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]

Tangent arcs are a type of halo, an atmospheric optical phenomenon, which appears above and below the observed Sun or Moon, tangent to the 22° halo. To produce these arcs, rod-shaped hexagonal ice crystals need to have their long axis aligned horizontally.[2]

  1. ^ "A magnificent halo". NOAA. 1980-12-21. Archived from the original on 2006-12-13. Retrieved 2007-04-14.
  2. ^ Les Cowley (?). "Tangent Arcs". Atmospheric Optics. Retrieved 2015-04-07.