A halo is a white or faintly coloured ring that sometimes appears around the sun; or in rare instances, the moon.
Haloes require ice crystals to form - either from high cirrus cloud of free falling crystals. Typically sunlight or moonlight is reflected by ice crystals producing a white halo. However, if the light rays strike the light at a particular angle, some light may be refracted. In such situations the halo will have a faint colouration.
The majority of ice crystals are hexagonal, or six-sided, and the most common angle of refraction through such a crystal is about 22 degrees. This is the most frequent type of halo and they are named 22° Haloes. Crystals with a different structure or that are at a different angle to the Sun may result in haloes that vary in size. Small haloes produced at 9° and larger ones at 46° are relatively common, and in some cases only part of a halo forms; appearing like an arc.
In the past haloes and other such atmospheric phenomena were used as an empirical form of weather forecasting before the development of meteorology. There is some degree of truth in this. The high cirrus cloud which contains the ice crystals required for haloes to form often signifies an approaching frontal system; however in many cases the front will be inactive or simply change course from the area - producing no rain.