Circumscribed halo

A circumscribed halo is a type of halo, an optical phenomenon typically in the form of a more or less oval ring that circumscribes the circular 22° halo centred on the Sun or Moon.[1] As the Sun rises above 70° it essentially covers the 22° halo.[2] Like many other halos, it is slightly reddish on the inner edge, facing the Sun or Moon, and bluish on the outer edge.

A circumscribed halo (outer ring) together with a 22° halo (inner ring).

A circumscribed halo (top) together with a circumhorizon arc (bottom)

The shape of the circumscribed halo is strongly dependent on the distance of the Sun or Moon above the horizon.[3] Its top and bottom (i.e., the points directly below and above the Sun or Moon) always lie directly tangential to the 22° halo, but its left and right sides take on different shapes depending on solar (or lunar) elevation. At an elevation between about 35°–50°, the sides form two distinct, downward-drooping "lobes" outside of the 22° halo. As the Sun or Moon rises higher (between c. 50°–70°), the drooping diminishes towards a more regular oval shape. At an elevation of c. 70° or more, the shape of the circumscribed halo approaches a circle, and as such becomes nearly indistinguishable from the 22° halo, only to be identified by its tendency to show more saturated colors than the latter.[2] When the Sun or Moon is at an elevation lower than c. 35°, the circumscribed halo breaks up into the upper tangent and lower tangent arcs.[4]