What is fireline intensity and its units?

• A. Heat released per unit length of fire front per unit time; Btu/ft·s (or kW/m in metric)
• B. Total heat released per unit area; Btu/ft^2
• C. Fire diameter per unit time; ft
• D. Energy per unit fuel mass; Btu/lb

Answer: (A)

Fireline intensity measures the energy released along the advancing fire front per unit length each second. Conceptually, as the fire front moves, energy is released along that line; combining the heat released per unit area with the rate of spread gives the energy release per length per time. The units reflect that, such as Btu per foot per second in imperial or kilowatts per meter in metric. This way, you can compare how fiercely different fronts are burning, regardless of how large the burned area is.

Why the other ideas don’t fit: heat released per unit area describes energy per area, not per length per time, so it doesn’t capture the line-based energy release rate. Fire diameter per unit time relates to how the flame size grows, not the energy release along the front. Energy per unit fuel mass is the fuel’s energy content, not the rate of energy release along a fire front.