Sun dog

Very bright sun dogs in Fargo, North Dakota. Also visible are parts of the 22° halo (the arcs passing through each sundog), a sun pillar (the vertical line) and the parhelic circle (the horizontal line).

A sun dog (or sundog) or mock sun, formally called a parhelion[1] (plural parhelia) in meteorology, is an atmospheric optical phenomenon that consists of a bright spot to the left or right 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.

Formation and characteristics

Right-hand sun dog in Salem, Massachusetts, Oct 27, 2012. Also visible are a Parry arc, an upper tangent arc, a 22° halo and part of the parhelic circle.

Sun dogs are commonly caused by the refraction and scattering of light from plate-shaped hexagonal ice crystals either suspended in high and cold cirrus or cirrostratus clouds, or drifting in freezing moist air at low levels as diamond dust.[2] The crystals act as prisms, bending the light rays passing through them with a minimum deflection of 22°. As the crystals gently float downwards with their large hexagonal faces almost horizontal, sunlight is refracted horizontally, and sun dogs are seen to the left and right of the Sun. Larger plates wobble more, and thus produce taller sundogs.[3]

Sun dogs are red-colored at the side nearest the Sun; farther out the colors grade through oranges to blue. The colors overlap considerably and are muted, never pure or saturated.[4] The colors of the sun dog finally merge into the white of the parhelic circle (if the latter is visible).[5]

The same plate-shaped ice crystals that cause sun dogs are also responsible for the colorful circumzenithal arc, meaning that these two types of halo tend to co-occur.[6] The latter is often missed by viewers, since it is located more or less directly overhead. Another halo variety often seen together with sun dogs is the 22° halo, which forms a ring at roughly the same angular distance from the sun as the sun dogs, thus appearing to interconnect them. As the Sun rises higher, the rays passing through the plate crystals are increasingly skewed from the horizontal plane, causing their angle of deviation to increase and the sun dogs to move farther from the 22° halo, while staying at the same elevation.[7]

It is possible to predict the forms of sun dogs as would be seen on other planets and moons. Mars might have sun dogs formed by both water-ice and CO2-ice. On the gas giant planets — Jupiter, Saturn, Uranus and Neptune—other crystals form clouds of ammonia, methane, and other substances that can produce halos with four or more sun dogs.[8]

Halo with sun dogs in Hesse, Germany - August 12, 2012
Other Languages
català: Parheli
čeština: Parhelium
dansk: Bisol
Deutsch: Nebensonne
eesti: Parheelia
español: Parhelio
français: Parhélie
한국어: 환일
Հայերեն: Պարհելիում
hrvatski: Lažno Sunce
Bahasa Indonesia: Parhelion
íslenska: Aukasól
italiano: Parelio
עברית: כלבי השמש
қазақша: Сағым күн
Latina: Parhelium
Lëtzebuergesch: Niewesonn
magyar: Melléknap
മലയാളം: സൺ ഡോഗ്സ്
Nederlands: Parhelium
日本語: 幻日
norsk: Bisol
norsk nynorsk: Bisol
português: Parélio
română: Halou
русский: Паргелий
slovenščina: Parhelij
srpskohrvatski / српскохрватски: Lažno Sunce
svenska: Vädersol
українська: Паргелій
Tiếng Việt: Mặt trời giả
žemaitėška: Saulabruolē
中文: 幻日