A Parry arc is a rare halo, an optical phenomenon which occasionally appears over a

22° halo A 22° halo is an atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 22° around the Sun or Moon. When visible around the Moon, it is also known as a moon ring or winter halo. It forms as sunlight o ...

together with an

upper tangent arc Tangent arcs are a type of halo, an atmospheric optical phenomenon, which appears above and below the observed Sun or Moon, tangent to the 22° halo. To produce these arcs, rod-shaped hexagonal ice crystals need to have their long axis aligned ...

Discovery

The halo was first described by Sir

William Edward Parry Sir William Edward Parry (19 December 1790 – 8 July 1855) was an 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 Pass ...

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

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

. 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, he made a drawing of the phenomenon. The drawing accurately renders 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. ...

, a

, a pair of

sun dog A sun dog (or sundog) or mock sun, also called a parhelion (plural parhelia) in meteorology, 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 a 22° ...

s, a

lower tangent arc Tangent arcs are a type of halo, an atmospheric optical phenomenon, which appears above and below the observed Sun or Moon, tangent to the 22° halo. To produce these arcs, rod-shaped hexagonal ice crystals need to have their long axis aligned ...

, a

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 su ...

, and a

circumzenithal arc The circumzenithal arc, also called the circumzenith arc (CZA), upside-down rainbow, and the Bravais arc,"Mémoire sur les halos et les phénomènes optiques qui les accompagnent", J. de l' École Royale Polytechnique 31(18), 1-270, A. Bravais, ...

. He did, however, get the

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 arc 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 result from ray entrance and exit through prism end faces ...

s (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 Ancient Greek, Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple polygon, simple (non-self-intersecting) hexagon is 720°. Regular hexa ...

column crystal A column or pillar in architecture and structural engineering is a structural element that transmits, through compression, the weight of the structure above to other structural elements below. In other words, a column is a compression member. ...

s, 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 Scalene may refer to: * A scalene triangle, one in which all sides and angles are not the same. * A scalene ellipsoid, one in which the lengths of all three semi-principal axes are different * Scalene muscles of the neck * Scalene tubercle The sc ...

hexagonal cross-section which are likely to be the cause. Parry arcs can be confused with either

Lowitz arc A Lowitz arc is an optical phenomenon that occurs in the atmosphere; specifically, it is a rare type of ice crystal halo that forms a luminous arc which extends inwards from a sun dog (parhelion) and may continue above or below the sun. Histor ...

s, and any of the odd radius halos produced by pyramidal crystals.

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