As the remnants of Superstorm Sandy approached us on Oct. 29, people in Wisconsin observed a halo on two consecutive nights. These halos resulted from the ice clouds generated from the storm.
A halo is a whitish ring that encircles but does not touch the sun or moon. It is an optical phenomenon that owes its existence to the bending of light by ice crystals, much like the “rainbow crystals” you may hang in your windows.
The most commonly observed halo is the 22 degree halo. This halo encircles the moon or sun at about a hand’s width from the center of the moon, if your arm is fully extended. Small column-like ice crystals form the halo. Light rays enter a crystal, bend or refract, and then refract again as they exit the crystal. Because the crystals are randomly oriented in space, there are many different directions from which light rays can enter the crystals. More light rays are refracted at this 22-degree angle than at any other, producing the concentration of light known as the halo.
If you were lucky, you may have seen shiny, colored regions at either side of the moon. These are called moondogs, and are another optical effect caused by refraction. Moondogs appear because hexagonal ice crystals in the high clouds tend to drift downward with their flat bases parallel to the ground. The sunlight passing through the crystal refracts sideways.