Photo: 22° Halo Captured Over Durham

From Wikipedia:

A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon (also called a moon ring or winter halo). It forms as sunlight is refracted in millions of randomly oriented hexagonal ice crystals suspended in the atmosphere. The halo is large; the radius is roughly the size of your outstretched hand at arms length.^[1]

Pathway of light through a hexagonal prism in the optimal angle resulting in minimum deviation.

As light passes through the 60° apex angle of the hexagonal ice prisms it is deflected twice resulting in deviation angles ranging from 22° to 50°. The angle of minimum deviation is almost 22° (or more specifically 21.84° on average; 21.54° for red light and 22.37° for blue light). This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish.^[2] As no light is refracted at angles smaller than 22° the sky is darker inside the halo.^[3] A 22° halo may be visible on as many as 100 days per year.^[1]

22° solar halo and parhelion (sun dog) inSalem, Massachusetts, Oct 27, 2012. Parry arc and Upper tangent arc are also visible.

In folklore, moon rings are said to warn of approaching storms. Like other ice halos, 22° halos appear when the sky is covered by thin cirrus clouds that often come a few days before a large storm front.^[4]

The similar phenomenon called coronas are produced by water droplets and they are much smaller and more colorful than 22° halos.^[5]