Which condition is described as causing a fire's rate of spread to double?

Which condition is described as causing a fire's rate of spread to double?

• A. Wind increases 2 mph up to 10 mph, then doubling every 10 mph
• B. Slope increases by 20%
• C. Temperature decreases by 10 degrees
• D. Humidity increases by 20%

Answer: (A)

Wind controls the rate at which a fire spreads more than any other factor. When wind strength rises, it feeds the flame with more oxygen, tilts the flame toward unburned fuels, and accelerates preheating and ignition of fuels ahead of the fire front. A common training rule of thumb uses a doubling effect: as wind increases, the rate of spread roughly doubles, with a step where small wind increases up to about 10 mph are followed by a doubling of ROS for each additional 10 mph. That makes this description the best fit because it directly ties wind speed to a rapid, multiplicative increase in spread rate, which isn’t described by the other factors. Slope can speed up spread by directing the fire upslope, but it doesn’t inherently double the ROS in a simple, universal way. Temperature dropping tends to slow or suppress spread, not double it. Higher humidity also tends to slow spread by keeping fuels moist.

Wind controls the rate at which a fire spreads more than any other factor. When wind strength rises, it feeds the flame with more oxygen, tilts the flame toward unburned fuels, and accelerates preheating and ignition of fuels ahead of the fire front. A common training rule of thumb uses a doubling effect: as wind increases, the rate of spread roughly doubles, with a step where small wind increases up to about 10 mph are followed by a doubling of ROS for each additional 10 mph. That makes this description the best fit because it directly ties wind speed to a rapid, multiplicative increase in spread rate, which isn’t described by the other factors.

Slope can speed up spread by directing the fire upslope, but it doesn’t inherently double the ROS in a simple, universal way. Temperature dropping tends to slow or suppress spread, not double it. Higher humidity also tends to slow spread by keeping fuels moist.