Tangent arc

To produce these arcs, rod-shaped hexagonal ice crystals need to have their long axis aligned horizontally.

As the Sun is seen to rise above the Earth's horizon, the curved wings of the arc lower towards the 22° halo while gradually becoming longer.

As the Sun reaches 29-32° over the horizon, it finally begins to widen and merge with the upper tangent arc to form the circumscribed halo.

[4] Both the upper and lower tangent arc form when hexagonal rod-shaped ice crystals in cirrus clouds have their long axis oriented horizontally.

Such a crystal configuration also produces other halos, including 22° halos and sun dogs; a predominant horizontal orientation is required to produce a crisp upper tangent arc.

An upper tangent arc appearing above San Francisco Bay and Marin County in California, at sunset on April 9, 2023.
An upper tangent arc with the Sun at an observed low altitude.
An upper tangent arc seen over the setting Sun at Santa Rosa Beach, Florida.
A halo display 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 ]
A 22° halo featuring two sun dogs on both sides, as well as an upper and a lower tangent arcs at the top and the bottom.