Parry & Tangent Arcs ~ Images by Steve Mattan (Photoblog) taken in Southampton, New Jersey, USA.

The lower "V" of a near sunset upper tangent arc is echoed by a similar but much rarer upper sunvex Parry arc".

All images ©Steve Mattan, shown with permission

Both arcs are produced by rays entering a side face of a horizontal column crystal and leaving through another inclined 60° to the first.

The only difference is that Parry oriented crystals do not take all rotational positions about their long axes. Instead an upper and lower prism face stays fixed horizontal.

The reason for this orientation is not properly understood. It is rare. We only see Parry arcs because their generation is extraordinarily efficient. Only a whiff of Parry oriented crystals is necessary.
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Whenever the sun is low and a tangent arc is visible always wait and search above it. You may be rewarded by a Parry arc. The two arcs quickly change shape and separation as the sun altitude changes. Check here for their shapes.