A half second exposure at ISO 1600 with Canon 350D. Wish I had a wide angle lens and I could have got the whole thing.

How are they formed

A halo (Greek: ἅλως; also known as a nimbus or Gloriole) is a ring of light that surrounds an object. Halos, also known as icebows, are also 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 optical 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 colours because of dispersion, similarly to the rainbow. Sometimes in very cold weather optical halos are formed by crystals close to ground level, called diamond dust. The crystals behave like jewels, refracting and reflecting sunlight between their faces, sending shafts of light in particular directions. Atmospheric phenomena such as halos were used as an empirical means of weather forecasting before meteorology was developed.