Upper tangent arc

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An upper tangent arc with the sun at a low altitude.

A halo phenomenon observed over the South Pole. Featured in the photo are several distinct phenomena: A parhelic circle (horizontal line), a 22° halo (circle) with a sundog (bright spot), and an upper tangent arc.
Photo: Cindy McFee, NOAA, December 1980.^[1]

An upper tangent arc is a halo, an atmospheric optical phenomenon which appears over and tangent to the 22° halo around the sun.

The shape of an upper tangent arc varies with the elevation of the sun; while the sun is low (less than 29–32°) it appears as an arc over the sun forming a sharp angle. As the sun rises, the curved wings of the arc lower towards the 22° halo while gradually becoming longer. As the sun rises over 29–32°, the upper tangent arc unites with the lower tangent arc to form the circumscribed halo.^[2]

Both the upper and lower tangent arc form when hexagonal rod-shaped ice crystals in cirrus clouds have their long axis oriented horizontally, while otherwise rotated in any direction. This orientation of the crystals also produces other halos, including 22° halos and sun dogs, but a predominant horizontal orientation is required to produce a crisp upper tangent arc. Like many other halos, upper tangent arcs grade from a red inner edge to a blue outer edge because red light is refracted more strongly than blue light.^[3]