What exactly is a sundog? You may have heard the term, and may have even witnessed the phenomenon without knowing what you were seeing. You also may have heard them referred to as "halos," and in some circles are even call "mock suns." Their scientific name is parhelia.
Sundogs are bright splashes of light seen on the sides of the Sun and are a type of ice crystal halo. When the Sun is close to the horizon, usually close in time to sunrise or sunset, and not blocked by anything such as trees or building, is an ideal time to see sundogs or halos. The observer must also be on the same horizontal plane as the ice crystals which form the sundog.
Winter also seems to be a favorable time, since the atmosphere is cold and dry. However, you can witness sundogs any time throughout the year when the conditions are right.
Sundogs are caused by sunlight passing through six sided ice crystals in the troposphere. These crystals are hexagonal prisms. Hexagonal prisms have a hexagonal top and bottom with six rectangular sides. These crystals tend to fall through the atmosphere similar to leaves falling from a tree with their flat side more or less faced down. Because of this, it allows the light from the Sun to be refracted into definite and specific patterns.
If the crystals fall in a random order, a halo is observed.
Sundogs are not rainbows. Rainbows are formed when the Sun's rays pass through drops of water. As the sun streams through the moist atmosphere after a storm, the water droplets act as prisms and appear as a rainbow. Rainbows always appear opposite from the Sun, and not around its edges.
Unlike rainbows, sundogs can be seen only by looking at the Sun (or in the case of the moondogs, looking at the moon). Sundogs can appear rainbow-like, as simple halos, or simply splashes of light, which are the result of refraction and reflection of solar or lunar light. They can appear as sun-like structures or comets, which in the past caused a lot of fear among people when they have been mistaken as supernatural events.
These crystals must be very small to produce these sundogs. They are found at many altitudes, but they must be in clouds that are 5 to 10 km above the ground to form sundogs and in cirrus or cirrostratus clouds. Frequently, two sundogs can be seen at the same time; one on each side of the sun. Crystals grown in cold, dry air display the most beautiful sundogs.