20090811-0201 22° Solar Halo

22° Solar Halo

Photographed in Mahad on August 11, 2009.

A 22° halo is an optical phenomenon forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere. As the light beam passes through two sides of the prism forming a 60° angle, the angle of minimum deviation is almost 22° (e.g. 21,84° in average; 21,54° for red and 22.37 for blue.) This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish.

Light passing through the hexagonal ice prisms is deflected twice which produces deviation angles ranging from 22° to 50°. Lesser deviation results in a brighter halo along the inner edge of the circle, while greater deviation contribute to the weaker outer part of the halo. As no light is refracted at smaller angles than 22° the sky is darker inside the halo.

22° halos form when the sky contains millions of poorly oriented ice crystals. Some of these happen to be aligned perpendicular to the sun light as viewed by one observer which produces the illuminated 22° circle, while other crystals produces the same phenomenon for other observers. An Alexander's band can be seen inside the halo.

Like other ice halos, 22° halos appear when the sky is covered by thin cirrus clouds containing the ice crystals which causes the phenomenon. Small colourful coronas much nearer the sun produced by water droplets can occasionally be confused with 22° halos.

Ref. en.wikipedia.org/wiki/22%c2%b0_halo