Skip to content
Upcoming courses

Wildfire: Difference between revisions

Tools
From Fire Safety Academy Wiki
VisualWikitext
Jan (talk | contribs)
Bots
492 edits
New FSE wiki article: Wildfire (written by Kimi Code CLI)
 
Mark (talk | contribs)
Bots
284 edits
Revise against the FSE wiki writing guidelines: corrections after independent fact-check (revised by Claude Opus 5.5)
 
(2 intermediate revisions by 2 users not shown)
Line 1: Line 1:
A '''Wildfire''' is a fire spreading through vegetation or other natural fuel, with behaviour shaped by fuel type and arrangement, terrain, wind and humidity. It can involve surface litter, brush and, in severe cases, tree crowns. Wildfires present different assessment problems from a single burning item in a building because their fuels and environmental conditions vary across the terrain.<ref name="Quintiere-2-255">Quintiere (2002), pp. 2-255–2-256.</ref>
A '''wildfire''', also called a wildland fire or bushfire, is a fire that spreads through vegetation. It can burn the brush on a forest floor, the crowns of trees, or gorse and heather.<ref name="Q-255">Quintiere (2002b), p. 2-255.</ref><ref name="IFEG-5">National Research Council of Canada et al. (2005), p. 1.6-5.</ref> How it spreads depends on the type and arrangement of the fuel, the terrain, the wind and the humidity. The flames heat the vegetation ahead of them by radiation and convection, and the wind can carry burning embers far ahead to start new fires.<ref name="Q-255" /> Forest fires spread at rates of the order of 1 to 100 cm per second, or about 0.6 to 60 m per minute.<ref name="Q-247">Quintiere (2002b), p. 2-247.</ref> Based on its research, the US Forest Service has introduced practical methods for estimating flame spread in forests under a variety of field conditions.<ref name="Q-255" />


Wildfire spread combines radiation, convection and the movement of burning material. Wind can carry large embers far from the main fire and start spot fires elsewhere. The wildland-urban interface introduces buildings and infrastructure into that environment, so exposure and ignition of structures must also be assessed.<ref name="Quintiere-2-255" /><ref name="FDS-iii">McGrattan et al. (2026), p. iii.</ref>
In fire safety engineering, the International Fire Engineering Guidelines (2005) note that flying brands and embers may have design implications in areas prone to bushfires.<ref name="IFEG-5" /> Wildfire smoke can also affect infrastructure. A US fire risk method for nuclear power plants assumes that heavy smoke from a large forest fire will make high-voltage transmission equipment trip off-line.<ref name="Nowlen">Nowlen et al. (2005b), p. T-14.</ref> Fire risk analysis counts wildland fires among the natural catastrophes that make zero fire risk unattainable.<ref name="Watts">Watts and Hall (2002), p. 5-4.</ref>


The supplied documents provide a basic account rather than a complete wildfire design method. This article therefore stays shorter than other Basic entries and does not invent fuel models, suppression tactics or regulatory requirements. More detailed wildfire behaviour and planning evidence is needed for project-specific assessment.<ref name="Quintiere-2-255" /><ref name="Hall-5-65">Hall and Ahrens (2002), pp. 5-65–5-67.</ref>
== How a wildfire spreads ==


== Fuel, terrain and fire environment ==
[[Flame spread]] is the movement of a flame that stays close to the solid or liquid fuel feeding it. In natural fires, it largely decides how the fire develops.<ref name="Q-246">Quintiere (2002b), p. 2-246.</ref> Fire spread over a fuel is often seen as a chain of ignitions, in which the flame heats and ignites the material just ahead of it.<ref name="Kanury">Kanury (2002a), p. 2-229.</ref> Growth is a race between spreading and burning. If the flame spreads quickly compared with the time the fuel needs to burn out, the fire is likely to accelerate.<ref name="Q-246" />


Fuel type and configuration influence how a wildfire starts and spreads. Fine surface material, brush and tree crowns offer different burning arrangements and heat-transfer paths. In severe fires, both the porous brush at the forest floor and the crowns can participate.<ref name="Quintiere-2-255" />
In a forest, the porous brush on the forest floor usually burns. In severe fires the crowns of the trees burn as well, or even on their own.<ref name="Q-255" /> Like most natural fires, a forest fire burns with diffusion flames, because no burner mixes the fuel and the air beforehand.<ref name="NUREG-3-12">Iqbal and Salley (2004), p. 3-12.</ref> Its advancing front is a long, narrow line of fire that draws in air from both sides as the hot gases rise, like a row of burning townhouses.<ref name="NUREG-9-2">Iqbal and Salley (2004), p. 9-2.</ref> A brush fire is an example of a fire whose energy output is small compared with its diameter, the opposite of an oil-well blowout fire, and most ordinary accidental fires lie in between.<ref name="Najafi">Najafi et al. (2007), p. 2-27.</ref>


