Editing Wildfire (section)

== Physical properties ==
{{See also|Combustion|Fire control|Heat wave|Firestorm}}
[[File:Comtrasts.jpg|thumb|A dirt road acted as a fire barrier in South Africa. The effects of the barrier can clearly be seen on the unburnt (left) and burnt (right) sides of the road.]]
Wildfires occur when all the necessary elements of a fire triangle come together in a susceptible area: an ignition source is brought into contact with a combustible material such as [[vegetation]] that is subjected to enough heat and has an adequate supply of oxygen from the ambient air. A high moisture content usually prevents ignition and slows propagation, because higher temperatures are needed to evaporate any water in the material and heat the material to its [[fire point]].<ref name="FireBehavior"/><ref name="NIFC">{{cite web|url=http://www.nifc.gov/preved/comm_guide/wildfire/fire_4.html|title=The Science of Wildland fire|publisher=National Interagency Fire Center|access-date=21 November 2008|archive-url=https://web.archive.org/web/20081105175208/http://www.nifc.gov/preved/comm_guide/wildfire/fire_4.html|archive-date=5 November 2008}}</ref>

Dense forests usually provide more shade, resulting in lower ambient temperatures and greater [[humidity]], and are therefore less susceptible to wildfires.<ref name="Graham, et al., 12">Graham, ''et al''., 12.</ref> Less dense material such as grasses and leaves are easier to ignite because they contain less water than denser material such as branches and trunks.<ref name="ReferenceA">''National Wildfire Coordinating Group Communicator's Guide For Wildland Fire Management'', 3.</ref> Plants continuously lose water by [[evapotranspiration]], but water loss is usually balanced by water absorbed from the soil, humidity, or rain.<ref>{{cite web | url = https://www.nbcnews.com/id/wbna27148069 | title = Ashes cover areas hit by Southern Calif. fires | agency = Associated Press | publisher = NBC News | date = 15 November 2008 | access-date = 4 December 2008 | archive-date = 30 November 2020 | archive-url = https://web.archive.org/web/20201130025208/https://www.nbcnews.com/id/wbna27148069 | url-status = live }}</ref> When this balance is not maintained, often as a consequence of [[drought]]s, plants dry out and are therefore more flammable.<ref name="Structure">{{cite web | url = http://www.fs.fed.us/projects/hfi/2003/november/documents/forest-structure-wildfire.pdf | title = Influence of Forest Structure on Wildfire Behavior and the Severity of Its Effects | date = November 2003 | publisher = US Forest Service | access-date = 19 November 2008 | url-status = live | archive-url = https://web.archive.org/web/20081217125731/http://www.fs.fed.us/projects/hfi/2003/november/documents/forest-structure-wildfire.pdf | archive-date = 17 December 2008 }}</ref><ref>{{cite web | url = http://www.fema.gov/hazard/wildfire/wf_prepare.shtm | title = Prepare for a Wildfire | publisher = Federal Emergency Management Agency (FEMA) | access-date = 1 December 2008 | archive-url = https://web.archive.org/web/20081029025706/https://www.fema.gov/hazard/wildfire/wf_prepare.shtm | archive-date = 29 October 2008 }}</ref>

A wildfire ''front'' is the portion sustaining continuous flaming combustion, where unburned material meets active flames, or the [[smolder]]ing transition between unburned and burned material.<ref>''Glossary of Wildland Fire Terminology'', 74.</ref> As the front approaches, the fire heats both the surrounding air and woody material through [[convection]] and [[thermal radiation]]. First, wood is dried as water is vaporized at a temperature of {{convert|100|C|F}}. Next, the [[pyrolysis]] of wood at {{convert|230|C|F|sigfig=2}} releases flammable gases. Finally, wood can smolder at {{convert|380|C|F|sigfig=2}} or, when heated sufficiently, ignite at {{convert|590|C|F|sigfig=1}}.<ref>de Sousa Costa and Sandberg, 229–230.</ref><ref>{{cite web | url = http://web.mit.edu/2.009/www/experiments/deathray/10_ArchimedesResult.html | title = Archimedes Death Ray: Idea Feasibility Testing | date = October 2005 | publisher = Massachusetts Institute of Technology (MIT) | access-date = 1 February 2009 | url-status = live | archive-url = https://web.archive.org/web/20090207164348/http://web.mit.edu/2.009/www/experiments/deathray/10_ArchimedesResult.html | archive-date = 7 February 2009 }}</ref> Even before the flames of a wildfire arrive at a particular location, [[heat transfer]] from the wildfire front warms the air to {{convert|800|C|F|sigfig=2}}, which pre-heats and dries flammable materials, causing materials to ignite faster and allowing the fire to spread faster.<ref name="ReferenceA" /><ref name="FireScars">{{cite web | url = http://www.esa.int/esaCP/SEMNJMV4QWD_Protecting_0.html | title = Satellites are tracing Europe's forest fire scars | publisher = European Space Agency | date = 27 July 2004 | access-date = 12 January 2009 | url-status = live | archive-url = https://web.archive.org/web/20081110172926/http://www.esa.int/esaCP/SEMNJMV4QWD_Protecting_0.html | archive-date = 10 November 2008 }}</ref> High-temperature and long-duration surface wildfires may encourage [[flashover]] or ''torching'': the drying of [[canopy (ecology)|tree canopies]] and their subsequent ignition from below.<ref>Graham, ''et al''., 10–11.</ref>

