A sun dog (or sundog) or mock sun, formally called a parhelion (plural parhelia) in meteorology, is an atmospheric optical phenomenon that consists of a bright spot to one or both sides of the Sun. Two sun dogs often flank the Sun within a 22° halo.
The sun dog is a member of the family of halos, caused by the refraction of sunlight by ice crystals in the atmosphere. Sun dogs typically appear as a pair of subtly colored patches of light, around 22° to the left and right of the Sun, and at the same altitude above the horizon as the Sun.
They can be seen anywhere in the world during any season, but are not always obvious or bright. Sun dogs are best seen and most conspicuous when the Sun is near the horizon.
In addition to the shape of the ice crystals and the angle between the facets, the orientation of the ice crystals also determines the kind of optical phenomenon that will be created.
Plate-shaped crystals float in the air horizontally like a leaf. On the other hand, column-shaped ice crystals tend to float in the air vertically.
The Different Kinds of Atmospheric Phenomena
Two kinds of halos are most commonly observed from Earth – the 22-degree and the 46-degree halos. In general, halos can be seen throughout the year, around the world. However, they are more often seen in the winter months because the cold weather creates better conditions for the formation of halo-generating ice crystals.
22-degree halos are formed when light passing through an ice crystal bends 22 degrees, while 46-degree halos occur when the light bends 46 degrees.
A sundog, also known as sun dog, mock sun or parhelion, consists of glowing spots around the sun. They are created by sunlight refracting off plate-shaped ice crystals in the cirrus clouds. Sundogs are some of the most frequently observed optical phenomena and can be observed throughout the year and anywhere in the world. They are also associated with 22-degree halos.
Sundogs tend to be most visible when the Sun is close to the horizon. The part of a sundog closest to the Sun tends to be red in color, while the areas further away from the Sun generally appear blue or green.
Light from the Moon also creates glowing spots on both sides of the Moon, called moondogs. Also known as mock moon or paraselene, moondogs are rarer than sundogs because they only occur when the Moon is full or close to being full.