Fire Safety at the Wildland/Urban Interface
How Fire Behaves in Nature
Fuels:Learn about Fuels
Terrain:Its a big word. It includes aspect, slope, position of fire, shape of the land and elevation. Aspect, the direction a slope faces, affects how much sun a landscape receives. The slope and aspect of the land have a lot to do with which plants will thrive there, which in turn determines the fuel. Southwest facing slopes tend to be the most dry. Slope and sunlight, or lack thereof, also have a lot to do with how moist the soil is and how much water is in the plants. Plants that are full of water are slower to ignite and require higher fire temperatures to burn.
The slope of the land during a fire affects the speed at which the fire advances (Figure 7.4). As heat rises from the fire it pre-heats the upslope fuels which makes the fire travel faster. On steep slopes, rising or wind-driven embers can cause a fire
Fire behavior triangle.
Illustration by Sarah Lynch-Walker.
also makes a difference. In hilly or mountainous country, wind will be channeled. A narrow valley or hollow can act as a chimney, drawing fire uphill. Fire can also travel more easily from one side of a narrow valley to another.
The effect of wind on slopes during a fire. Illustration by Matthew Gillespie.
Weather:Weather is the most variable aspect of fire behavior. When you listen to news reports of a firefight you often hear about the winds or nightfall giving firefighters difficulties or sometimes advantages in their efforts to contain the fire. Wildland fires are affected by wind, temperature, humidity and precipitation. It is easy to understand how winds can fan the flames, but other factors such as air temperature and humidity can all play a role too.
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