DEFINITION OF SUN HALOS

A number of unusual phenomena are related to

the interaction of the Sun’s rays with ice crystals in the upper

atmosphere. A ring of light that circles and extends outward

from the Sun, or the Moon, is known as a halo. The halo

forms when the Sun’s or Moon’s rays get refracted by ice crystals

in high-level cirriform clouds. Most sun halos form at an

angle of 22° from the Sun because randomly oriented small ice

particles refract the light at this angle. Occasionally a 46° halo

is visible, formed when sub-horizontally oriented columnar ice

crystals refract the light at this higher angle. Most sun halos

are simply bright bands of light but some exhibit rainbow-like

zones of color. These form when the light is dispersed by the

ice crystals and light of different wavelengths (colors) is

refracted by different amounts depending on its speed.

Atmospheric crystals sometimes cause hexagonal or

platy ice crystals to fall slowly through the atmosphere, and

this vertical motion causes the crystals to become uniformly

oriented with their long dimensions in a horizontal direction.

This orientation prevents light that is refracted through the

ice crystals from forming a halo, but when the Sun approaches

the horizon it causes two bright spots or colored bright

spots to appear on either side of the sun. These spots are

commonly called sundogs, or parhelia.

Sun pillars are a similar phenomenon but are formed by

light that is reflected off the ice crystals instead of refracted

through them. In this case, usually at sunset or sunrise, the

Sun’s rays reflect off the sub-horizontally oriented ice crystals

and form a long column of light extending downward from

the Sun.

See also RAINBOW.

sun pillars See SUN HALOS.

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