Infralateral Arc Plus

A magnificent and rare infralateral arc seemingly hangs above an alpine valley. Willem Koppenol imaged this as part of a wider diamond dust halo display pictured in an earlier OPOD.

Look closely to its left to see a faint colouration. Peter Paul Hattinga Verschure spotted this and identified it as a supralateral arc. A bright example of the pair is here.

©Willem Koppenol, shown with permission.
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Infralateral arcs and their cousins, supralateral arcs, glint their colours twice as far from the sun as the more commonplace 22° halo.

Both are produced by sun rays passing through hexagonal column ice crystals oriented by aerodynamic drag to have their long axes nearly horizontal.

The infralateral arc results from rays that enter a near vertical hexagonal end face and leave through a side face.

The arc is strongly coloured with the hues widely spread by the refraction that is in effect through a 90° prism.

Supralateral arc - a very faint section of one shows on the images to the left of the infralateral arc - have the reverse ray path. Rays enter a side face and leave through a near vertical one.