Upper and lower tangent arcs

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 observed Sun forming a sharp angle. 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 rises over 29–32°, the upper tangent arc unites with the lower tangent arc to form the circumscribed halo.[3]

The lower tangent arc is rarely observable, appearing under and tangent to a 22° halo centred on the Sun. Just like upper tangent arcs, the shape of a lower arc is dependent on the altitude of the Sun. As the Sun is observed slipping over Earth's horizon the lower tangent arc forms a sharp, wing-shaped angle below the Sun. As the Sun rises over Earth's 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.[4]

Since by definition, the Sun elevation must exceed 22° above the horizon, most observations are from elevated observation points such as mountains and planes.[4]