Tecnobosque Wildfire Knowledge Graph · Fire Behaviour
Reference Concept · v1.0Definition → relationship → calculation → evidence context
The rate of spread (ROS) describes how rapidly a wildfire extends its horizontal dimensions. In forward-spread engineering calculations, it commonly means the horizontal speed of a specified fire-front segment over a defined time interval.
ROS · fire spread rate
What is rate of spread?
A wildfire does not normally have one universal speed. Different parts of the perimeter can advance at different rates because fuels, wind, slope, fire orientation and local interactions vary around the incident.
For engineering and reconstruction work, ROS should therefore be attached to a defined object: for example, the head-fire front between two mapped positions from 14:10 to 14:22. The observation may be measured directly, reconstructed from time-stamped perimeter data or produced by a model; those evidence types should not be treated as interchangeable.
Why it matters
ROS is a core input to fire-behaviour interpretation, progression reconstruction and equations such as Byram's fireline-intensity relationship. A single spread value without spatial and temporal context can be technically precise but misleading.
Rate of spread answers a simple question—how fast is the fire front moving?—but the number only becomes meaningful when the segment, direction, observation period and units are stated. A head fire, flank and rear can all move at different speeds at the same time.
ROS = distance travelled ÷ elapsed timeFor linear front spread, distance and time must describe the same fire segment, direction and observation window. NWCG also uses the broader term 'rate of spread' for some perimeter- or area-growth descriptions; this Reference Concept focuses on linear front motion unless otherwise stated.
Horizontal movement rate for the stated fire segment and interval.
Horizontal distance between compatible front positions or observation points.
Time separating the two observations used to calculate the spread rate.
A reconstructed head-fire segment
Suppose two time-stamped perimeter observations indicate that the same head-fire segment advanced 240 m in 12 minutes.
ROS = 240 m ÷ 12 minROS = 20 m/min20 m/min ÷ 60 = 0.333 m/s20 m/min × 60 ÷ 1000 = 1.20 km/hThe arithmetic is straightforward. The difficult part is proving that both mapped positions refer to the same advancing front and that their timestamps and geometry are sufficiently reliable.
What changes rate of spread?
Wind can increase forward spread, alter flame tilt and create strong directional differences around the perimeter.
Upslope spread can accelerate because flames and convective heating are brought closer to unburned fuels.
Drier fine fuels generally require less energy before ignition and can support faster propagation under otherwise comparable conditions.
Fuel type, loading, particle size, packing, continuity and vertical arrangement influence how efficiently heat is transferred to new fuel.
Head, flank and backing portions of the same fire can have very different spread rates even under the same broad weather conditions.
Spot fires and abrupt transitions can make overall fire growth jump ahead of a continuously advancing front, so simple linear ROS may not describe the whole event.
Rate of Spread in context
The portion with the strongest forward advance in the dominant spread direction. It is often the segment meant when a forward ROS is reported, but that assumption should be stated.
The lateral portions of the fire perimeter. Flank spread is usually evaluated separately from head-fire movement.
The portion moving opposite the dominant spread direction or into the wind/down slope. Its speed can be much lower than the head-fire rate.
A fire can expand in perimeter length or burned area even when no single linear ROS captures the whole geometry. Always match the unit to the quantity being described.
How to interpret rate of spread
- State the fire segment or direction: head, flank, rear/backing or another explicitly mapped section.
- State the observation interval. An average over 60 minutes can hide short periods of acceleration or deceleration.
- State the units and measurement basis. Convert units only after confirming the underlying quantity is the same.
- Identify whether the value is observed, reconstructed, derived or modelled.
- Carry uncertainty from position and timestamp errors into any downstream calculation that uses ROS.
Use the calculator as a transparent technical aid and keep its assumptions explicit.Open Rate of Spread Calculator →
Rate of Spread in real wildfire records
In a documented wildfire reconstruction, ROS can be estimated from successive perimeter positions, satellite detections, aerial mapping, camera observations or other time-stamped evidence. Each source has its own spatial and temporal limitations.
Tecnobosque Wildfire Black Box therefore treats spread values as claims tied to evidence and time, rather than as timeless properties of an incident. A later, better-supported reconstruction can refine an earlier estimate without erasing what was known at the time.
What rate of spread cannot tell you by itself
- A simple distance-over-time calculation does not predict future wildfire behaviour.
- Average ROS can conceal rapid changes in wind, slope exposure, fuels, suppression effects or fire-atmosphere interaction.
- Spotting can create new ignitions well ahead of the main front; that process is not represented by continuous front speed alone.
- Mapped perimeters and satellite detections have spatial and temporal uncertainty. Apparent movement can include mapping differences as well as true fire spread.
- ROS from one segment should not be transferred to another location or time without a defensible reason.
Rate of Spread FAQ
What is wildfire rate of spread?
It describes how rapidly a wildfire extends its horizontal dimensions. For linear forward-spread calculations, it is the distance travelled by a specified fire-front segment divided by elapsed time.
What units are used for rate of spread?
Common linear units include metres per second, metres per minute, feet per minute and chains per hour. Area-growth descriptions can use area per unit time, so the quantity must be stated explicitly.
Is rate of spread the same everywhere on a wildfire?
No. The head, flanks and rear can advance at different speeds, and the rate can also change over time as wind, fuels, slope and fire behaviour change.
How do you calculate linear rate of spread?
For two compatible observations of the same fire-front segment, divide the horizontal distance travelled by the elapsed time: ROS = distance ÷ time.
Is rate of spread the same as fireline intensity?
No. Rate of spread is a movement rate. Fireline intensity is a heat-release rate per unit length of fire front and can use ROS as one of its inputs in Byram's relationship.
Can rate of spread predict where an active wildfire will be later?
Not by itself. A past or current ROS does not guarantee future behaviour. Forecasting requires models and assumptions about fuels, weather, terrain and fire processes, and operational decisions should rely on official fire authorities.