Halo (optical phenomenon)
A halo is a ring, arc, or other luminous pattern around the Sun or Moon, produced when light interacts with ice crystals in the atmosphere.
Refraction: The change in a wave’s direction as it passes between materials with different refractive indices. Light bends as it enters and leaves ice crystals, producing many common halo angles.
22° halo: A circular halo with an angular radius of about 22 degrees around the Sun or Moon. This is the most familiar ring, formed by refraction through randomly oriented hexagonal ice crystals.
Cirrus cloud: A high, thin cloud composed mainly of ice crystals. Cirrus clouds often supply the crystals that create halos around the Sun or Moon.
Corona (optical phenomenon): A set of colored rings close to the Sun or Moon, produced mainly by diffraction through small water droplets or ice particles. Coronas form by diffraction and lie close to the light source, unlike most ice-crystal halos.
Hexagonal ice crystal: An ice crystal with a six-sided hexagonal structure, common in cirrus clouds. Its hexagonal faces create the characteristic angular paths that form halos.
46° halo: A large, faint halo with an angular radius of about 46 degrees around the Sun or Moon. It shares the 22-degree halo’s ice-crystal source but follows a different path through the crystal.
Ice crystal: A solid particle of water whose molecules form an ordered crystalline structure. Atmospheric ice crystals are the material that redirects light into halo patterns.
Rainbow: A colored arc produced when sunlight is refracted, internally reflected, and dispersed in water droplets. Rainbows arise from liquid droplets and appear opposite the Sun, rather than around it.
Parry arc: A rare halo arc formed by sunlight passing through horizontally oriented, plate-like ice crystals. It shows how a particular crystal orientation produces a distinctive arc.
Sun dog: A bright colored patch that appears beside the Sun, usually near the 22-degree halo. Sun dogs are localized parts of the broader family of ice-crystal halos.