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22 Degree Halo
a ring of light 22 degrees from the sun or moon
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 color.
|[Image: 22 degree halo (45K)]
|Halos form when light from the sun or moon is
by ice crystals associated with thin, high-level clouds (like
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.
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.
refractions bend the light by 22 degrees from its
producing a ring of light
observed at 22 degrees from the sun or moon.
[Image: tangent arc (56K)]
|A tangent arc is a patch of bright light that is occasionally observed
along a halo. This occurs
when sunlight is
refracted by falling hexagonal "pencil-shaped"
ice crystals whose long axes are oriented horizontally.
air, dust, haze
46 degree halo