Terrain changes flame contact and convective heating. A slope can tilt flames towards unburned fuel and increase preheating there, while wind can have a similar or reinforcing effect. The direction of spread must therefore be evaluated with the terrain, not only with the fuel name.<ref name="Quintiere-2-255" /><ref name="Simcox-3-208">Simcox, Wilkes, and Jones (2002), p. 3-208.</ref>
The SFPE Handbook gives spread rates of the order of 1 to 100 cm per second for forest and urban fires, the same order as upward flame spread on thick solids. Flames creeping sideways or downwards over a thick solid move at about 0.1 cm per second, and smouldering at only 0.001 to 0.01 cm per second.<ref name="Q-247" /> For the same heating, flames spread faster over a fuel of lower density.<ref name="Q-247" /> An approximate correlation by Thomas, quoted for wildland fires, wood cribs and urban conflagrations, makes the spread rate rise with wind speed. The rate falls as the fuel bed becomes denser.<ref name="Q-255" />


Humidity and weather alter the environment in which fuels dry and burn. Quintiere lists fuel configuration, terrain, wind and humidity as the main influences in the source treatment. Those variables must be represented for the particular location and time.<ref name="Quintiere-2-255" />
== Wind, slope and radiation ==


The fire environment can change across short distances. Fuel breaks, vegetation changes and terrain alter the available heat-transfer paths. A break that interrupts one fire may be insufficient for another, because its required size depends on fire intensity and heat transfer.<ref name="Quintiere-2-255" />
Flame spread in a forest depends on radiant heat transfer and on convective heating caused by the wind or by the slope of the terrain.<ref name="Q-255" /> When the flow runs in the direction of spread, the flame and its hot gases heat a stretch of unburnt fuel about as long as the flame, 0.1 to 10 m. A flame spreading against the flow heats only 1 to 3 mm ahead of it. Such wind-aided spread can accelerate and therefore appears more rapid.<ref name="Q-250">Quintiere (2002b), pp. 2-250–2-251.</ref> A slope works in a similar way, and forest fires are known to accelerate uphill because their flames lean towards the slope. Analysis of the King's Cross underground fire in London showed the same effect: flames on a wooden escalator lay down in its trench instead of rising vertically.<ref name="Cox">Cox and Kumar (2002), p. 3-207.</ref>
[[File:fse-wildfire-v1.svg|thumb|upright=1.5|alt=Wind and slope tilt flames towards uphill vegetation while a fuel gap interrupts one path.|Wildfire spread depends on fuel, terrain and wind. Based on Quintiere (2002b), p. 2-255 and Cox and Kumar (2002), p. 3-207.]]


The source documents do not provide a universal fuel-moisture or fuel-load value for these assessments. They instead support an explanation of the physical inputs that must be obtained from an appropriate wildland-fire method.<ref name="Quintiere-2-255" /><ref name="Hall-5-65" />
[[Flame radiation]] preheats the fuel ahead of the front, and as the fuel warms up through radiation and convection from the fire, the spread accelerates.<ref name="Q-247" /> Radiation dominates heat transfer at high temperatures, from about 400 °C, because its intensity rises with the fourth power of the temperature.<ref name="VR-5.4">Van Rossum (2027), §5.4.</ref> How much of it reaches a target depends on the [[view factor]]. This is larger for a bigger radiating surface and smaller at a greater distance or a more oblique angle. A radiating surface of only 5 m² can fill almost the whole view of a target close to it, while the sun is enormous but too far away to have a large view factor.<ref name="VR-5.4" /> Radiation also lets a fire cross gaps, and the jump of a forest fire across a firebreak is generally viewed as ignition by radiation.<ref name="Kanury" />


== Flame spread and heat transfer ==
== Firebrands and fuel breaks ==


[[Flame spread]] is the movement of flame through a fuel arrangement. Wildfire can spread when heat from the flame and hot gases ignites vegetation ahead of the burning front. Wind and slope influence that process through convective heating and flame orientation.<ref name="Quintiere-2-255" />
[[Firebrands]] are flaming or smouldering objects carried through the air from a fire, which can sometimes ignite combustibles far away. An ember is a particle of solid material that gives off radiant energy because of its temperature or because it is burning at its surface.<ref name="NUREG-F">Iqbal and Salley (2004), pp. F-17, F-22, F-24, F-29.</ref> In a forest fire, wind currents can carry large embers great distances and start spot fires at other locations.<ref name="Q-255" />


Radiative heating can preheat fuel before direct flame contact. [[Flame radiation]] therefore contributes to ignition and to the conditions experienced by people and structures at a distance. The required assessment should consider both the flame front and the heat reaching exposed targets.<ref name="Quintiere-2-255" /><ref name="Hamins-29">Hamins et al. (2005), p. 29.</ref>
The brands are lifted by the rising hot gases of the fire and then fall onto combustible material. This mechanism has been observed in warehouse fires and wildland fires, both of which involve cellulosic materials. In NIST tests with burning office workstations, by contrast, there was no evidence that flying brands played a role. There the fire spread through burning carpet, and in one test the video record suggests that radiation from the hot smoke layer ignited a workstation.<ref name="Hamins">Hamins, Maranghides, McGrattan, Ohlemiller, and Anleitner (2005), p. 30.</ref>
[[File:fse-wildfire-v2.svg|thumb|upright=1.5|alt=Wind carries burning brands over a fuel break to ignite a separate spot fire.|Firebrands create discontinuous spread beyond the main fire front. Based on Quintiere (2002b), p. 2-255 and Hamins, Maranghides, McGrattan, Ohlemiller, and Anleitner (2005), p. 30.]]


