Sundogs appear frequently at McMurdo. The most brilliant ones occur on a cold sunny morning or evening, when the sun is near the horizon and the air is loaded with ice crystals. The ice crystals refract (bend) the sunlight causing an image of the sun to appear on either or both sides of the sun.

We also see rings of light surrounding the sun or moon. These rings are called halos. Most halos appear as bright white rings but in some instances, the dispersion of light as it passes through ice crystals can cause a halo to have color.

Halos form when sunlight or moonlight is refracted by ice crystals associated with thin, high-level clouds (like cirrostratus clouds). A 22° halo is a ring of light 22 degrees from the sun (or moon) and is the most common type of halo observed.

Light undergoes two refractions as it passes through an ice crystal and the amount of bending that occurs depends upon the diameter of the crystal. In the case of a 22° halo, we will focus on hexagonal ice crystals with diameters less than 0.5 micrometers.

A 22° halo develops when light enters one side of a columnar ice crystal and exits through another side. The light is refracted when it enters the ice crystal and once again when it leaves the ice crystal.

The two refractions bend the light by 22° from its original direction, producing a ring of light observed at 22° from the sun or moon.

A tangent arc is a patch of bright light that is occasionally observed along a halo. This occurs when sunlight is refracted by falling hexagonal "pencil-shaped" ice crystals whose long axes are oriented horizontally.

So, there you have it, a technical explanation of something quite simple and beautiful to behold. On some days the halos are quite vivid, but ususally they are faint and difficult to see. With everything basically white in color in the Antarctic, looking up at the sun's brilliance is not an easy thing to do.