Sundogs, Pillars or My!

Have you ever seen a "sundog"? How about a "pillar"? Well, maybe. What are they? Glad you asked. Basically, water droplets and ice crystals in the atmosphere refract and reflect light resulting in amazing optical phenomena. Are you ready to investigate further?


Image of a halo with sundogs.
 

"Sundogs" are also called "mock suns" or "parhelia" (from the Greek meaning with or beside the sun). The University of Illinois web site has a graphic illustrating how one is formed. Hexagonal plate-like ice crystals in the atmosphere, with their flat face horizontally oriented, refract or bend the sunlight 22 degrees leading to the formation of sundogs-bright spots on either side of the sun. The web abounds with pictures of sundogs (i.e. sundogs, January 28, Cool Atmospheric Phenomena and Sundogs over VLA). When light is refracted as it passes from one medium to another, a rainbow effect maybe observed due to dispersion. Therefore, it is not uncommon to see a sundog brightly coloured or all the colours of the rainbow! When sunlight is refracted by ice crystals randomly oriented, 22 degree halos occur. This time, light is refracted twice, once when entering and once when leaving the crystals. On Jan 11th, 1999, approximately 24 halos were observed at the South Pole. Was it a world’s record?

A "sun pillar" (or spectacular sun pillar) is a column or shaft of light reaching above or below the light source. In this case, plate or column shaped ice crystals reflect (or light pillars) the light. Pillars usually occur at sunset or sunrise, when the sun is low on the horizon. The web site reflection of light illustrates what happens when light is reflected. The reflected ray is the same colour as the incident ray. Pillars can be coloured, taking on the same colour as the light being reflected.

The sundogs and pillars do not just result from light refracted or reflected from the sun. One might chance to see a moondog or moon pillar or a Venus pillar (Venus Pillars and Dogs).