A 22° halo is the optical phenomenon that appears as a large ring with an apparent radius of about 22 degrees around the Sun. It can also form around the Moon and is produced when light passes through millions of suspended hexagonal ice crystals in high, cold clouds.
The ring is usually associated with thin cirrus or cirrostratus clouds. As sunlight enters and leaves the ice crystals, refraction bends the light by angles that concentrate brightness roughly 22 degrees from the Sun. Viewed at arm’s length, the radius is approximately the width of an outstretched hand.
A 22° halo is often confused with a corona, but the two have different causes and appearances. Halos form through ice-crystal refraction and are relatively large; coronas are smaller, more colorful rings produced by diffraction through water droplets or tiny ice particles. A sundog is a bright patch beside the Sun, while a sun pillar is a vertical column of light.
Moon rings are sometimes treated as storm warnings because the clouds that produce them can precede weather fronts. However, a halo alone is not a reliable forecast of approaching bad weather.