Circumscribed Halo (EE07)

On the 22° halo lies a type of halo that can take on various forms. Depending on the sun's altitude, a different image results. With a solar altitude of up to about 29°, this halo phenomenon is called the lower and upper tangent arc of the 22° halo, above that height it is known as a circumscribed halo. When the sun is low, the two branches of the upper tangent arc form a sharp angle. As the sun's altitude increases, the branches approach the 22° halo more and more. At the same time, they become longer and meet at a solar altitude of about 29° with the lower tangent arc. The resulting closed ring is called a circumscribed halo. It touches the 22° halo above and below the sun. Initially, the circumscribed halo is still oval. As the sun's altitude increases, it becomes more circular and approaches the 22° halo. At a solar altitude of 70°, the circumscribed halo nearly coincides with the small ring. The circumscribed halo, like the tangent arcs, is more intensely colored than the 22° halo. It is reddish towards the sun and bluish on the outside. When the sun is high, it lies close to the 22° halo. It can then be recognized by brightenings above and below the sun and by stronger colors. Sometimes only these brightenings are visible. In Central Europe, the sun reaches an altitude of about 29° in the winter half of the year only briefly around noon or not at all. The circumscribed halo is therefore mainly seen around noon in the summer half of the year.

The contact arcs and the circumscribed halo are created in horizontally oriented column crystals. The refracting angle of the prism is 60°. The light enters a prism surface and exits at the next but one. The light follows the same path with the parhelia and the 22° halo. The different types of halos arise only from the different orientation of the crystals.