arc of contact

A halo phenomenon that consists of an arc (or arcs) tangent to the 22° halo. The form varies with solar elevation. At low elevations, both upper and lower contact arcs are convex towards the Sun. The lower arc is then normally hidden below the horizon but may be visible from an aircraft. At higher elevations the upper arc becomes concave. The two arcs bend and, at an elevation of about 40°, merge into a single arc, known as the circumscribed halo. At c.70° elevation, the arc can no longer be distinguished from the 22° halo.