Wildfire Rate of Spread (ROS) Calculator
Understanding Rate of Spread (ROS) in Wildfire Management
The Rate of Spread (ROS) is a critical metric used by fire behavior analysts and incident commanders to predict how quickly a fire front moves across a landscape. It is typically measured in meters per minute (m/min) or chains per hour (ch/h), where one chain equals 66 feet or approximately 20 meters.
Key Factors Influencing Spread
Calculating the ROS is not just about measuring distance over time; it involves understanding the environment. The primary factors include:
- Fuel Type: Grass fires spread much faster than timber fires due to the "fine" nature of the fuel.
- Wind Speed: Wind provides oxygen and tilts flames forward, pre-heating fuel ahead of the fire.
- Slope: Fire spreads faster uphill because the flames are closer to the fuel on the rising ground.
- Fuel Moisture: Dry fuels ignite and burn significantly faster than those with high moisture content.
The ROS Calculation Formula
The basic mathematical formula for observed rate of spread is:
ROS = Distance / Time
To predict future spread, professionals use the Rothermel surface fire spread model, which incorporates wind and slope adjustment factors. This calculator uses a simplified adjustment where wind speed and slope percentage provide a multiplier to the base observed rate.
If a fire travels 600 meters in 20 minutes, the observed ROS is 30 meters per minute.
If you are in a standard forestry unit, this translates to 90 chains per hour (since 1 m/min ≈ 3 chains/hour). If a 20 km/h wind picks up on a 15% slope, that spread rate could easily double or triple.
Why Monitoring ROS is Crucial
Knowing the ROS allows emergency services to determine evacuation timelines. If a fire is 2 kilometers from a town and spreading at 10 m/min, it will reach the perimeter in 200 minutes (3.3 hours). This data is the difference between a successful evacuation and a disaster.