22° Halo
22 degree halo is the most common type of halo observed.
Halo is a ring of light surrounding the sun or the 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 color.
Halos form when light from the sun or moon is refracted by ice crystals at high level clouds like cirrostratus clouds. A 22 degree halo is ring of light 22 degree from the sun or moon. It is formed by hexagonal ice crystals with diameters less than 20.5 micrometers.
A 22 degree halo develops when light enters one side of a columnar ice crystal and exits through anothe side. The light is refracted when it enters the ice crystal and once again when it leaves the ice crystal.
references:
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/opt/ice/halo/22.rxml
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