at Otway Massif, Grosvenor Mountains, Antarctica by Monika Kress.
shown with permission.
Circling the sun at top right is the familiar
22° halo and slightly beyond it is a bright
sundog. These halos are produced by sun rays passing between
ice crystal faces tilted 60° to one another. The 60° prism splits
the light into colours.
Why are the colours of the arc at lower left spread more widely? This
is a rare infralateral arc. It is produced by rays passing between
crystal facets angled 90° to each other. The wider prism strongly refracts
and splits the light.