A Sundog occurs when the sunlight refracts through ice crystals in the clouds which creates a rainbow effect. They are seen around 22˚ to the left or right of the Sun and are often in pairs, either side of the Sun but sometimes there's just one.
The Sun Halo is a circle of light which creates a ring around the Sun at 22˚ and is related to the Sundog phenomenon. The halo is often seen much higher in the sky than the Sundog. Halos are seen more in the winter months because the cold weather creates better conditions to form crystals.
Moon Halos are also created by a similar set of conditions. To read about Moon Halos see our Astro MOON DIARY 2019