December 28, 2004
These unusual columns of light pierce the night sky like a solar pillar accents a morning or evening sky (see yesterday's Earth Science Picture of the Day). The causes are unique, but the effects are similar. A solar or Sun pillar is caused by the Sun’s rays reflecting off the undersides of ice crystals stacked on top of each other in the lower troposphere. In the case of a light pillar, the light source isn't the Sun, but rather artificial lighting, such as the street lights above, reflecting off ice crystals close to the ground. For artificial pillars to form, the air near the ground must be sufficiently cold and the lower atmosphere quite stable. I've even seen pillars caused by car headlights. When you first notice these light shafts, you might think they're the northern lights, but then you'll observe that they're stationary -- the only thing that may vary is their intensity. The colour of the pillars will change depending on the source of the artificial light. See also the Earth Science Picture of the Day for May 4, 2004.
This photo was captured from the north end of Peterborough, Ontario on December 27, 2000. On this night, the pillars only lasted for about a half hour. When a light wind changed the orientation of the crystals, they quickly disappeared.Related Links: