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   Parry Arcs  

The changing aspect of Parry arcs with solar altitude. Faint 22� halos and upper tangent arcs (UTA) are included for reference in these HaloSim simulations. At 0� a 'sunvex' Parry arc is almost coincident with the UTA. As the sun rises the sunvex arc separates upward and a 'suncave' Parry arc appears and approaches the UTA. Above 25� a lower sunvex arc becomes visible.   

To see this rare halo, anticipate its appearance at the sun's actual altitude - distinguish also between it and halos from pyramidal crystals or even Lowitz arcs.

Ray paths for suncave and sunvex upper and lower Parry arcs.
First recorded in 1820 by Parry during his search for the North West Passage, the arcs are rare. The column crystals with their long axes nearly horizontal which form tangent arcs are even further constrained in the Parry orientation. Parry oriented crystals have their long axes and their upper and lower prism side faces nearly horizontal.

The high degree of orientation produces many different and rare halos. Parry arcs proper result from ray paths through prism side faces inclined at 60�. There are four possibilities and the Parry arcs are called 'suncave' or 'sunvex' depending whether they are concave or convex with respect to the sun.