February 12, 2001
The photograph above shows a sun pillar (the column of red light above the horizon) on a stratocumulus cloud deck in the waning light of a winter's day. A sun pillar is a reflection phenomena, so only one color is involved, whereas with refraction, in general, multiple colors are observed. The red color results from the longer path-length of the sunlight (the Sun is below the horizon). Ice crystals in the stratocumulus clouds are oriented more or less horizontally (like six-sided dinner plates), in such a way that their faces reflect the sunlight. When sun pillars are observed, the sunlight is typically reflected off the bottom faces of hexagonal plate crystals. However, in some cases there may be an internal reflection - light is transmitted through a side face, bounces off of a bottom face and then out through the adjacent side face. The angle of the light that leaves the crystal is the same as the angle of light that enters it.