Editing Fog (section)

== Formation ==
{{see also|Cloud physics}}
[[File:FogParticles.jpg|thumb|right|Minute droplets of water constitute this after-dark radiation fog, with an ambient temperature of {{cvt|-2|C|F|0}}. Their [[motion blur|motion trails]] are captured as streaks.]]
[[File:FogParticlesHighSpeed.jpg|thumb|right|A close-up view of water droplets forming fog. Those [[defocus aberration|outside]] the camera lens's [[depth of field]] appear as [[backscatter (photography)|orbs]].]]

Fog forms when the difference between air temperature and [[dew point]] is less than {{cvt|2.5|C-change|lk=on}}.<ref>{{cite web|url=http://glossary.ametsoc.org/wiki/Fog|title=Fog – AMS Glossary|access-date=16 March 2013|url-status=live|archive-url=https://web.archive.org/web/20130327012718/http://glossary.ametsoc.org/wiki/Fog|archive-date=27 March 2013}}</ref><ref>{{cite web |url=https://www.weather.gov/media/zhu/ZHU_Training_Page/fog_stuff/fog_guide/fog.pdf |title=Fog |publisher=National Weather Service |date=2022}}</ref> Fog begins to form when [[water vapor]] [[condensation|condenses]] into tiny water droplets that are suspended in the air. Some examples of ways that water vapor is condensed include wind convergence into areas of upward motion;<ref name="convection">{{cite book|author=Robert Penrose Pearce|year=2002|url=https://books.google.com/books?id=QECy_UBdyrcC&pg=PA66 |title=Meteorology at the Millennium|publisher=Academic Press|page=66|isbn=978-0-12-548035-2 |access-date=2 January 2009}}</ref> precipitation or [[virga]] falling from above;<ref>{{cite web|website=[[National Weather Service]] Office, Spokane, Washington|url=http://www.wrh.noaa.gov/otx/outreach/ttalk/virga.php |title=Virga and Dry Thunderstorms|url-status=live|archive-url=https://web.archive.org/web/20090522112015/http://www.wrh.noaa.gov/otx/outreach/ttalk/virga.php|archive-date=22 May 2009}}</ref> daytime heating evaporating water from the surface of oceans, water bodies, or wet land;<ref>{{cite web|author1=Bart van den Hurk |author2=Eleanor Blyth |year=2008 |url=http://www.knmi.nl/~hurkvd/Loco_workshop/Workshop_report.pdf |title=Global maps of Local Land-Atmosphere coupling |publisher=KNMI |access-date=2 January 2009 |url-status=dead |archive-url=https://web.archive.org/web/20090225074154/http://www.knmi.nl/~hurkvd/Loco_workshop/Workshop_report.pdf |archive-date=25 February 2009 }}</ref> [[transpiration]] from plants;<ref>{{cite web|author1=Krishna Ramanujan |author2=Brad Bohlander |year=2002 |url=http://www.gsfc.nasa.gov/topstory/20020926landcover.html|title=Landcover changes may rival greenhouse gases as cause of climate change|publisher=[[National Aeronautics and Space Administration]] [[Goddard Space Flight Center]] |access-date=2 January 2009 |url-status=dead |archive-url = https://web.archive.org/web/20080603022239/http://www.gsfc.nasa.gov/topstory/20020926landcover.html |archive-date = 3 June 2008}}</ref> cool or dry air moving over warmer water;<ref>{{cite web|author=[[National Weather Service]] JetStream|year=2008|url=http://www.srh.weather.gov/srh/jetstream/synoptic/airmass.htm|title=Air Masses|access-date=2 January 2009|url-status=dead|archive-url=https://web.archive.org/web/20081224062959/http://www.srh.weather.gov/srh/jetstream/synoptic/airmass.htm|archive-date=24 December 2008}}</ref> and lifting air over mountains.<ref name="MT">{{cite web|author=Michael Pidwirny|year=2008|url= http://www.physicalgeography.net/fundamentals/8e.html |title=CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes|publisher=Physical Geography|access-date=1 January 2009|url-status=live|archive-url= https://web.archive.org/web/20081220230524/http://www.physicalgeography.net/fundamentals/8e.html |archive-date=20 December 2008}}</ref> Water vapor normally begins to condense on [[Cloud condensation nuclei|condensation nuclei]] such as dust, ice, and salt in order to form clouds.<ref>{{cite web|url=http://glossary.ametsoc.org/wiki/Front |website=Glossary of Meteorology|date=25 April 2012|publisher=[[American Meteorological Society]]|title= Front |url-status=live|archive-url=https://web.archive.org/web/20181010010455/http://glossary.ametsoc.org/wiki/Front |archive-date= 10 October 2018}}</ref><ref name="DR">{{cite web|author=Roth, David M.|title=Unified Surface Analysis Manual|date=14 December 2006|access-date=9 October 2010 |publisher= [[Hydrometeorological Prediction Center]] |url=http://www.wpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf |url-status=live|archive-url=https://web.archive.org/web/20060929004553/http://www.hpc.ncep.noaa.gov/sfc/UASfcManualVersion1.pdf |archive-date=29 September 2006}}</ref> Fog, like its elevated cousin [[Stratus cloud|stratus]], is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass.<ref>{{cite web|author=FMI|year=2007|url= http://www.zamg.ac.at/docu/Manual/SatManu/main.htm?/docu/Manual/SatManu/CMs/FgStr/backgr.htm |title=Fog And Stratus – Meteorological Physical Background|publisher=Zentralanstalt für Meteorologie und Geodynamik|access-date=7 February 2009|url-status=live|archive-url=https://web.archive.org/web/20110706085616/http://www.zamg.ac.at/docu/Manual/SatManu/main.htm?%2Fdocu%2FManual%2FSatManu%2FCMs%2FFgStr%2Fbackgr.htm|archive-date=6 July 2011}}</ref>

Fog normally occurs at a [[relative humidity]] near 100%.<ref name="gliess">Gleissman, Stephe (2007). ''Agroecology: the ecology of [[sustainable food systems]].'' [[CRC Press]]. p. 73. {{ISBN|0849328454}}.</ref> This occurs from either added moisture in the air, or falling ambient air temperature.<ref name="gliess" /> However, fog can form at lower humidities and can sometimes fail to form with relative humidity at 100%. At 100% relative humidity, the air cannot hold additional moisture, thus the air will become [[supersaturation|supersaturated]] if additional moisture is added.

Fog commonly produces [[precipitation]] in the form of [[drizzle]] or very light snow. Drizzle occurs when the humidity attains 100% and the minute cloud droplets begin to coalesce into larger droplets.<ref>Allred, Lance (2009). ''Enchanted Rock: A Natural and Human History.'' [[University of Texas Press]]. p. 99. {{ISBN|0292719639}}.</ref> This can occur when the fog layer is lifted and cooled sufficiently, or when it is forcibly compressed from above by descending air. Drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point.

The thickness of a fog layer is largely determined by the altitude of the inversion boundary, which in coastal or oceanic locales is also the top of the [[marine layer]], above which the air mass is warmer and drier. The inversion boundary varies its altitude primarily in response to the weight of the air above it, which is measured in terms of atmospheric pressure. The marine layer, and any fog-bank it may contain, will be "squashed" when the pressure is high and conversely may expand upwards when the pressure above it is lowering.