Parry arc
Encyclopedia
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...
, an optical phenomenon
Optical phenomenon
An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....
which occasionally appears over a 22° halo
22° halo
A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere...
together with an upper tangent arc
Upper tangent arc
An upper tangent arc is a halo, an atmospheric optical phenomenon which appears over and tangent to the 22° halo around the sun.The shape of an upper tangent arc varies with the elevation of the sun; while the sun is low it appears as an arc over the sun forming a sharp angle...
.
Discovery
The halo was first described by Sir William Edward ParryWilliam Edward Parry
Sir William Edward Parry was an English rear-admiral and Arctic explorer, who in 1827 attempted one of the earliest expeditions to the North Pole...
(1790-1855) in 1820 during one of his Arctic expeditions in search for the Northwest Passage
Northwest Passage
The Northwest Passage is a sea route through the Arctic Ocean, along the northern coast of North America via waterways amidst the Canadian Arctic Archipelago, connecting the Atlantic and Pacific Oceans...
. On April 8, under harsh conditions while his two ships were trapped by ice forcing him to winter over at Melville Island
Melville Island, Canada
Melville Island is a vast, uninhabited member of the Canadian Arctic Archipelago with an area of . It is the 33rd largest island in the world and Canada's eighth largest island. Melville Island is shared by the Northwest Territories, which is responsible for the western half of the island, and...
in the northern Canadian Arctic Archipelago
Canadian Arctic Archipelago
The Canadian Arctic Archipelago, also known as the Arctic Archipelago, is a Canadian archipelago north of the Canadian mainland in the Arctic...
, he made a drawing of the phenomenon. The drawing accurately renders 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....
, a 22° halo
22° halo
A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere...
, a pair of sun dog
Sun dog
A sun dog or sundog, scientific name parhelion ; , also called a mock sun or a phantom sun, is an atmospheric phenomenon that creates bright spots of light in the sky, often on a luminous ring or halo on either side of the sun.Sundogs may appear as a colored patch of light to the left or right of...
s, a lower tangent arc
Lower tangent arc
A lower tangent arc is a rarely observable halo, an optical phenomenon appearing under and tangent to a 22° halo centred around the sun.Just like upper tangent arcs, the shape of a lower tangent arc is dependent on the altitude of the sun. As the sun slips over the horizon the lower tangent arc...
, a 46° halo
46° halo
A 46° halo is a rare optical halo centred on the sun. At sun elevations between 15-27°, it is often confused with the more colourful and frequently observed supralateral and infralateral arcs. It is named for crossing the parhelic circle 46° from the sun....
, and a circumzenithal arc
Circumzenithal arc
The circumzenithal arc or circumzenith arc , also called the Bravais' arc, is an optical phenomenon similar in appearance to a rainbow; but it arises from refraction of sunlight through horizontally-oriented ice crystals, generally in cirrus clouds, rather than from raindrops...
. He did, however, get the upper tangent arc
Upper tangent arc
An upper tangent arc is a halo, an atmospheric optical phenomenon which appears over and tangent to the 22° halo around the sun.The shape of an upper tangent arc varies with the elevation of the sun; while the sun is low it appears as an arc over the sun forming a sharp angle...
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 arc
Infralateral arc
An infralateral arc is a rare halo, an optical phenomenon appearing similar to a rainbow under a white parhelic circle...
s, but, as thing were, probably correctly drawn subhelic arc
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...
s. (both produced by the same crystal orientation but with light passing through different faces of the crystals.)
Formation
Parry arcs are generated by double-oriented hexagonal 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..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
Scalene may refer to:* A scalene triangle, one in which all sides are different* A scalene ellipsoid, one in which the lengths of all three semi-principal axes are different* Scalene muscles of the neck...
hexagonal cross-section which are likely to be the cause .
Parry arcs can be confused with either upper tangent arc
Upper tangent arc
An upper tangent arc is a halo, an atmospheric optical phenomenon which appears over and tangent to the 22° halo around the sun.The shape of an upper tangent arc varies with the elevation of the sun; while the sun is low it appears as an arc over the sun forming a sharp angle...
s, Lowitz arcs, and any of the odd radius halos produced by pyramidal crystals.
External links
- Close-up of an upper Parry arc by Bruce Grieg (GIFGIFThe Graphics Interchange Format is a bitmap image format that was introduced by CompuServe in 1987 and has since come into widespread usage on the World Wide Web due to its wide support and portability....
) - Halo Reports - Photo by Joe MacGregor of a rare lower Parry sunvex arc in Antarctica (Blogg)