{{anchor|Spot|Spotting|Spot fire}} Wildfires have a rapid ''forward rate of spread'' (FROS) when burning through dense uninterrupted fuels.<ref>{{cite web|url=http://www.flash.org/resources/files/WildfireBrochure.pdf|title=Protecting Your Home From Wildfire Damage|publisher=Florida Alliance for Safe Homes (FLASH)|access-date=3 March 2010|page=5|url-status=live|archive-url=https://web.archive.org/web/20110719000918/http://www.flash.org/resources/files/WildfireBrochure.pdf|archive-date=19 July 2011}}</ref> They can move as fast as {{convert|10.8|km/h|mph}} in forests and {{convert|22|km/h|mph}} in grasslands.<ref>Billing, 5–6</ref> Wildfires can advance tangential to the main front to form a ''flanking'' front, or burn in the opposite direction of the main front by ''backing''.<ref>Graham, ''et al''., 12</ref> They may also spread by ''jumping'' or ''spotting'' as winds and vertical [[convection]] columns carry ''firebrands'' (hot wood embers) and other burning materials through the air over roads, rivers, and other barriers that may otherwise act as [[firebreak]]s.<ref name="underfire">{{cite magazine | url = http://ngm.nationalgeographic.com/2008/07/fire-season/shea-text.html | title = Under Fire | last = Shea | first = Neil | magazine = National Geographic | date = July 2008 | access-date = 8 December 2008 | archive-url = https://web.archive.org/web/20090215065522/http://ngm.nationalgeographic.com/2008/07/fire-season/shea-text.html | archive-date = 15 February 2009 }}</ref><ref>Graham, ''et al''., 16.</ref> Torching and fires in tree canopies encourage spotting, and dry ground fuels around a wildfire are especially vulnerable to ignition from firebrands.<ref>Graham, ''et al''., 9, 16.</ref> Spotting can create spot fires as hot embers and firebrands ignite fuels downwind from the fire. In [[bushfires in Australia|Australian bushfires]], spot fires are known to occur as far as {{convert|20|km|0}} from the fire front.<ref name="Kilmore-East">{{cite book
 |chapter     = Volume 1: The Kilmore East Fire
 |title       = 2009 Victorian Bushfires Royal Commission
 |publisher   = Victorian Bushfires Royal Commission, Australia
 |date        = July 2010
 |chapter-url = http://www.royalcommission.vic.gov.au/commission-reports/final-report/volume-1/chapters/the-kilmore-east-fire
 |isbn        = 978-0-9807408-2-0
 |access-date  = 26 October 2013
|archive-url  = https://web.archive.org/web/20131029190327/http://www.royalcommission.vic.gov.au/commission-reports/final-report/volume-1/chapters/the-kilmore-east-fire
 |archive-date = 29 October 2013
}}</ref>

Especially large wildfires may affect air currents in their immediate vicinities by the [[stack effect]]: air rises as it is heated, and large wildfires create powerful [[updraft]]s that will [[advection|draw in]] new, cooler air from surrounding areas in [[thermal column]]s.<ref>''National Wildfire Coordinating Group Communicator's Guide For Wildland Fire Management'', 4.</ref> Great vertical differences in temperature and humidity encourage [[pyrocumulus cloud]]s, strong winds, and [[fire whirl]]s with the force of tornadoes at speeds of more than {{convert|80|km/h|mph|sigfig=1}}.<ref>Graham, ''et al''., 16–17.</ref><ref>Olson, ''et al.'', 2</ref><ref>{{cite web | url = http://www.nwcg.gov/pms/pubs/newshelt72.pdf | title = The New Generation Fire Shelter | page = 19 | publisher = National Wildfire Coordinating Group | date = March 2003 | access-date = 16 January 2009 | url-status = live | archive-url = https://web.archive.org/web/20090116133450/http://www.nwcg.gov/pms/pubs/newshelt72.pdf | archive-date = 16 January 2009 }}</ref> Rapid rates of spread, prolific crowning or spotting, the presence of fire whirls, and strong convection columns signify extreme conditions.<ref>''Glossary of Wildland Fire Terminology'', 69.</ref>

=== Intensity variations during day and night ===
[[File:Incendio en Caracas (4515878847).jpg|thumb|A wildfire in Venezuela during a [[drought]]]]Intensity also increases during daytime hours. Burn rates of smoldering logs are up to five times greater during the day due to lower humidity, increased temperatures, and increased wind speeds.<ref>de Souza Costa and Sandberg, 228</ref> Sunlight warms the ground during the day which creates air currents that travel uphill. At night the land cools, creating air currents that travel downhill. Wildfires are fanned by these winds and often follow the air currents over hills and through valleys.<ref>''National Wildfire Coordinating Group Communicator's Guide For Wildland Fire Management'', 5.</ref> Fires in Europe occur frequently during the hours of 12:00&nbsp;p.m. and 2:00&nbsp;p.m.<ref>San-Miguel-Ayanz, ''et al.'', 364.</ref> Wildfire suppression operations in the United States revolve around a 24-hour ''fire day'' that begins at 10:00&nbsp;a.m. due to the predictable increase in intensity resulting from the daytime warmth.<ref>''Glossary of Wildland Fire Terminology'', 73.</ref>