How does weather (temperature and humidity) influence fuel moisture and thus ROS?

• A. Higher temperature and lower humidity lower fuel moisture and can increase ROS; higher humidity raises moisture and can reduce ROS
• B. Weather has no effect on ROS
• C. ROS is influenced by wind but not by temperature or humidity
• D. Low temperature always increases ROS

Answer: (A)

Weather shifts fuel moisture, and fuel moisture levels control how quickly a fire can spread. When air is hot and dry, fuels lose water to the atmosphere more readily, lowering their moisture content. This makes fuels easier to ignite and burn hotter and more freely, which increases the rate of spread. In contrast, cooler air with higher humidity reduces or slows moisture loss, helping fuels stay moist and resist ignition, which slows the fire and lowers the rate of spread. In short, hot, dry conditions dry fuels and tend to raise ROS, while humid conditions keep fuels moist and tend to reduce ROS.