How Fire Behaves in the Wild
As we learned in the first chapter, 96% of fires are started by some type of human action. Fire behavior describes how fuels ignite, flames develop, and fires spread. The things which most influence fire behavior in the wildlands are fuel, weather, and terrain. These three elements make up the fire behavior triangle (Figure 7.3).
Fire behavior triangle.
Illustration by Sarah Lynch-Walker
- Fuel: There is a difference between fire and flame. Flames indicate that there is fire, but fire can be burning in the form of embers without any visible flame. Fires can spread without flames being seen.
The fuel is ignited as the fire spreads by one of the three methods just mentioned (radiation, convection or conduction). The quality of the fuel (e.g., highly combustible resinous plants like pines or less combustible plants like succulents) has a lot to do with how well the fire can spread. Fires that burn hot enough to ignite logs get hotter from the combustion of the dense fuel in the log.
The shape of the fuel can also have a lot to do with its combustibility. Needles and fine foliaged plants will ignite more quickly than densely leaved plants or tree trunks, and, as discussed above, the arrangement of the fuels influences fire behavior. Are they continuous? Are they piled in a way that a lot of air can move through them? Are they arranged vertically so fires can easily climb up?
- Terrain: 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 to leap to the top. Land shape 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.
- 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.
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.
The effect of wind on slopes during a fire.
Illustration by Matthew Gillespie.
Every year we seem to hear about wildfires in California or “out west”. Because of climate and vegetation differences, we know that some areas are more likely to have frequent fires than other areas. Fires in the Mid-Atlantic States are not as frequent, large, or intense as they are in much of the west. Still, if there’s a fire near your house or mine, it’s a concern. And, it won’t be too much longer until insurance companies will begin to rate areas for fire prevalence just like they do for flood prone areas. They will make their decisions based on how fire prone an area is, and whether there is a history of fires along with the extent of damage from them. To decide what to do at your home to prepare for fire, it would be good to know when your locality is fire prone as well as how damaging those fires might be.
Fire season is the time or times of year that wildland fires are usually most likely to start and spread. Fire seasons vary across the country. Many people in the Mid-Atlantic are surprised to find that their primary wildland fire season is late winter to early spring when most plant moisture is still down in the roots and not in stalks, trunks, and branches. Trees have not leafed out, so sunlight from the lengthening days gets to the forest floor and fields. The ground is covered with dead grasses or leaves, pine needles, twigs and branches. Windy days are common. The dry, abundant fuels need only an ignition source. There is usually a secondary fire season in the fall, roughly mid-October through November. Plants lose leaves and shut down for the winter and October tends to be a dry month. By December, shortening daylight and cooler temperatures reduce the danger of wildfires.
You’ll notice that I’m describing the primary fire season “late winter to early spring”. That will vary with just where you live. Are you near sea level? Or, are you living on the side of a mountain? Elevation, of course, makes a big difference in when true seasonal changes occur and when the wildlands are more likely to burn. Winter ends earlier in the Tidewater area of Virginia than it does in the Blue Ridge, and spring comes much later in the high country. Indicators of reduced fire danger in the Mid-Atlantic are when the marshes have greened-up, or the deciduous trees around your home or cabin are fully leafed out and green.
“Fire Season” may also be defined with specific dates for legal reasons. For instance, many states, including Virginia, have restrictions on open burning at certain times of the year. Localities may impose additional restrictions, often also by date. Keep in mind, however, that even if it’s before or after a legal “fire season”, your neighborhood could be more likely to burn than not. Under the right conditions, especially in drought years, there can be fire almost any time. On the other hand, well-timed rains and snows can make a fire season a fleeting event some years.
We already discussed in the first chapter how important human activities are in starting wildland fires, but many fires are started during fire season by mistake. For example, smoking a cigarette outside and flicking the butt is the cause of many fires each year. You are just tempting a fire to start when that butt lands on a mulched bed near the house or in dry grass or leaves. At our house one winter we started a fire by dumping ashes from the woodstove out on the ground. The ashes had been out of the stove and in the ash bucket for at least 3 days, but when we spread the ashes on our garden bed, the dry winds fanned some coals back to life and caught our dried grass on fire!