Sun Column

Solar Pillar

Description

A light pillar is an atmospheric optical phenomena in which a vertical beam of light appears o extend above and/or below the light source. The effect is created by the reflection of light from tiny ice crystals that are suspended in the atmosphere or that comprise high altitude clouds such as cirrostratus or cirrus. If the light comes from the Sun (usually when it is near or even below the horizon), the phenomenon is called a sun pillar or solar pillar. Light pillars can also be caused by the Moon or terrestrial sources such as street lights or erupting volcanoes.

Sun pillar photographed near San Francisco (Source: Wikicommons, Author: Brocken Inaglory)

Formation

Since they are caused by the interaction of light with ice crystals, light pillars belong to the family of halos. The crystals responsible for light pillars usually consist of flat, hexagonal plates, which tend to orient themselves more or less horizontally as they fall through the air. Each flake acts as a tiny mirror which reflects light sources that are appropriately positioned below it (see drawing), and the presence of flakes at a spread of altitudes causes the reflection to be elongated vertically into a column. The larger and more numerous the crystals, the more pronounced this effect becomes. More rarely, column-shaped crystals can cause light pillars as well. In very cold weather, the ice crystals can be suspended near the ground, in which case they are referred to as "Diamond dust".

Formation of sun pillar: Light from source reflects off ice crystals in high level cloud towards observer creating visual illusion of pillar of light over the light source.

Unlike a light beam, a light pillar is not physically located above or below the light source. Its appearance as a vertical line is an Optical illusion, resulting from the collective reflection off the ice crystals; but only those that are in the common vertical plane, direct the light rays towards the observer. This is similar to viewing a light source on a body of water. Ripples on the surface of the water reflect the light source in many directions, and those that happen to be aimed at the viewer, combine to form a bright line pointing toward the light source.