Solar Light Pillars, North Bay, Ontario, Canada

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. Their collective surfaces act as a giant mirror, which reflects the light source upwards and/or downwards into a virtual image. As the crystals are disturbed by turbulence, the angle of their surfaces deviates some degrees from the horizontal orientation, causing the reflection (i.e. the light pillar) to become elongated into a column. The larger 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.
Unlike a light beam, a light pillar is not physically located above or below the light source. Its appearance of a vertical column is an optical illusion, resulting from the collective reflection off the ice crystals, only those of which that appear to lie in a vertical line direct the light rays towards the observer (similar to the reflection of a light source in a body of water).