Halo of 22°
From AMS Glossary
halo of 22°
A halo in the form of a circle, or portion of a circle, with an angular radius of about 22° about a light source, such as the sun or moon.
This is the most common of all halos. The sky is darker just to the inside of the halo than it is to the outside. The halo exhibits a pale coloration from a reddish tint on the inside fading to a bluish white on the outside. The 22° halo is explained by the refraction of light that enters one prism face and leaves by the second prism face beyond, thus being refracted by a prism with an effective angle of 60°. The angle of minimum deviation for an ice prism of this prism angle is about 22°, so such light does not appear inside the halo, accounting for the darker region there. The minimum angle of deviation varies slightly with wavelength, with the longer wavelengths being deviated least. This causes the reddish inner edge, outside which the additional contributions from light of increasingly shorter wavelengths decrease the color purity. The orientation that a crystal must have to contribute light to the halo depends on both the elevation angle of the light source and the portion of the halo in question, but the probability that a crystal will have a particular orientation depends upon its type and size. Consequently, it is frequently the case that for a given population of crystals and sun height, only a portion of the halo will be seen, while a change in one or the other might enable the full circle to form. It is not the case, despite being frequently asserted, that a view of the full circle requires crystals to have random orientations.