Fire spread can also be intermittent. A spot fire may start away from the main front and develop separately before later interacting with it. Treating the entire event as one smooth line of flame would miss that discontinuous behaviour.<ref name="Quintiere-2-255" />
[[Fire spread]] through vegetation can be controlled with breaks in the fuel. The size of break needed to stop a fire depends on the size of the fire and on how it transfers heat.<ref name="Q-255" /> According to a glossary definition quoted in NUREG-1805, controlling a fire means completing a control line around it. The line also has to enclose any spot fire from it and any interior island to be saved. Unburnt areas next to the line on the fire side are also burnt out, and hot spots that threaten the line are cooled.<ref name="NUREG-F" />


[[Fire spread]] between fuels is not always a simple contiguous path. NIST's workstation experiments distinguish radiation from the hot layer from flame spread by a continuously burning item. That evidence concerns a compartment, but it illustrates why ignition mechanism must be established rather than assumed.<ref name="Hamins-29" />
Fire can also persist in the forest floor. The duff layer on a forest floor, a porous layer of decaying organic material, is often set smouldering by a passing forest fire.<ref name="Ohlemiller">Ohlemiller (2002), p. 2-206.</ref> This [[smouldering]] is very irregular in extent. It can re-ignite a flaming fire, and it also affects whether dormant seeds survive and the forest regenerates.<ref name="Ohlemiller" />


The source describes practical models for forest-fire behaviour but does not reproduce their equations or input data. Those methods require a suitable fuel, weather and terrain description and should not be replaced by an invented Basic-level correlation.<ref name="Quintiere-2-255" />
== Buildings at the wildland-urban interface ==


== Firebrands and spot fires ==
Where wildland borders built-up areas, at the [[wildland-urban interface]], a vegetation fire can reach buildings.<ref name="IFEG-5" /><ref name="FDS-ix">McGrattan et al. (2026), p. ix.</ref> The International Fire Engineering Guidelines (2005) list three routes for fire spread from a burning building to a neighbouring building or property boundary: burning embers and other debris, radiation, and direct flame contact.<ref name="IFEG-7">National Research Council of Canada et al. (2005), p. 1.6-7.</ref> For adjacent buildings, fire can even spread by radiation through the glass of closed windows without the glass breaking.<ref name="IFEG-7" /> The same guidance notes that flying brands and embers may need to be considered if fire spread to adjoining property is to be limited. This may have design implications in areas prone to bushfires.<ref name="IFEG-5" /> [[Fire spread between buildings]], for example across a city, involves radiant heating to ignition.<ref name="Kanury" />
[[File:fse-wildfire-v3.svg|thumb|upright=1.5|alt=Vegetation fire exposes a building by direct flames, radiant heat and firebrands.|Routes of fire spread to buildings: direct flame contact, radiation and firebrands. The guidance names these routes for fire spreading from a burning building and notes that embers may need to be considered in areas prone to bushfires. Based on National Research Council of Canada et al. (2005), pp. 1.6-5 and 1.6-7, and Quintiere (2002b), p. 2-255.]]


[[Firebrands]] are burning particles carried away from the main fire by convection and wind. They can land on combustible material and start spot fires at distant locations. This transport mechanism can bypass the area that would otherwise interrupt direct flame spread.<ref name="Quintiere-2-255" /><ref name="Hamins-29" />
== Predicting wildfire spread ==


Large embers can travel great distances in the cited account. Their effect depends on where they land, their remaining energy and the condition of the receiving fuel. A firebreak alone therefore cannot demonstrate protection from every ignition path.<ref name="Quintiere-2-255" />
The Fire Dynamics Simulator (FDS), a computer fire model, includes a wildland fire spread model, and the US Forest Service supported the development of its sub-models for the wildland-urban interface.<ref name="FDS-ix" /> FDS can represent grasses, leaves and burning brands as particles, so that their heating and decomposition are calculated directly.<ref name="FDS-96">McGrattan et al. (2026), pp. 96–97.</ref><ref name="FDS-135">McGrattan et al. (2026), pp. 135–136.</ref> Alternatively, a spread-rate model replaces these details with an empirical spread rate, which greatly reduces the complexity of the model.<ref name="FDS-135" /> By default, this model uses the formulae of Rothermel. The spread rate without wind or slope is then the ratio between the energy reaching the unburnt fuel and the energy needed to ignite it, both expressed through properties of the fuel bed.<ref name="FDS-137">McGrattan et al. (2026), p. 137.</ref>


NIST's workstation analysis describes burning brands as a mechanism observed in warehouse and wildland fires involving cellulosic material. It also reports that brands were not the mechanism in that particular video record. This distinction illustrates the need for evidence of the mechanism in the actual incident.<ref name="Hamins-29" />
FDS offers the 13 fuel models of Rothermel and Albini, and factors for wind and slope increase the spread rate at the head of the fire. The original Rothermel model counts wind and slope only when they point in the direction of spread, whereas FDS also handles wind and slope that point in different directions.<ref name="FDS-137" /> The fire front grows as a set of ellipses with a head, flanks and a back that spread at different rates. The ellipses are stretched in the direction set by the wind and the slope, and their ratio of length to breadth is limited to between 1 and 8.<ref name="FDS-135" /> By default, the wind is taken from a standard weather station measurement at 6.1 m above the ground and converted to mid-flame height.<ref name="FDS-138">McGrattan et al. (2026), p. 138.</ref>


