A halo is an optical phenomenon caused by light interacting with ice crystals floating in the atmosphere (usually from the Sun or Moon). Halos come in a variety of shapes and sizes, ranging from colorful or white rings to arcs and spots in the sky. Many of these may be seen near the Sun or Moon, but others can be found elsewhere or even in the other direction of the sky. The circular halo (officially termed the 22° halo), light pillars, and sun dogs are among the most well-known halo forms, but many more exist; some are very frequent, while others are (extremely) rare.
Even though it is one of the most common types of halo, the exact shape, and orientation of the ice crystals responsible for the 22° halo are the topic of debate. Hexagonal, randomly oriented columns are usually put forward as the most likely candidate, but this explanation presents problems, such as the fact that the aerodynamic properties of such crystals leads them to be oriented horizontally rather than randomly. Alternative explanations include the involvement of clusters of bullet-shaped ice columns.
Light is deflected twice as it travels through the hexagonal ice prisms’ 60° apex angle, resulting in deviation angles ranging from 22° to 50°. The minimal deviation angle is almost 22° (or, more precisely, 21.84° on average; 21.54° for red light and 22.37° for blue light). Because of this wavelength-dependent difference in refraction, the inner edge of the circle is crimson and the outside edge is blue.
The ice crystals in the clouds all deviate the light in the same way, but only those from a specific ring at 22 degrees contribute to the effect for an observer at a certain distance. The sky is darker inside the halo because no light is refracted at angles smaller than 22°.
The corona is another phenomenon that produces a ring around the Sun or Moon and is frequently mistaken with the 22° halo. However, unlike the 22° halo, it is created by water droplets rather than ice crystals, therefore it is considerably smaller and more colorful.