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".
|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.