Tangent arcs

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tangent arcs

A halo in the form of an arc tangent to a circular halo.

The most common of these are the upper and lower tangent arcs to the halo of 22°. They form separate arcs when the sun is low, but as the sun climbs they join to produce the circumscribed halo. These arcs are explained by refraction through the 60° prism sides of columnar crystals oriented with their long axes horizontal.
See 46° lateral arcs.