{{Short description|Reflections of a light source created by ice crystals in the air}} [[File:Light Pillars and Diamond Dust in London Ontario Canada.jpg|thumb|Light pillars in London, Ontario, Canada]] [[File:Sun pillar and kitesurfers.jpg|thumb|Sun pillar in San Francisco, California]] A '''light pillar''' or '''ice pillar''' is an atmospheric optical phenomenon in which a vertical beam of light appears to extend above and/or below a light source. The effect is created by the reflection of light from tiny ice crystals that are suspended in the atmosphere or that compose high-altitude clouds (e.g. cirrostratus or cirrus clouds).<ref name="OMM">{{cite web|url-status=dead |url=http://www.eumetcal.org/euromet/glossary/sunpilla.htm|title=Colonne lumineuse|publisher=World Meteorological Organization|archive-date=23 September 2016|archive-url=https://web.archive.org/web/20160923160931/http://www.eumetcal.org/euromet/glossary/sunpilla.htm}}</ref> If the light comes from the Sun (usually when it is near or even below the horizon), the phenomenon is called a '''sun pillar''' or '''solar pillar'''. Light pillars can also be caused by the Moon or terrestrial sources, such as streetlights and erupting volcanoes.<ref>{{cite web |title=Light Pillar over Volcanic Etna |url=https://apod.nasa.gov/apod/ap211115.html |website=apod.nasa.gov |publisher=NASA |access-date=2 June 2024 |date=15 November 2021}}</ref>
== Formation == thumb|Scheme of light pillars 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. 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. The larger and more numerous the crystals, the more pronounced this effect becomes. More rarely, column-shaped crystals can cause light pillars as well.<ref>{{cite web |date= |title=Sun pillars from column crystals |url=https://www.atoptics.org.uk/halo/colpill.htm |access-date=29 September 2025 |website=www.atoptics.org.uk}}</ref> In very cold weather, the ice crystals can be suspended near the ground, in which case they are referred to as diamond dust.<ref>{{cite web|url=http://apod.nasa.gov/apod/ap131218.html|title=APOD: 2013 December 18 – Light Pillars over Finland|website=apod.nasa.gov}}</ref>
Unlike a light beam, a light pillar is not physically located above or below the light source. 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). This is similar to viewing a light source on a body of water. Ripples on the surface of the water reflect the light source in many directions, and those that happen to be aimed at the viewer, combine to form a bright line pointing toward the light source.<ref>{{cite web |date= |title=Light Pillars |url=https://www.atoptics.org.uk/halo/lpil.htm |access-date=29 September 2025 |website=www.atoptics.org.uk}}</ref>
== See also == * Crepuscular rays * Diamond dust * Halo * Light beam * Sun dog * Crown flash * False sunrise * False sunset
== References == {{Reflist}}
== External links == {{Commons|Sun pillar}} * [https://www.atoptics.org.uk/halo/pillar.htm Pillars]. Atmospheric Optics. Explanations (10 pages) and many images. * [http://www.islandnet.com/~see/weather/eyes/pillars.htm Light Pillars]: An Introduction to Sun Pillars and Related Phenomena. The Weather Doctor's Weather Eyes. Another nice explanation, all on one page * [http://www.smh.com.au/news/science/fabulous-frozen-frames/2006/10/31/1162278140099.html Fabulous frozen frames] – Sydney Morning Herald. November 1, 2006 * [http://antwrp.gsfc.nasa.gov/apod/ap081215.html A Sun Pillar Over North Carolina]. NASA Astronomy Picture of the Day, 15 December 2008
Category:Atmospheric optical phenomena Pillar Category:Light sources Category:Snow or ice weather phenomena