Sun dog

A bright, often colored patch of light that appears beside the Sun when sunlight refracts through ice crystals in the atmosphere.

Atmospheric refraction: The bending of light as it passes through layers or particles in Earth’s atmosphere. Refraction through ice crystals redirects sunlight into the bright patches.
Refraction: The change in direction of a wave as it passes between materials with different wave speeds. Sun dogs are a visible result of light changing direction inside ice.
Moon dog: A bright patch beside the Moon, produced when moonlight refracts through atmospheric ice crystals. It is the nighttime counterpart of a sun dog, formed by the same optical process.
Atmospheric optics: The study of optical phenomena produced by interactions between light and Earth’s atmosphere. Sun dogs are a recognizable example of atmospheric optics.
Hexagonal ice crystal: An ice crystal whose molecular structure forms a six-sided hexagonal lattice. Its hexagonal prism faces refract sunlight into the characteristic 22-degree halo region.
Dispersion (optics): The separation of light into its component wavelengths because different wavelengths refract by different amounts. Dispersion spreads a sun dog’s colors, with red typically nearest the Sun.
Halo (optical phenomenon): A ring or arc of light around the Sun or Moon, commonly formed by atmospheric ice crystals. Sun dogs belong to the broader family of ice-crystal halos.
Cloud observation: The observation and classification of clouds using their appearance, structure, and atmospheric setting. Sun dogs can signal ice crystals in high or cold clouds.
Parry arc: A rare, brightly colored halo arc produced by sunlight passing through horizontally oriented plate-shaped ice crystals. It shares the role of crystal orientation in shaping atmospheric light displays.
Cloud ice crystal: A microscopic ice particle suspended in a cloud, often forming a hexagonal prism or plate. Sun dogs require suitably shaped ice crystals suspended in the atmosphere.

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Light pillarRelated: The same horizontally aligned plates can produce sun dogs as well as reflected pillars.