Parry arcs can be confused with either upper tangent arcs, Lowitz arcs, and any of the odd radius halos produced by pyramidal crystals. Complex halo combination 03

Notes

References

* (Including a computer simulation recreating the halo observed by Parry.) * * {{cite web , url = http://www.meteoros.de/arten/ee27e.htm , title = Parry Arc , publisher = Arbeitskreis Meteore e.V. , accessdate = 2007-04-23

External links

Halo Reports
– Photo by Joe MacGregor of a rare lower Parry sunvex arc in Antarctica

Atmospheric optical phenomena

Discovery

Formation

See also

Notes

References

External links