Halos are optical phenomena that appear as bright circles or rings around a light source, often seen around the sun or moon. They are caused by the refraction, reflection, and dispersion of light through ice crystals in the atmosphere. This effect is commonly observed in cirrus clouds, which contain these ice crystals. Halos can take various forms, including 22-degree halos, which are the most common, and parhelia, also known as sun dogs, which appear as bright spots on either side of the halo. These phenomena are often used in weather prediction, as their presence can indicate changes in atmospheric conditions.