Firebrands can make the hazard geographically discontinuous. Assessment must consider both the area near the main fire and potential landing zones for transported burning material. The exposure of those zones depends on their fuel and the heat delivered by the brand.<ref name="Quintiere-2-255" /><ref name="Hamins-29" />
In FDS, the time a patch of vegetation burns is assumed to depend mainly on the ratio of surface to volume of its fuel particles.<ref name="FDS-140">McGrattan et al. (2026), p. 140.</ref> In the particle approach, vegetation is described by this surface-to-volume ratio, its packing ratio and an assumed shape. How strongly it absorbs radiation is measured from the share of sunlight that passes through it, like the fraction of blue sky seen through a tree canopy.<ref name="FDS-96" /> In a coupled mode, the burning vegetation releases fuel gas that burns in the model, and the resulting plume interacts with the surrounding air.<ref name="FDS-139">McGrattan et al. (2026), p. 139.</ref>


The supplied sources do not give a universal travel distance or ignition probability for firebrands. Those outcomes depend on particle properties, fire behaviour, wind and receiving fuel, so no numerical distance is stated here.<ref name="Quintiere-2-255" />
Such predictions are only as good as their input, because a fire model predicts fire effects from data chosen by the user. Its estimates are technically credible only when the conditions for which the model was developed are not violated. Model outputs are single values that do not show the uncertainty in the inputs.<ref name="Notarianni">Notarianni (2002), p. 5-43.</ref>


== Wildland-urban interface ==
== Suppression ==


The [[Wildland-urban interface]] is where vegetation fires interact with buildings and other developed assets. FDS research topics explicitly include wildland and wildland-urban interface fires, with attention to heat and mass transfer and prediction of fire growth. Those research areas show that the interface introduces coupled fuel and structure problems.<ref name="FDS-iii" />
One of the glossary definitions quoted in NUREG-1805 describes fire suppression as all the work of confining and extinguishing wildland fires.<ref name="NUREG-F" /> In the United States, wildfire control has its own NFPA standards. Examples listed in NUREG-1805 are NFPA 295, Standard for Wildfire Control (1998 edition), and NFPA 1051 on the professional qualifications of wildland fire fighters (2002 edition). Others cover wildland fire apparatus (NFPA 1906, 2001 edition) and protective clothing and equipment for wildland fire fighting (NFPA 1977, 1998 edition).<ref name="NUREG-E">Iqbal and Salley (2004), pp. E-6, E-10, E-12.</ref> Putting out the flames is not always the end, because smouldering in the duff layer can re-ignite a flaming fire.<ref name="Ohlemiller" />


Structures can be exposed to radiation, flames and firebrands. The relevant assessment should consider which mechanism delivers enough energy to ignite the building or its contents. Treating every structure loss as direct contact with the main flame front would miss the other pathways.<ref name="Quintiere-2-255" /><ref name="Hamins-29" />
Class A foams, intended for ordinary combustibles such as wood,<ref name="NUREG-B58">Iqbal and Salley (2004), pp. B-58–B-59.</ref> have been used widely in wildland fire suppression.<ref name="Scheffey">Scheffey and Hickey (2002), p. 4-144.</ref> Their success with fires in natural cover fuels led to research on their use in buildings.<ref name="Scheffey" /> In wildland use they are generally mixed with water at 0.1 to 1.0 per cent and applied with compressed air foam systems through hose lines.<ref name="NUREG-B60">Iqbal and Salley (2004), p. B-60.</ref> Foam keeps air away from the fire, produces steam, penetrates crevices and protects exposed material that is not yet burning. It is, however, unstable. Physical or mechanical forces break it down, heat drives out its water, and turbulent air can blow light foam away from the burning area.<ref name="NUREG-B58" />
 
The condition of vegetation and other fuels around a structure affects exposure. Gaps can reduce direct flame spread, but the required break depends on fire intensity and heat-transfer characteristics. Firebrands can still create ignition beyond that gap.<ref name="Quintiere-2-255" />
 
Evidence from compartment fires can help explain radiation and ignition but cannot replace wildland-specific inputs. A room's hot layer and a terrain fire differ in fuel geometry, weather and scale. Models must therefore be chosen for the actual environment.<ref name="Hamins-29" /><ref name="FDS-iii" />
 
The supplied sources do not provide a complete defensible-space or construction standard for the interface. No universal clearance, façade treatment or building-protection requirement is therefore stated. Such provisions must come from the applicable wildfire and building guidance.<ref name="Hall-5-65" /><ref name="Quintiere-2-255" />
 
== Suppression and exposure protection ==
 
Suppression can aim to control spread, protect exposed material or extinguish burning fuel. NUREG-1805 describes foam for wildland use involving ordinary combustible materials, with compressed-air or foam equipment used through hose lines. Those examples concern the cited Class A foam applications rather than every wildfire situation.<ref name="NUREG-B60">U.S. Nuclear Regulatory Commission (2005), p. B-60.</ref>
 
