22-Degree Halo

A 22-degree halo is an atmospheric optical phenomenon consisting of a ring with an apparent radius of about 22 degrees around the Sun or Moon, sometimes called a sun halo or, around the Moon, a moon ring, storm ring, or winter halo. It forms when sunlight or moonlight is refracted by millions of hexagonal ice crystals suspended in the atmosphere, which bend the light so that it concentrates into a ring at roughly 22 degrees from the light source, about the span of an outstretched hand at arm's length. Because the refraction is wavelength dependent, the inner edge of the ring often looks reddish and the outer edge bluish, and the sky inside the halo appears noticeably darker since light does not refract at angles smaller than 22 degrees. It is sometimes confused with a corona, a similar-looking ring produced by water droplets rather than ice crystals that is smaller and more vividly colored.