One of the most common atmospheric displays is the 22° circular halo, such as this one that I photographed in Kanata on Saturday. Created by sunlight refracting through hexagonal ice crystals suspended in the atmosphere, they are visible throughout the year and are easiest to spot when the sky is covered with a thin layer of high altitude cirrus clouds. The halo is always a distance of 22° from the sun, or about the distance between your thumb and pinkie finger when your arm is fully extended with all your fingers spread. Although the hexagonal ice crystals may be randomly-oriented, the diameters of the crystals are less than 20.5 micrometers.
A 22° circular halo develops when light enters one side of the hexagonal column and exits through another side. The light is refracted twice: one when it enters the ice crystal, and once again when it leaves the ice crystal. These two refractions bend the light by 22° from its original direction.
I believe the brightness at the top of the halo is all that is visible of an upper tangent arc, a patch of bright light that touches the top of the 22° circular halo and, in better conditions, forms a pair of “wings” curving outward. A tangent arc develops when sunlight is refracted by falling hexagonal “pencil-shaped” ice crystals whose long axes are oriented horizontally.
For more information on halos and other atmospheric phenomena, this website provides some great information and photographs of various sun dogs, halos, and other fascinating lights in the sky.