The cited Class A foams are generally mixed at 0.1 to 1.0 percent in water for those applications. Foam can help exclude air and cool material, but heat, airflow and mechanical forces can disrupt the blanket. The required formulation and application depend on the fuel and fire conditions.<ref name="NUREG-B60" /><ref name="NUREG-B58">U.S. Nuclear Regulatory Commission (2005), pp. B-58–B-59.</ref>
 
Exposure protection must consider where heat and firebrands travel. Water or another suitable agent may protect unburned material even where the main fire is not extinguished. Its delivery and persistence must be assessed under wind and fire conditions.<ref name="NUREG-B58" /><ref name="NUREG-B60" />
 
Suppression decisions also affect responders and other people. Smoke, heat, changing wind and difficult terrain can alter access and exposure. The supplied sources do not provide a complete operational wildfire suppression protocol, so no universal attack sequence is given.<ref name="Quintiere-2-255" /><ref name="Hall-5-65" />
 
== Evidence and assessment limits ==
 
Wildfire assessment requires appropriate fuel, weather, terrain and exposure data. Incident observations, laboratory tests and field data can contribute different evidence, but their representativeness must be checked. A model chosen for a building or another terrain cannot be assumed to apply directly.<ref name="Hall-5-65" /><ref name="FDS-iii" />
 
FDS research includes sub-models for wildland and interface fires, which is a different modelling domain from its compartment applications. A computational result depends on those sub-models and their inputs, not only on the software name. Validation must address the actual fire environment.<ref name="FDS-iii" /><ref name="Notarianni-5-43">Notarianni (2002), pp. 5-43–5-44.</ref>
 
Firebrands and spot fires add uncertainty to the location and timing of ignition. A scenario should therefore include possible landing zones and alternative spread paths. A single continuous fire-front map may not capture those outcomes.<ref name="Quintiere-2-255" /><ref name="Hamins-29" />
 
The available documents do not supply a full wildfire prevention, suppression or WUI planning method. Additional wildland-fire behaviour, firebrand exposure, responder safety and local regulatory sources would be needed for a more detailed treatment. Those gaps should remain visible in any subsequent engineering assessment.<ref name="Hall-5-65" /><ref name="Quintiere-2-255" />


== See also ==
== See also ==


* [[Firebrands]]
* [[Facade fire]]
* [[Flame spread]]
* [[Warehouse fire]]
* [[Wildland-urban interface]]
* [[Car park fire]]


== Notes ==
== Notes ==
Line 85: Line 62:
== References ==
== References ==


* Hall, J. R., Jr., & Ahrens, M. J. (2002). Data for engineering analysis. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* Cox, G., & Kumar, S. (2002). Modeling enclosure fires using CFD. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 3-194–3-218). National Fire Protection Association; Society of Fire Protection Engineers.
* Hamins, A., Maranghides, A., McGrattan, K. B., Ohlemiller, T. J., & Anleitner, R. L. (2005). ''Experiments and modeling of multiple workstations burning in a compartment'' (NIST NCSTAR 1-5E). National Institute of Standards and Technology.
* Hamins, A., Maranghides, A., McGrattan, K. B., Ohlemiller, T. J., & Anleitner, R. L. (2005). ''Experiments and modeling of multiple workstations burning in a compartment'' (NIST NCSTAR 1-5E). National Institute of Standards and Technology.
* McGrattan, K., Hostikka, S., Floyd, J., McDermott, R., Vanella, M., Mueller, E., & Paul, C. (2026). ''Fire Dynamics Simulator technical reference guide: Volume 1: Mathematical model'' (6th ed.). National Institute of Standards and Technology.
* Iqbal, N., & Salley, M. H. (2004). ''Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program'' (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
* Notarianni, K. A. (2002). Uncertainty. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* Kanury, A. M. (2002a). Flaming ignition of solid fuels. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 2-229–2-245). National Fire Protection Association; Society of Fire Protection Engineers.
* Quintiere, J. G. (2002). Surface flame spread. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* McGrattan, K., Hostikka, S., Floyd, J., McDermott, R., Vanella, M., Mueller, E., & Paul, C. (2026). ''Fire Dynamics Simulator technical reference guide: Vol. 1. Mathematical model'' (6th ed., NIST Special Publication 1018-1). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.SP.1018
* Simcox, S., Wilkes, N. S., & Jones, I. P. (2002). Computer simulation of smoke movement in buildings. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* Najafi, B., Joglar, F., & Dreisbach, J. (2007). ''Verification and validation of selected fire models for nuclear power plant applications: Vol. 1. Main report'' (NUREG-1824; EPRI 1011999). U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research; Electric Power Research Institute.
* U.S. Nuclear Regulatory Commission. (2005). ''Fire dynamics tools: Quantitative fire hazard analysis methods for the U.S. nuclear regulatory commission fire protection inspection program'' (NUREG-1805). Author.
* National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). ''International fire engineering guidelines'' (2005 ed.). Australian Building Codes Board.
* Notarianni, K. A. (2002). Uncertainty. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 5-40–5-64). National Fire Protection Association; Society of Fire Protection Engineers.
* Nowlen, S. P., Najafi, B., Wyant, F. J., Forester, J., Kazarians, M., Kolaczkowski, A., Joglar, F., Funk, D., Hannaman, G., & Anoba, R. (2005b). ''EPRI/NRC-RES fire PRA methodology for nuclear power facilities: Vol. 2. Detailed methodology'' (NUREG/CR-6850; EPRI 1011989). Electric Power Research Institute; U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research.
* Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 2-200–2-210). National Fire Protection Association; Society of Fire Protection Engineers.
* Quintiere, J. G. (2002b). Surface flame spread. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 2-246–2-257). National Fire Protection Association; Society of Fire Protection Engineers.
* Scheffey, J. L., & Hickey, H. E. (2002). Foam system calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 4-123–4-148). National Fire Protection Association; Society of Fire Protection Engineers.
* Van Rossum, E. (2027). ''Introduction to Fire Safety Engineering''. Fire Safety Academy.
* Watts, J. M., Jr., & Hall, J. R., Jr. (2002). Introduction to fire risk analysis. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), ''SFPE handbook of fire protection engineering'' (3rd ed., pp. 5-1–5-7). National Fire Protection Association; Society of Fire Protection Engineers.


