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
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
Ray paths for suncave and sunvex upper and lower Parry arcs.
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.