A display observed by William Edward Parry on April 8, 1820 while icebound
off Melville Island in the Canadian Arctic.
Parry's two small wooden ships comprising
a British expedition to find the North West Passage had been trapped in the ice
since the previous September. They had endured a harsh winter and April
continued frigid. They were not to escape for a further three months,
narrowly avoiding being frozen in for a second winter.
It says much for Parry's
fortitude and skills that he was able in these conditions to observe very accurately
the halo display.
He drew a 22° halo touching the horizon
and an outer 46° halo. A parhelic
circle passed through the sun and extended around the sky. An upper
tangent arc was clearly marked. The tip of the lower
tangent arc was the bright area on the horizon directly below the sun.
halo was topped by a circumzenithal arc.
The previously unrecorded arc directly above the upper tangent arc is now called
a Parry Arc and the crystal orientation producing
it and other rare arcs is the "Parry orientation".
Parry drew two other arcs extending outwards at the horizon from the 46° halo
and these were earlier attributed to infralateral arcs incorrectly drawn by Parry.
However, Michael Schroeder points out that the HaloSim simulation shows strong
arcs extending from the horizon just as Parry drew them. These are subhelic
arcs produced by rays entering the end faces of singly oriented column
crystals, being internally reflected twice and then leaving through the opposite
Parry drew the parhelic circle parallel to the horizon as indeed it is, the 'camera'
projection of the simulation renders it curved. The display was simulated by
poorly (randomly) oriented, plate and singly oriented column crystals. Only a
few percent of Parry oriented crystals were necessary to produce the arc that
bears his name.