[[Category:Special Hazards & Building Types]]
[[Category:Special Hazards & Building Types]]

Latest revision as of 19:33, 2 November 2026

A wildfire, also called a wildland fire or bushfire, is a fire that spreads through vegetation. It can burn the brush on a forest floor, the crowns of trees, or gorse and heather.[1][2] How it spreads depends on the type and arrangement of the fuel, the terrain, the wind and the humidity. The flames heat the vegetation ahead of them by radiation and convection, and the wind can carry burning embers far ahead to start new fires.[1] Forest fires spread at rates of the order of 1 to 100 cm per second, or about 0.6 to 60 m per minute.[3] Based on its research, the US Forest Service has introduced practical methods for estimating flame spread in forests under a variety of field conditions.[1]

In fire safety engineering, the International Fire Engineering Guidelines (2005) note that flying brands and embers may have design implications in areas prone to bushfires.[2] Wildfire smoke can also affect infrastructure. A US fire risk method for nuclear power plants assumes that heavy smoke from a large forest fire will make high-voltage transmission equipment trip off-line.[4] Fire risk analysis counts wildland fires among the natural catastrophes that make zero fire risk unattainable.[5]

How a wildfire spreads

Flame spread is the movement of a flame that stays close to the solid or liquid fuel feeding it. In natural fires, it largely decides how the fire develops.[6] Fire spread over a fuel is often seen as a chain of ignitions, in which the flame heats and ignites the material just ahead of it.[7] Growth is a race between spreading and burning. If the flame spreads quickly compared with the time the fuel needs to burn out, the fire is likely to accelerate.[6]

In a forest, the porous brush on the forest floor usually burns. In severe fires the crowns of the trees burn as well, or even on their own.[1] Like most natural fires, a forest fire burns with diffusion flames, because no burner mixes the fuel and the air beforehand.[8] Its advancing front is a long, narrow line of fire that draws in air from both sides as the hot gases rise, like a row of burning townhouses.[9] A brush fire is an example of a fire whose energy output is small compared with its diameter, the opposite of an oil-well blowout fire, and most ordinary accidental fires lie in between.[10]

The SFPE Handbook gives spread rates of the order of 1 to 100 cm per second for forest and urban fires, the same order as upward flame spread on thick solids. Flames creeping sideways or downwards over a thick solid move at about 0.1 cm per second, and smouldering at only 0.001 to 0.01 cm per second.[3] For the same heating, flames spread faster over a fuel of lower density.[3] An approximate correlation by Thomas, quoted for wildland fires, wood cribs and urban conflagrations, makes the spread rate rise with wind speed. The rate falls as the fuel bed becomes denser.[1]

Wind, slope and radiation

Flame spread in a forest depends on radiant heat transfer and on convective heating caused by the wind or by the slope of the terrain.[1] When the flow runs in the direction of spread, the flame and its hot gases heat a stretch of unburnt fuel about as long as the flame, 0.1 to 10 m. A flame spreading against the flow heats only 1 to 3 mm ahead of it. Such wind-aided spread can accelerate and therefore appears more rapid.[11] A slope works in a similar way, and forest fires are known to accelerate uphill because their flames lean towards the slope. Analysis of the King's Cross underground fire in London showed the same effect: flames on a wooden escalator lay down in its trench instead of rising vertically.[12]

Wind and slope tilt flames towards uphill vegetation while a fuel gap interrupts one path.
Wildfire spread depends on fuel, terrain and wind. Based on Quintiere (2002b), p. 2-255 and Cox and Kumar (2002), p. 3-207.

Flame radiation preheats the fuel ahead of the front, and as the fuel warms up through radiation and convection from the fire, the spread accelerates.[3] Radiation dominates heat transfer at high temperatures, from about 400 °C, because its intensity rises with the fourth power of the temperature.[13] How much of it reaches a target depends on the view factor. This is larger for a bigger radiating surface and smaller at a greater distance or a more oblique angle. A radiating surface of only 5 m² can fill almost the whole view of a target close to it, while the sun is enormous but too far away to have a large view factor.[13] Radiation also lets a fire cross gaps, and the jump of a forest fire across a firebreak is generally viewed as ignition by radiation.[7]

Firebrands and fuel breaks

Firebrands are flaming or smouldering objects carried through the air from a fire, which can sometimes ignite combustibles far away. An ember is a particle of solid material that gives off radiant energy because of its temperature or because it is burning at its surface.[14] In a forest fire, wind currents can carry large embers great distances and start spot fires at other locations.[1]

The brands are lifted by the rising hot gases of the fire and then fall onto combustible material. This mechanism has been observed in warehouse fires and wildland fires, both of which involve cellulosic materials. In NIST tests with burning office workstations, by contrast, there was no evidence that flying brands played a role. There the fire spread through burning carpet, and in one test the video record suggests that radiation from the hot smoke layer ignited a workstation.[15]

Wind carries burning brands over a fuel break to ignite a separate spot fire.
Firebrands create discontinuous spread beyond the main fire front. Based on Quintiere (2002b), p. 2-255 and Hamins, Maranghides, McGrattan, Ohlemiller, and Anleitner (2005), p. 30.

