A light pillar is an atmospheric optical phenomenon in the form of a vertical band of light which appears to extend above and/or below a light source. The effect is created by the reflection of light from numerous tiny ice crystals suspended in the atmosphere or clouds. The light can come from the Sun (usually when it is near or even below the horizon) in which case the phenomenon is called a sun pillar or solar pillar. It can also come from the Moon or from terrestrial sources such as streetlights.
Since they are caused by the interaction of light with ice crystals, 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.