Wildland Firefighting Practice Exam

How does a relative humidity of 40% or more affect grass burning?

1. Grass burns quickly

2. Grass may burn readily immediately

3. Grass may not burn at all for an hour

4. Grass is immune to fire

The correct answer is: Grass may not burn at all for an hour

A relative humidity of 40% or more indicates a higher moisture content in the air, which significantly influences the burning characteristics of grass. When the relative humidity is high, it tends to slow down the rate at which grass can ignite and burn. The moisture in the grass itself, combined with the atmospheric humidity, can create conditions where the grass does not easily catch fire. In these conditions, grass may require a longer period of time to dry out before it can ignite effectively. This means that, in some instances, grass may not burn readily for an hour or even longer because the moisture levels are too high for combustion to occur. This is particularly relevant in wildland fire management, where understanding humidity's impact on fire behavior is crucial for predictions and safety measures. The higher relative humidity helps to inhibit ignition and slow fire spread, making it a critical factor to consider in fire behavior assessments.