Fire spread through vegetation can be controlled with breaks in the fuel. The size of break needed to stop a fire depends on the size of the fire and on how it transfers heat.[1] According to a glossary definition quoted in NUREG-1805, controlling a fire means completing a control line around it. The line also has to enclose any spot fire from it and any interior island to be saved. Unburnt areas next to the line on the fire side are also burnt out, and hot spots that threaten the line are cooled.[14]

Fire can also persist in the forest floor. The duff layer on a forest floor, a porous layer of decaying organic material, is often set smouldering by a passing forest fire.[16] This smouldering is very irregular in extent. It can re-ignite a flaming fire, and it also affects whether dormant seeds survive and the forest regenerates.[16]

Buildings at the wildland-urban interface

Where wildland borders built-up areas, at the wildland-urban interface, a vegetation fire can reach buildings.[2][17] The International Fire Engineering Guidelines (2005) list three routes for fire spread from a burning building to a neighbouring building or property boundary: burning embers and other debris, radiation, and direct flame contact.[18] For adjacent buildings, fire can even spread by radiation through the glass of closed windows without the glass breaking.[18] The same guidance notes that flying brands and embers may need to be considered if fire spread to adjoining property is to be limited. This may have design implications in areas prone to bushfires.[2] Fire spread between buildings, for example across a city, involves radiant heating to ignition.[7]

Vegetation fire exposes a building by direct flames, radiant heat and firebrands.
Routes of fire spread to buildings: direct flame contact, radiation and firebrands. The guidance names these routes for fire spreading from a burning building and notes that embers may need to be considered in areas prone to bushfires. Based on National Research Council of Canada et al. (2005), pp. 1.6-5 and 1.6-7, and Quintiere (2002b), p. 2-255.

Predicting wildfire spread

The Fire Dynamics Simulator (FDS), a computer fire model, includes a wildland fire spread model, and the US Forest Service supported the development of its sub-models for the wildland-urban interface.[17] FDS can represent grasses, leaves and burning brands as particles, so that their heating and decomposition are calculated directly.[19][20] Alternatively, a spread-rate model replaces these details with an empirical spread rate, which greatly reduces the complexity of the model.[20] By default, this model uses the formulae of Rothermel. The spread rate without wind or slope is then the ratio between the energy reaching the unburnt fuel and the energy needed to ignite it, both expressed through properties of the fuel bed.[21]

FDS offers the 13 fuel models of Rothermel and Albini, and factors for wind and slope increase the spread rate at the head of the fire. The original Rothermel model counts wind and slope only when they point in the direction of spread, whereas FDS also handles wind and slope that point in different directions.[21] The fire front grows as a set of ellipses with a head, flanks and a back that spread at different rates. The ellipses are stretched in the direction set by the wind and the slope, and their ratio of length to breadth is limited to between 1 and 8.[20] By default, the wind is taken from a standard weather station measurement at 6.1 m above the ground and converted to mid-flame height.[22]

In FDS, the time a patch of vegetation burns is assumed to depend mainly on the ratio of surface to volume of its fuel particles.[23] In the particle approach, vegetation is described by this surface-to-volume ratio, its packing ratio and an assumed shape. How strongly it absorbs radiation is measured from the share of sunlight that passes through it, like the fraction of blue sky seen through a tree canopy.[19] In a coupled mode, the burning vegetation releases fuel gas that burns in the model, and the resulting plume interacts with the surrounding air.[24]

Such predictions are only as good as their input, because a fire model predicts fire effects from data chosen by the user. Its estimates are technically credible only when the conditions for which the model was developed are not violated. Model outputs are single values that do not show the uncertainty in the inputs.[25]

Suppression

One of the glossary definitions quoted in NUREG-1805 describes fire suppression as all the work of confining and extinguishing wildland fires.[14] In the United States, wildfire control has its own NFPA standards. Examples listed in NUREG-1805 are NFPA 295, Standard for Wildfire Control (1998 edition), and NFPA 1051 on the professional qualifications of wildland fire fighters (2002 edition). Others cover wildland fire apparatus (NFPA 1906, 2001 edition) and protective clothing and equipment for wildland fire fighting (NFPA 1977, 1998 edition).[26] Putting out the flames is not always the end, because smouldering in the duff layer can re-ignite a flaming fire.[16]

