On the
22° ring is a type of halo that can take on a variety of forms. Depending on the height of the sun, it presents a different picture. At a sun height of up to about 29°, this halo phenomenon is referred to as the lower and upper tangent arc of the 22° ring, and at heights above this, as a circumscribed halo. When the sun is low, the two branches of the upper tangent arc form an acute angle. When the sun is very low, the arc has the shape of a V or a pair of wings pointing upwards. It can then be confused with the tip of a
light pillar or with the
Parry arc. Together with the 22° ring and the sundogs, the upper tangent arc is one of the most common halos. It can also be observed around the moon. As the sun's height increases, the branches becom closer to the 22° ring. At the same time, they grow longer and meet at a sun height of about 29° with the
lower tangent arc. The resulting closed ring is called a
circumscribed halo. It touches the 22° ring above and below the sun. Initially, the circumscribed halo is still oval. As the sun's height increases, it becomes progressively more circular and comes closer to the 22° ring. At a sun height of 70°, the circumscribed halo almost merges with the small ring. The circumscribed halo, like the tangent arcs, is more strongly colored than the 22° ring. Towards the sun, it is reddish and bluish on the outside.
The contact arcs and the circumscribed halo are formed in
horizontally oriented column crystals. The refracting angle of the prism is 60°. The light enters a prism face and exits at the next but one. The light takes the same path with the
parhelia and the 22° ring. The various types of halos result only from the different orientation of the crystals.
