Right Angled Faces ~ Two halo arcs imaged by Bill Purdy at Livingston, New Jersey, USA. The sun was low in the west and the camera was pointing overhead. ©Bill Purdy, shown with permission.
The brighter arc is the more familiar. It is a circumzenithal arc or ‘smile in the sky’ seen overhead maybe once a month on average when the sun is low . Whenever it is glimpsed (it is a will-o’-the-wisp halo) look for a fainter downwards curving companion. That is the grandly named supralateral arc, too often mistaken for the hardly ever seen 46° halo.

The circumzenithal arc is made by hexagonal plate shaped ice crystals drifting with their large faces horizontal. The supralateral comes from hexagonal columns drifting with their long axes horizontal.

What they have in common is that they are both formed by sun rays refracted between crystal faces at right angles to one another. And their colours are bright and widely separated.

The colour separation results from the effective ray passage through a 90° prism. Everyday 22° halos and sundogs formed by 60° prism rays have less separated colours. Bill’s lower image shows a fragment of a supralateral arc (right) and 22° halo (left) – compare the colour widths.


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