Class A foams, intended for ordinary combustibles such as wood,[27] have been used widely in wildland fire suppression.[28] Their success with fires in natural cover fuels led to research on their use in buildings.[28] In wildland use they are generally mixed with water at 0.1 to 1.0 per cent and applied with compressed air foam systems through hose lines.[29] Foam keeps air away from the fire, produces steam, penetrates crevices and protects exposed material that is not yet burning. It is, however, unstable. Physical or mechanical forces break it down, heat drives out its water, and turbulent air can blow light foam away from the burning area.[27]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Quintiere (2002b), p. 2-255.
  2. ↑ 2.0 2.1 2.2 2.3 National Research Council of Canada et al. (2005), p. 1.6-5.
  3. ↑ 3.0 3.1 3.2 3.3 Quintiere (2002b), p. 2-247.
  4. ↑ Nowlen et al. (2005b), p. T-14.
  5. ↑ Watts and Hall (2002), p. 5-4.
  6. ↑ 6.0 6.1 Quintiere (2002b), p. 2-246.
  7. ↑ 7.0 7.1 7.2 Kanury (2002a), p. 2-229.
  8. ↑ Iqbal and Salley (2004), p. 3-12.
  9. ↑ Iqbal and Salley (2004), p. 9-2.
  10. ↑ Najafi et al. (2007), p. 2-27.
  11. ↑ Quintiere (2002b), pp. 2-250–2-251.
  12. ↑ Cox and Kumar (2002), p. 3-207.
  13. ↑ 13.0 13.1 Van Rossum (2027), §5.4.
  14. ↑ 14.0 14.1 14.2 Iqbal and Salley (2004), pp. F-17, F-22, F-24, F-29.
  15. ↑ Hamins, Maranghides, McGrattan, Ohlemiller, and Anleitner (2005), p. 30.
  16. ↑ 16.0 16.1 16.2 Ohlemiller (2002), p. 2-206.
  17. ↑ 17.0 17.1 McGrattan et al. (2026), p. ix.
  18. ↑ 18.0 18.1 National Research Council of Canada et al. (2005), p. 1.6-7.
  19. ↑ 19.0 19.1 McGrattan et al. (2026), pp. 96–97.
  20. ↑ 20.0 20.1 20.2 McGrattan et al. (2026), pp. 135–136.
  21. ↑ 21.0 21.1 McGrattan et al. (2026), p. 137.
  22. ↑ McGrattan et al. (2026), p. 138.
  23. ↑ McGrattan et al. (2026), p. 140.
  24. ↑ McGrattan et al. (2026), p. 139.
  25. ↑ Notarianni (2002), p. 5-43.
  26. ↑ Iqbal and Salley (2004), pp. E-6, E-10, E-12.
  27. ↑ 27.0 27.1 Iqbal and Salley (2004), pp. B-58–B-59.
  28. ↑ 28.0 28.1 Scheffey and Hickey (2002), p. 4-144.
  29. ↑ Iqbal and Salley (2004), p. B-60.

References

  • Cox, G., & Kumar, S. (2002). Modeling enclosure fires using CFD. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-194–3-218). National Fire Protection Association; Society of Fire Protection Engineers.
  • Hamins, A., Maranghides, A., McGrattan, K. B., Ohlemiller, T. J., & Anleitner, R. L. (2005). Experiments and modeling of multiple workstations burning in a compartment (NIST NCSTAR 1-5E). National Institute of Standards and Technology.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Kanury, A. M. (2002a). Flaming ignition of solid fuels. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-229–2-245). National Fire Protection Association; Society of Fire Protection Engineers.
  • McGrattan, K., Hostikka, S., Floyd, J., McDermott, R., Vanella, M., Mueller, E., & Paul, C. (2026). Fire Dynamics Simulator technical reference guide: Vol. 1. Mathematical model (6th ed., NIST Special Publication 1018-1). National Institute of Standards and Technology. https://doi.org/10.6028/NIST.SP.1018
  • Najafi, B., Joglar, F., & Dreisbach, J. (2007). Verification and validation of selected fire models for nuclear power plant applications: Vol. 1. Main report (NUREG-1824; EPRI 1011999). U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research; Electric Power Research Institute.
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Notarianni, K. A. (2002). Uncertainty. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 5-40–5-64). National Fire Protection Association; Society of Fire Protection Engineers.
  • Nowlen, S. P., Najafi, B., Wyant, F. J., Forester, J., Kazarians, M., Kolaczkowski, A., Joglar, F., Funk, D., Hannaman, G., & Anoba, R. (2005b). EPRI/NRC-RES fire PRA methodology for nuclear power facilities: Vol. 2. Detailed methodology (NUREG/CR-6850; EPRI 1011989). Electric Power Research Institute; U.S. Nuclear Regulatory Commission, Office of Nuclear Regulatory Research.
  • Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-200–2-210). National Fire Protection Association; Society of Fire Protection Engineers.
  • Quintiere, J. G. (2002b). Surface flame spread. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-246–2-257). National Fire Protection Association; Society of Fire Protection Engineers.
  • Scheffey, J. L., & Hickey, H. E. (2002). Foam system calculations. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-123–4-148). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
  • Watts, J. M., Jr., & Hall, J. R., Jr. (2002). Introduction to fire risk analysis. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 5-1–5-7). National Fire Protection Association; Society of Fire Protection Engineers.
Retrieved from "https://fire-safety.academy/w/index.php?title=Wildfire&oldid=2797"