Light pillar

Light pillars can also be caused by the Moon or terrestrial sources, such as streetlights and erupting volcanoes.

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.

[3] In very cold weather, the ice crystals can be suspended near the ground, in which case they are referred to as diamond dust.

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 (See drawing).