Sun dogs, also called parhelia or sometimes mock suns, are bright, colourful patches of sky lying at the same elevation as the sun, just outside the 22º halo or the 46º halo (the latter being rarer and fainter). They are caused by sunlight refracted by the ice crystals in cirrus clouds, and are just one of the beautiful optical phenomena people miss out on if they don't look at the sky enough. They are actually quite common, especially in cold weather.

As with the circumzenith arc, the ice crystals needed to cause sun dogs are horizontally aligned hexagonal plate crystals. When the sun is very close to the horizon, they lie on top of the halo, if it is visible; the higher it climbs, the further they get; when the sun is higher than 60º they are not visible at all.

Although they hardly ever look much like real dogs, many sun dogs do have long, bluish-white tails.

In the frigid grasp of winter, when your nares are briefly sticking shut with each inhalation, and you’ve dispensed with gloves in favor of the huddled warmth of mittens, there is a visual redemption for the coldness endured. This is the weather for sundogs, pale, wraith-like images escorting the sun across the brilliant blue sky.

If you haven’t seen them, they are short arcs of light flanking the sun, often appearing as part of a faint halo totally encircling the sun. The most striking ones occur when the sun is near the horizon and the air is loaded with ice crystals, specifically tiny (less than 0.5 micrometers in diameter) hexagonal ice crystals suspended in thin, nearly invisible cirrostratus clouds. These ice crystals refract the sunlight at an angle of about 158º, resulting in the appearance of sundogs about 22º from the sun.* The amount of refraction is dependant on size of the crystals, so instead of a precise, neat rainbow, you get stretched pale images.
Sundogs are seen in short arcs in a horizontal plane with the sun because the pencil-like ice crystals predominantly tumble through the sky with their long axis oriented horizontally. The faint halo seen with the sundogs is from the crystals that are at the correct orientation for their location to refract light to your eye.

So there you are, a technical explanation for a simple apparition.
Have fun looking, but remember, don’t stare at the sun.

*The math: Line of sight is 180º ; subtract the 158º refraction and you get the image at 22º.

A variety of the sundog is the 120° parhelion. These are caused by the same hexagonal, plate-shaped ice crystals in the same orientation, but the plates must be sufficiently thick to allow sunlight to reflect off of two adjacent internal faces before exiting a base face.

They appear as glowing white spots at the same elevation as the sun, 120° to the left and/or right. If needle-shaped ice crystals are also present, you may see a parhelic arc that you can follow across the sky to see where the 120° parhelia should be. They are rarely as bright as primary parhelia, but as they are visually isolated from the sun and its major halos, they can be quite arresting, floating there like an eye of the unblinking sky.


Sources:
  • http://www.sundog.clara.co.uk/halo/120pars.htm
  • http://www.meteoros.de/arten/ee18e.htm

Sun"dog` (?), n. (Meteorol.)

A luminous spot occasionally seen a few degrees from the sun, supposed to be formed by the intersection of two or more halos, or in a manner similar to that of halos.

 

© Webster 1913


Sun"dog`, n. (Meteor.)

A fragmentary rainbow; a small rainbow near the horizon; -- called also dog and weathergaw.

 

© Webster 1913

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