A
sun pillar is an ice-based
atmospheric effect. It appears as a column of light above or below the
sun when it is low in the sky. The former are more common and called
upper sun pillars and are most common after
sunset. The latter are called
lower sun pillars. Pillars adopt the coloration of their light source, and in conjuntion with the rich hues of sunset can be quite beautiful.
Sun pillars occur when plate-shaped ice crystals are uniformly distributed above or below (respectively) the sun, with their long axes horizontal. The primary shape of the effect is caused by simple reflection off of the base facets of the plates. Either the presence of needle shaped crystals or the dispersal of the sunlight through the edge facets of plates can widen and diffuse the effect, often in a slight (and metaphorically fitting) entasis of the columnar shape or, in extreme cases, a full horizontal diffusion. Less distinct pillars can be caused by similarly-oriented snow cystals.
The same effect is called by different names depending on the source of light. It is generically called a light pillar when it occurs with other sources such as lampposts. It is called a lunar pillar or moon pillar if the source is the moon. One online image even shows a low rising Venus as the source.
Conditions pending, pillars can be as long as 30° in the sky, but are more often 5 to 10°.
When both upper and lower sun pillars are visible at the same time as strong parhelia, the halo phenomenon appears as a giant cross in the sky. Such sights had a massive impact on medieval Christians such as Hildegard von Bingen, who felt it was a sure sign from God.
Similar conditions can result in subsuns and subparhelia.
Sources
- http://amsglossary.allenpress.com/glossary/search?p=1&query=sun+pillar
- http://www.meteoros.de/arten/ee08e.htm
- http://www.meteoros.de/arten/ee09e.htm
- The Venus pillar is at
http://www.sundog.clara.co.uk/halo/venpill.htm