Parry & Upper Tangent Arcs Imaged by Ian Wright (more images) at Chesterfield, England on May 13, 2010.
©Ian Wright, shown with permission.

A low sun is needed to capture this rare sight. Here sunset was approaching with the sun only ~7° high.

The pure coloured upper 'V' is a sunvex Parry arc. The noticeably less purely coloured lower 'V' is the much more common upper tangent arc that at its lowest extremity always touches the 22° halo. A very faint 22° halo is visible at right.

Both are produced by hexagonal column ice crystals in high and cold cirrus cloud.

Aerodynamic drag aligns the columns as they drift very slowly downwards relative to air currents within the cloud. The drag aligns the crystal long axis so that it is horizontal to within a degree or less.

Parry oriented crystals have a further - and improbable - orientation restriction. A side face is maintained nearly horizontal.

The orientation differences give rise to the two separate arcs from a ray that passes through two side faces inclined with a 60° prism wedge between them.

Colour purity?
Parry arcs have pure colours because the 60° prism wedge through which their rays pass has its apex vertical. The dispersed colours therefore have little overlap.

In contrast, the upper tangent arc rays pass through a prism wedge that can rotate around the crystal long axis. Rays from the millions of differently oriented crystals overlap and the colours are mixed.


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