A halo is a ring of light surrounding the sun or moon. Most halos appear as bright white rings but in some instances, the dispersion of light as it passes through ice crystals found in upper level cirrus clouds can cause a halo to have colour.
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Halos form when light from the sun or moon is refracted by ice crystals associated with thin, high-level clouds, like cirrostratus clouds, which are commonly found between 5-10kms in the troposphere.
There are several types of halos, but in this article we will look at the two most common ones seen in Western Australia.
The 22 degree Halo
A 22 degree halo is a ring of light 22 degrees from the sun (or moon) and is the most common type of halo observed and is formed by hexagonal ice crystals with diameters less than 20.5 micrometers (1mm = 1000 micrometers)
Light undergoes two refractions as it passes through an ice crystal and the amount of bending that occurs depends upon the ice crystal’s diameter.
A 22 degree halo develops when light enters one side of a columnar ice crystal and exits through another side. The light is refracted when it enters the ice crystal and once again when it leaves the ice crystal.
The two refractions bend the light by 22 degrees from its original direction, producing a ring of light observed at 22 degrees from the sun or moon.
The 46 degree Halo
A 46 degree halo is a ring of light observed 46 degrees from the sun or moon. Although they are less common than 22 degree halos, the process by which they form is similar.
What determines if a 46 degree halo or a 22 degree halo develops is the path of the light as it passes through hexagonal ice crystals. A 22 degree halo results from “in one side, out another side”, whereas a 46 degree halo results from “in one side, out the bottom“.
These ice crystals are hexagonal-shaped columns with diameters between 15 and 25 micrometers and have an appearance resembling tiny pencils.
A 46 degree halo develops when light enters one side of a columnar ice crystal and exits from either the top or bottom face of the crystal. The light is refracted twice as it passes through the ice crystal and the two refractions bend the light by 46 degrees from its original direction. This bending produces a ring of light observed at 46 degrees from the sun or moon.
Sometimes, you will notice that the inside of the halo appears darker. This is because light is deflected by the ice into a wide range of directions larger than 22°, but no light goes below that angle. This means that no light is added by the ice into the inside of the halo, at angles lower than 22°. Technically, the sky inside the halo is the same brightness as it would otherwise have been without the ice.
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The text and diagrams in this article are used with permission under non-commercial rights from the University of Illinois WW2010 Project. All photos are copyright and remain the property of named photographer.