A halo ;also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky. They can also form around artificial lights in very cold weather when ice crystals called diamond dust are floating in the nearby air.

There are many types of ice halos. They are produced by the ice crystals in cirrostratus clouds high (5–10 km, or 3–6 miles) in the uppertroposphere. The particular shape and orientation of the crystals is responsible for the type of halo observed. Light is reflected andrefracted by the ice crystals and may split up into colors because of dispersion. The crystals behave like prisms and mirrors, refracting and reflecting sunlight between their faces, sending shafts of light in particular directions.

Atmospheric phenomena such as halos were used as part of weather lore as an empirical means of weather forecasting beforemeteorology was developed.

They often do mean that rain is going to fall within the next 24 hours as the cirrostratus clouds that cause them can signify an approaching frontal system.

http://en.wikipedia.org/wiki/Halo