How to Calculate Rate of Fire Spread

Calculating the rate of fire spread fundamentally involves measuring how quickly the fire front moves over a specific distance or how far it travels in a set amount of time. It is essentially the speed at which a wildfire advances.

Methods for Calculating Rate of Fire Spread

According to common methods used in fire science, the rate of fire spread can be calculated using two primary approaches:

1. Measuring Time Between Two Known Locations

This method involves identifying two points along the path of the fire that are a known distance apart. You then measure the time it takes for the fire front to travel from the first point to the second.

• Process:
  1. Select Point A and Point B with a measured distance (Distance) between them.
  2. Record the time the fire front reaches Point A (Time A).
  3. Record the time the fire front reaches Point B (Time B).
  4. Calculate the time taken: Time = Time B - Time A.
• Formula: Rate of Spread = Distance / Time

2. Measuring Distance Travelled Between Two Known Times

This method involves observing the fire at a specific time, noting its location (or a feature it has reached), and then observing it again at a later known time to see how far it has advanced.

• Process:
  1. At Time 1, observe the fire front's location (or a reference point Location 1).
  2. At Time 2 (later than Time 1), observe the fire front's new location (Location 2).
  3. Measure the distance the fire has travelled: Distance = Distance between Location 1 and Location 2.
  4. Calculate the time elapsed: Time = Time 2 - Time 1.
• Formula: Rate of Spread = Distance / Time

Key Considerations

• Units: The rate of spread is typically expressed in units of distance per unit of time, such as meters per minute (m/min), feet per minute (ft/min), or chains per hour (ch/hr).
• Observation Period: As noted in the reference, shorter observation periods tend to provide a more 'instantaneous' observation of the rate of spread, reflecting conditions at that specific moment. Longer periods might average out variations due to changes in fuel, weather, or topography.
• Fire Front: The "fire front" is typically defined as the leading edge of the fire.
• Accuracy: The accuracy of the calculation depends on the precision of the distance measurements and time recordings.

Summary Table

Method	Description	Calculation Basis	Formula
Time Between Known Locations	Measure time for fire to move between two points of known separation.	Known Distance / Time	Distance / Time
Distance Travelled Between Known Times	Measure distance fire moves during a specific, known time interval.	Distance / Known Time	Distance / Time

In essence, both methods apply the fundamental speed calculation: distance divided by time. The choice often depends on the observational capabilities and the specific situation.