A halo is a ring of light encircling and extending away from the sun or moon. The display of light is produced when sunlight or moonlight is refracted off tiny ice crystals contained within cirrostratus clouds. The most common type of halo produced by the refraction of light through ice crystals is a 22º halo. A less common type of halo is a 46º halo that forms in a similar way that 22º halos form; however, the shape and orientation of the ice crystals from which they form differ from those of a 22º halo. A halo can also form around artificial lights in the presence of ice crystals.
Process of formation of a halo
A halo is an optical display of light produced around the sun or moon by the refraction of light through ice crystals contained within cirrostratus clouds. A halo usually forms as a circle surrounding the sun or moon; however, a halo may also be created around other sources of light given that the weather is cold and ice crystals are suspended in the atmosphere. There are many types of halos and most of them are produced when cirrostratus clouds containing ice crystals form in the upper troposphere. The characteristic form of a halo is determined by the orientation and shape of the ice crystals.
A halo begins to form when cirrostratus clouds cover the sky at about 5-10 km (3-6 miles) of altitude. These types of clouds are so thin that the sun or moon can be clearly observed through them. The ice crystals within cirrostratus clouds bend the light passing through them, forming a halo. A 22º halo is created by the minimum deviation of refracted light off hexagonal ice crystals. As the light gets refracted through the ice crystals, light dispersion causes the inner edge of the halo to turn red while the outer edge of the halo is blue.
A 46º halo is a ring of light that forms around the sun or moon. The process of its formation is similar to that of a 22º halo; however, the orientation and shape of the ice crystals from which they form is different from those of the 22º halo. A 46º halo is produced at minimum light deviation through 90º prisms, which results from randomly aligned ice crystals in the sky. The 90º dispersion causes this halo to be more widely dispersed than the 22º halo, producing more diffused light on the outer edges.
Upper tangent arc
This type of halo usually is produced tangentially to the 22º halo in the sun or moon. A tangent arc forms when hexagonal pencil-like ice crystals in cirrostratus clouds descend with their long axes oriented horizontally. Like any other type of halo, a tangent arc usually shows a reddish inner edge color and a bluish outer edge color due to more intense refraction of red light than blue light. The horizontal orientation of the ice crystals also produces other types of halos known as sun dogs, which appear as a pair of bright patches on either side of the sun.
A sun dog or perihelion, meaning “beside the sun,” is an optical phenomenon that creates a pair of bright colored spots on either side of the sun. Sun dogs are often seen 22º apart from each other and at the same distance above the horizon as the sun. their color usually vary from red on the inner side to blue on the outer side. Sun dogs are caused by the horizontal orientation of ice crystals contained in cirrus clouds or by a type of ice crystals called diamond dust during very cold weather. These crystals bend light horizontally creating sun dogs.
The formation of a halo around the sun or moon indicates that cirrus clouds are present in the sky. Often a halo around the moon predicts the development of a storm front. A halo is as large as the tips of your thumb and little fingers stretched at arms length. Extending your arm like this, the tips of your fingers should be roughly 20º. You should always protect your eyes from the sun, when observing a halo. It’s preferable to observe the halo around the sun by hiding the sun from behind the edge of a building.