Sundogs are an atmospheric phenomenon you may never have heard of, let alone seen. Sundogs, also known as 'parahelia' or 'mock suns', are a pair of bright spots that form on either side of the sun. The name 'parahelia' literally means 'para'-'beside' and 'helion'-'the sun'. The name 'mock suns' comes from the fact that they are frequently the same size as the sun and just about as bright.
Sundogs are always situated on a line through the sun parallel with the horizon. They form at a distance of about 22 degrees from the sun, depending on how high the sun is above the horizon. To get an approximation of that distance, put your hand out in front of you at arms length, palm down. Now stretch your thumb and little finger out at right angles as far as you can. Place your thumb on the sun, and you will find a sundog about at your little fingertip. Remember, there are two, one on each side.
At their best, sundogs have a white-blue tail that extends outward from the sun parallel to the horizon. Frequently, small arcs of rainbow-like color will extend upward and downward in an arc from the sundog, with the reddish colors nearest the sun. The colors can be as bright as a rainbow or they can be subdued, pastel colors. Often, sundogs are so bright the observer is dazzled.
Sundogs are formed by refraction of light inside of ice crystals. When ice crystals form, they form like hexagonal (six-sided) plates. If the crystals are the right size, (about 30 micrometers or larger) and oriented so their flat sides are horizontal, they fall through the air like leaves. Sunlight striking the crystal is bent 22 degrees, and sundogs form. If the orientation of the crystals is random and not horizontal, a halo will form around the sun at a distance of 22 degrees, but the bright spost that define sundogs will not be there.
Some people mistake the colored arc of a sundog for a rainbow. Three major things distinguish sundogs from rainbows. For one, rainbows are formed by light passing through water droplets, whereas sundogs form by light passing through ice crystals. Secondly, if you look at a rainbow, the sun is at your back, but if you look at sundogs, you are facing the sun. Third, the angle of refraction for sundogs is 22 degrees, the angle of refraction for a rainbow is 48 degrees.
Related phenomenon are moondogs, also called 'paraselene' (selene - moon). These are virtually identical to sundogs, except they form around the moon. Because the moon is not as bright, moondogs are more subdued and the colored arcs are sometimes less obvious.
Believe it or not, sundogs are much more common than rainbows. There are certain conditions that must exist for rainbows to form. Water droplets must be present and they must be fairly large. Cloud droplets are 100 to 1000 times too small. The sun must be close to the horizon: rainbows do not form at midday when the sun is high in the sky. Sundogs, on the other hand, can form anywhere, because even at the equator, ice can form at extreme altitudes. The height of the sun is less critical too. Rainbows are not seen during the winter months in large portions of the world because the temperature doesn't get high enough. But this is a prime time for sundog formation.
Statistics show that sundogs occur on the average of twice a week in many locals. Rainbows, on the other hand, generally occur on the average of 10 to 20 times a year in those same areas.
There may be several reasons sundogs are not observed very much. Because many sundogs occur when it is cold, most people do not linger outside to observe them. Further, because one must face the sun to see sundogs, most people would rather protect their eyes than look towards the sun. The colored arcs, too, are sometimes mistaken for rainbows, so some sundogs are simply mislabeled.
Here is a link to a photograph of sundogs that are formed during the winter.
Atmospheric optical displays, which include sundogs, rainbows, halos and many other phenomena, are all very beautiful. So get out there and see what there is to see! Enjoy!