Define rate of spread (ROS) in S-390 terms.

• A. The linear distance the fire front advances per unit time
• B. The speed of wind through the fire area
• C. The total heat released per unit length of fire front per unit time
• D. The angular rotation of the fire plume

Answer: (A)

Rate of spread is the linear distance the fire front advances per unit time. In practice, you track how far the leading edge of the fire moves over a set period (for example, feet per minute or meters per minute). This measurement translates the combined effects of wind, slope, and fuels into a single, actionable rate that helps with planning suppression actions and predicting fire progression. For instance, if the front moves 300 feet in 5 minutes, the ROS is 60 feet per minute. The other concepts listed—wind speed, total heat released per unit length of fire front per unit time, and angular rotation of the plume—are related but describe different fire characteristics (air movement, energy output along the front, and plume behavior, respectively) and do not define how fast the fire front itself is moving.