Wikipedia:Picture of the day/January 28, 2005
Halos are optical phenomena that appear near or around the Sun or Moon, and sometimes near other strong light sources such as street lights. There are many types of halos, but they are mostly caused by ice crystals in cold cirrus clouds located high (5-10 km, or 3-6 miles) in the upper troposphere. The particular shape and orientation of the crystals is responsible for the type of halo observed. Light is reflected and refracted by the ice crystals and may split up into colors because of dispersion, similarly to the rainbow.