A sundog is an atmospheric phenomenon that creates bright spots of light in the sky, often on a luminous ring or halo on either side of the sun.
Sundogs may appear as a coloured patch of light to the left or right of the sun, 22° distant and at the same distance above the horizon as the sun, and in ice halos. They can be seen anywhere in the world during any season, but they are not always obvious or bright. Sundogs are best seen and are most conspicuous when the sun is low.
Sundogs are commonly made by the refraction of light from plate-shaped hexagonal ice crystals in high and cold cirrus clouds or, during very cold weather, these ice crystals are called diamond dust, and drift in the air at low levels. These crystals act as prisms, bending the light rays passing through them with a minimum deflection of 22°. If the crystals are randomly oriented, a complete ring around the sun is seen — a halo. But often, as the crystals sink through the air, they become vertically aligned, so sunlight is refracted horizontally — in this case, sundogs are seen.
As the sun rises higher, the rays passing through the crystals are increasingly skewed from the horizontal plane. Their angle of deviation increases, and the sundogs move further from the sun. However, they always stay at the same elevation as the sun.
Sundogs are red-coloured at the side nearest the sun. Farther out the colours grade through oranges to blue. However, the colors overlap considerably and so are muted, never pure or saturated. The colors of the sundog finally merge into the white of the parhelic circle (if the latter is visible).
It is theoretically possible to predict the forms of sundogs as would be seen on other planets and moons. Mars might have sundogs formed by both water-ice and CO2-ice. On the giant gas planets — Jupiter, Saturn, Uranus and Neptune — other crystals form the clouds of ammonia, methane, and other substances that can produce halos with four or more sundogs.
| image: Very bright sundogs in Fargo, North Dakota.
Sundogs, also known as Mock Suns or Parhelia, are a pair of brightly coloured spots on either side of the Sun. They are most easily seen when the Sun is low. The sundog in the photo was observed November 23, 2011 near Donnelly Dome in Alaska, US.
Sundogs form when sunlight is refracted by ice crystals with diameters larger than 30 micrometers. They are visible when the Sun is near the horizon and on the same horizontal plane as the observer and the ice crystals. When the sunlight passes through the ice crystals, it is bent by 22° before reaching the observer’s eyes, just like what happens with 22° haloes. The bending of the light causes sundogs to form.
Sundogs differ from haloes in the preferential orientation of the ice crystals. If the hexagonal ice crystals are oriented with their flat faces horizontal, then a sundog is seen. If the crystals are randomly oriented, a halo is seen.