A lower tangent arc is a rarely observable halo, an optical phenomenon appearing under and tangent to a 22° halo centred around the sun.
Just like upper tangent arcs, the shape of a lower tangent arc is dependent on the altitude of the sun. As the sun slips over the horizon the lower tangent arc forms a sharp, wing-shaped angle below the sun. As the sun rises above over the horizon, the arc first folds upon itself and then takes the shape of a wide arc. 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. The lower tangent arc is rarely visible as it only appears when the sun elevation exceeds 22°. Most observations thus are from elevated observation points such as mountains and planes. [1]
To produce lower tangent arcs rod-shaped hexagonal ice crystals need to have their long axis aligned horizontally. [2]