Rate of spread is defined as the speed at which a fire front advances. Which four factors influence it?

• A. Rate of spread is the time between ignition and extinction; influenced by soil type.
• B. Rate of spread is the distance a fire travels in a day; influenced by topography.
• C. Rate of spread is the speed at which a fire front advances; influenced by wind, slope, fuels, and weather (including fuel moisture).
• D. Rate of spread measures flame height; influenced by humidity and rainfall.

Answer: (C)

Rate of spread is the speed at which a fire front advances. The four factors that influence it are wind, slope, fuels, and weather (including fuel moisture). Wind pushes flames and heat toward unburned fuels, helping ignition and driving the front forward. Slope, especially upslope, causes heat to preheat fuels above the fire, making the front move faster uphill. Fuels determine how readily they ignite and sustain burning—their type, arrangement, and moisture content affect both ignition and burning rate. Weather, including temperature, humidity, and precipitation, controls how dry fuels stay and how quickly heat is transferred, so hot, dry conditions with low fuel moisture typically boost the spread.

The first option describes rate as the time between ignition and extinction and cites soil type as an influence, which misdefines rate of spread. The second option treats rate as distance traveled in a day and relies on topography alone, missing the dynamic factors that drive spread rate. The fourth option links rate to flame height and emphasizes humidity and rainfall, which relate more to fire intensity than the speed of the front.