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Contents
  1. 1How a wildfire spreads
  2. 2Wind, slope and radiation
  3. 3Firebrands and fuel breaks
  4. 4Buildings at the wildland-urban interface
  5. 5Predicting wildfire spread
  6. 6Suppression
  7. 7See also
  8. 8Notes
  9. 9References

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]

For fire safety engineering, wildfires matter where vegetation borders buildings, because fire can reach a building through embers, radiation or direct flame contact.[2][4] Their 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.[5] Fire risk analysis counts wildland fires among the natural catastrophes that make zero fire risk unattainable.[6] The SFPE Handbook does not describe wildfire research in detail and refers to practical methods for estimating spread in forests developed by the US Forest Service.[1]

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.[7] 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.[8] 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.[7]

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.[9] 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.[10] 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.[11]

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. The handbook also compares spread rates for wood cribs, gorse and heather, and urban fires in Japan at different wind speeds.[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.[12] 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.[13]

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.[14] 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.[14] Radiation also lets a fire cross gaps, and the jump of a forest fire across a firebreak is generally viewed as ignition by radiation.[8]

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.[15] 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 through radiation from the hot smoke layer.[16]

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.[15]

Fire can also stay in the ground. The duff layer on a forest floor, a porous layer of decaying organic material, is often set smouldering by a passing forest fire.[17] 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.[17]

Buildings at the wildland-urban interface

Where wildland borders built-up areas, at the wildland-urban interface, a vegetation fire can reach buildings.[2][18] 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.[4] 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] For adjacent buildings, fire can even spread by radiation through the glass of closed windows without the glass breaking.[4]

Vegetation fire exposes a building by direct flames, radiant heat and firebrands.
Buildings at the wildland-urban interface face several ignition pathways. Based on National Research Council of Canada et al. (2005), p. 1.6-7 and Quintiere (2002b), p. 2-255.

Fire growth across separate combustibles, such as the buildings of a city, involves radiant heating to ignition.[8] In New Zealand, the rules on fire spread between buildings are written in terms of radiation. Building Code clause C3.6, applied in C/VM2 (second edition, 2025), limits the radiation from a fire in a building. At the relevant property boundary it may not exceed 30 kW/m², and at 1 m beyond the boundary 16 kW/m². Under clause C3.7, external walls closer than 1 m to the boundary must be non-combustible or must resist ignition under 30 kW/m² for 15 or 30 minutes, depending on the importance level of the building.[19] These criteria are written for a fully developed fire in a building that exposes a neighbouring building, not for burning vegetation.[19]

The design fire behind these rules assumes that the unprotected wall areas of a burning building emit 83 to 144 kW/m² without sprinklers and 58 to 101 kW/m² with them, depending on the fire load. C/VM2 also comments that this design fire assumes firefighters will be available after some time to prevent fire spread to neighbouring property.[20] The commentary on the first edition of C/VM2 notes that ordinary timber cladding ignites in less than 15 minutes when exposed to 30 kW/m². Non-combustible materials can be assumed not to ignite up to that level. It adds that the design values do not guarantee that fire spread is prevented in every situation, partly because they assume timber cladding with a moisture content of 15 per cent. In New Zealand, fire spread to adjacent property occurred in about 3 per cent of all structure fires between 2005 and 2010.[21]

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.[18] FDS can represent grasses, leaves and burning brands as particles, so that their heating and decomposition are calculated directly.[22][23] Alternatively, a spread-rate model replaces these details with an empirical spread rate, which greatly reduces the complexity of the model.[23] 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.[24]

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.[24] 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.[23] The wind is taken from a standard weather station measurement at 6.1 m above the ground and converted to mid-flame height.[25]

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.[26] 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.[22] In a coupled mode, the burning vegetation releases fuel gas that burns in the model, and the resulting plume interacts with the surrounding air.[27]

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.[28]

Suppression

One of the glossary definitions quoted in NUREG-1805 describes fire suppression as all the work of confining and extinguishing wildland fires.[15] 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).[29] Putting out the flames is not always the end, because smouldering in the duff layer can re-ignite a flaming fire.[17]

Class A foams, intended for ordinary combustibles such as wood, have been used widely in wildland fire suppression.[30] Their success with fires in natural cover fuels led to research on their use in buildings.[31] 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.[32] 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.[30]

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. ↑ 4.0 4.1 4.2 National Research Council of Canada et al. (2005), p. 1.6-7.
  5. ↑ Nowlen et al. (2005b), p. T-14.
  6. ↑ Watts and Hall (2002), p. 5-4.
  7. ↑ 7.0 7.1 Quintiere (2002b), p. 2-246.
  8. ↑ 8.0 8.1 8.2 Kanury (2002a), p. 2-229.
  9. ↑ Iqbal and Salley (2004), p. 3-12.
  10. ↑ Iqbal and Salley (2004), p. 9-2.
  11. ↑ Najafi et al. (2007), p. 2-27.
  12. ↑ Quintiere (2002b), pp. 2-250–2-251.
  13. ↑ Cox and Kumar (2002), p. 3-207.
  14. ↑ 14.0 14.1 Van Rossum (2027), §5.4.
  15. ↑ 15.0 15.1 15.2 Iqbal and Salley (2004), pp. F-17, F-22, F-24, F-29.
  16. ↑ Hamins, Maranghides, McGrattan, Ohlemiller, and Anleitner (2005), p. 30.
  17. ↑ 17.0 17.1 17.2 Ohlemiller (2002), p. 2-206.
  18. ↑ 18.0 18.1 McGrattan et al. (2026), p. ix.
  19. ↑ 19.0 19.1 Ministry of Business, Innovation and Employment (2025b), p. 43.
  20. ↑ Ministry of Business, Innovation and Employment (2025b), p. 44.
  21. ↑ Ministry of Business, Innovation and Employment (2013), p. 23.
  22. ↑ 22.0 22.1 McGrattan et al. (2026), pp. 96–97.
  23. ↑ 23.0 23.1 23.2 McGrattan et al. (2026), pp. 135–136.
  24. ↑ 24.0 24.1 McGrattan et al. (2026), p. 137.
  25. ↑ McGrattan et al. (2026), p. 138.
  26. ↑ McGrattan et al. (2026), p. 140.
  27. ↑ McGrattan et al. (2026), p. 139.
  28. ↑ Notarianni (2002), p. 5-43.
  29. ↑ Iqbal and Salley (2004), pp. E-6, E-10, E-12.
  30. ↑ 30.0 30.1 Iqbal and Salley (2004), pp. B-58–B-59.
  31. ↑ Scheffey and Hickey (2002), p. 4-144.
  32. ↑ 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
  • Ministry of Business, Innovation and Employment. (2013). Commentary for Building Code clauses C1–C6 and Verification Method C/VM2.
  • Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
  • 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.
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