What causes a Halo around the Moon?
While sitting in my den watching television recently, a friend of mine from North Carolina called with the following statement. “My dad was out in the yard last night and looked up and noticed a very distinct halo around the moon, and he wanted to know what caused it?” My friend’s inquiry prompted this week’s science question of the week. A halo is a ring of light that surrounds either the sun or moon and typically appears as bright white rings. However, some halos can also have color patterns. A halo is an optical phenomenon similar in concept to a rainbow but also very different. Halos form when the sunlight or moonlight is refracted or bent by ice crystals associated with thin-high level clouds, like cirrus or cirrostratus. The most common type of halo is the 22 degree halo. In this halo, a ring of light 22 degrees from the sun or moon is projected by hexagonal (i.e. six-side) ice crystals with diameters less than 20.5 micrometers. As light passes through the crystal, it is bent or
