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.
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. As no light is refracted at angles smaller than 22° the sky is darker inside the halo. A 22° halo may be visible on as many as 100 days per year.
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.