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Fire Safety at the Wildland/Urban Interface

How Fire Behaves in Nature

    • Fuels: Learn about Fuels

    • Terrain: It’s 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

Figure 7.3

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.

    Figure 7.4

    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.

• Wind: Strong winds can literally fan the flames by pumping more oxygen into an area or by clearing out smoke to allow more oxygen in the air near the flame. Wind can also push the flames toward fresh fuel sources (Figure 7.4). In certain cases wind can blow burning embers ahead of the main fire, starting "spot fires". Many roofs far from the primary fire are ignited by windblown firebrands. Blowing wind can also help to dry out fuel making it much easier to ignite.

Wildland fires can generate their own wind. The fire superheats the air causing it to rapidly rise. This causes air to be drawn into the lower pressure area caused by the rising mass of hot air, and can further fan the flames and provide more oxygen. A chain reaction like this intensifies as the fire intensifies.

Sunlight heats the ground during the day and causes the warm air to rise, allowing air currents to travel up slope. At nightfall, the opposite can happen when the ground cools and the air currents fall down the slopes. Fires can change direction between day and night by this mechanism.

• Temperature: Temperature acts upon the spread of fire because the temperature of the fuel affects how quickly it will reach the ignition point and burn. The air temperature can change the temperature of fuel, but because fuels are also heated by the sun, fuels in sunlight will be warmer than fuels in the shade. As a result, fires can ignite more easily in sun than in shade.

• Humidity: Humidity is a measure of the amount of moisture in the air. This moisture dampens the fuel. Damp fuel has to be heated enough to drive off some water in order to ignite, so high humidity is capable of slowing the spread of flames. Because humidity is greater at night, fires will often burn less intensely at that time under normal circumstances, and therefore will not travel a great distance. Temperature and humidity changes most dramatically affect the finer fuels like dried leaves, pine needles, and grasses.

 

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Last modified: 09/10/13