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.