A lower (4,6) Parry arc is a rare sight.
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Parry arcs are produced by unusually oriented column crystals with their long axis and two prism side faces near horizontal.
The orientation is rare but Parry arcs are less rare than they might be because the ray path is efficient and gives bright, colourful and concentrated arcs.
There are four possible Parry arc ray paths. The visibility and shape of each arc depends strongly on the sun's altitude as this controls the angle of rays incident on the crystals. |
