ph . Astronomy . Galleries . Atmospheric phenomena . 22° solar halo

A 22° ice halo around the Sun.

Halos, cousins to rainbows, can sometimes be seen around the Sun and the Moon. They are caused by light refracted by ice crystals. Such ice halos form in skies in which there are cirrus clouds because these clouds are composed of tiny ice crystals (unlike most other clouds which consist of drops of water).

Ice crystals have an hexagonal shape, and may be thin segments (like an 8-sided coin) or longer columnar crystals (not unlike a pencil). When light is refracted by these columnar crystals, a halo is formed. On the other hand, light passing through a thin-segment crystal causes a perhelion, or a circumzenithal arc. (Light reflected off the top of a thin crystal forms a subsun, while reflection off the bottm surface causes a sun pillar.)

A fully-formed halo, such as in the photo above, appears as a diffuse ring of light, about 1.5° wide, with an angular radius of about 22° (hence the name, a "22° ice halo"). The inner edge of the halo is quite distinct; particularly when seen around the Sun it is markedly red. The outer edge tends to be indistinct and colourless; very occasionally it may be faintly blue.

More often than not, only a partial halo is seen, because the cirrostratus in which it is formed does not cover enough of the sky for a complete ring to form. The thickness of the cloud also plays a role. As a warm front moves in, and the cloud thickens, the halo becomes fainter and finally disappears. A slow-moving weather front can produce a halo visible for several hours.

To form a halo, there must be a uniform distribution of randomly oriented crystals throughout the cloud in which it is seen, which will ensure the light is scattered in every possible direction. However, random orientation is possibly only if crystals tumble as they fall, and this is something that only the smallest crystals can do.

The remarkable photograph above clearly shows a complete halo, with a markedly red inner edge. It was taken on 2005 November 10, and I received it via e-mail. Unfortunately, the photographer was not identified. The photo was apparently sent to a newspaper (or was taken by a newspaper photographer) with a query as to the origin of the halo. If anyone knows who took the photo, please let me know.

Keywords: Astronomy, astrophotography, atmospheric phenomena, sunset, sunrise