Parry arc

The halo was first described by Sir William Edward Parry (1790–1855) in 1820 during one of his Arctic expeditions in search for the Northwest Passage.

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.

[1] 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.

Parry arcs are the result of light passing through two side faces forming a 60° angle.

[2][3] 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.

A halo display photo documented by Cindy McFee, NOAA , December 1980 at the South Pole Station . Several distinct halo phenomena are featured in the photo: Two sun dogs (bright spots), a parhelic circle (horizontal line), a 22° halo (circle), and an upper tangent arc and a Parry arc (top).
Complex Moon halo. Visible are: halo , Parry arc , upper tangent arc and Moon dogs .
Upper tangent arc and sunvex Parry arc seen over Santa Rosa Beach, Florida .
Detail of a Parry arc along with an upper tangent arc , both visible between a 22° halo and a 46° halo .