Surface fire rate of spread
Ascoli, D.1
1DISAFA, University of Torino
Last updated: 28/09/2026
Recommended citation:
Ascoli, D. 2026. Surface fire rate of spread. In: Pere Casals, Davide Ascoli, Lena Vilà-Vilardell, Imma Oliveras, Madoka Krick (eds.) 2026. Methods handbook for characterizing fire use behavior, assessing fire effects, and monitoring soil and vegetation responses at the stand level. FIRE-ADAPT project (number 101086416), deliverable D1.1
Indicator
Fire intensity: Rate of spread
Brief introduction
Rate of spread (ROS) is a key descriptor of surface fire behaviour because it controls fire growth and, together with flame dimensions and fuel consumption, drives fire intensity and potential effects. Fire behaviour and ROS are strongly influenced by fuel properties, weather (especially wind speed and direction), and topography, which interactions make fire spread highly variable in space and time. In experimental burns, ROS is commonly estimated by timing the advance of the fire front across pre-set markers at fixed distances. However, linear arrays of points implicitly assume a spread direction parallel to the line connecting points, an assumption that can be violated when wind shifts or fuels are patchy. To cope with within-burn heterogeneity and variable spread direction, Simard et al. (1984) proposed estimating ROS from the times of fire arrival at the vertices of a triangle, providing an average spread rate that is independent of the instantaneous direction of fire propagation.
Method specifications
Method type
Soft
Hard
Method timing
Before
fire
During
fire
After fire
(short-term)
After fire
(mid-term)
After fire
(long-term)
Field skills
Low
Medium
Hard
Lab skills
Low
Medium
Hard
Material cost
Low
Medium
Hard
Time cost
Low
Medium
Hard
Calculation expertise
Low
Medium
Hard
Equipment
Measuring tape, rods and flags to lay out triangles and label vertices
Datalogging timers connected to thermocouples
Thermocouple extension wires (≈ 1 m) and connectors (0.4 mm in Ø)
Batteries and synchronisation software for simultaneous start/reset
Small hand tools to bury meters (trowel) and protect data loggers
Field notebook: data sheets for triangle geometry (b, c, A) and sensor IDs
Experimental design
Equilateral triangles are established within the experimental burn plot to define microplots (Fernandes et al. 2000, Smith et al. 1991) for fire behaviour assessment. Example: triangles 5 m sides (microplot scale), and additional larger equilateral triangles (20 m sides) can be used to estimate plot-scale ROS (see image). A thermocouple is installed at each vertex. Before ignition, all dataloggers are buried in the soil with only the thermocouple junction exposed above the surface in the expected fire path. Timers are synchronised.
Methodology
- Lay out each triangle and label the three vertices. Record the base side length b (between vertices 1 and 2), the adjacent side length c, and the included angle A (for equilateral triangles: b = c = side length; A = 60°).
- Install one thermocouple at each vertex: bury the datalogger, leave the thermocouple junction exposed above the ground, and ensure it is in contact with the flame front as it passes.
- Synchronise and start all dataloggers before ignition.
- After the fire passes, retrieve meters and download the time of fire arrival at each vertex (t1, t2, t3) based on a predefined temperature threshold (e.g., 200 °C).
- Compute ROS for each triangle using the Simard procedure (see calculations below) and summarise results as mean ROS per triangle (microplot) and/or per plot (average ROS among triangles).
Calculations
To estimate the direction of spread relative to the base line (Θ) and the rate of spread (ROS) within the triangle (Simard et al. 1984)1.
\Theta = \tan^{-1}\left[ \left(\frac{t_3-t_2}{t_2-t_1}\right) \left(\frac{b}{c\sin A}\right) -\frac{1}{\tan A} \right]
1 For equilateral triangles, A = 60° and b = c, simplifying field measurements. ROS represents an average spread rate across the triangle area, and accuracy decreases if the fire front is strongly fragmented at the scale of the triangle or if one vertex sensor fails.
Relation with other methods
This triangle-based timing approach provides ROS estimates that are not constrained by a priori assumptions about fire spread direction. ROS can be used to estimate fireline intensity. Since thermocouples allow calculating time above a threshold temperature, the same setup can be used to estimate flame residence time at fixed points, supporting integrated characterisation of fire behaviour (e.g., coupling ROS with intensity metrics). Inside the triangle ecological effects can be measured for correlation studies.
Illustrations
Data templates
NA
Further information
Fernandes P., Catchpole W.R., Rego F.C., 2000b. Shrubland Fire Behaviour Modelling with Microplot Data. Canadian Journal Forest Research 30: 889-899.
Simard, A.J., Eenigenburg, J.E., Adams, K.B., Nissen, R.L., & Deacon, A.G. (1984). A general procedure for sampling and analysing wildland fire spread. Forest Science, 30(1), 51–64.
Smith J.K., Laven R.D., Omi P.N., 1993. Microplot sampling of fire behavior on Populus tremuloides stand in North central Colorado. International Journal of Wildland Fire 3(2): 85-94.