What Do I Need to Know About Fires?

Fire is the combustion of fuel and requires fuel, oxygen and a heat source to be sustained. When the fuel is exhausted the fire will stop burning, much in the same way a fireplace stops burning when the logs are all consumed. Oxygen is in the air and is required for the fire to burn. Lack of oxygen can cause a fire to smolder or go completely out. This is how the dampers work on a wood stove. Adding more oxygen, like blowing on a campfire can stoke up the fire. Wind can fan the flames of a fire into a hotter or larger fire. Heat is generated by fire. It is what makes a fire spread, but some type of heat is required to start the fire in the first place.

The heat or catalyst can be anything from a lightning strike to a carelessly discarded cigarette, a trash fire gone awry, or even a car with a hot muffler parked on dry grass.



Figure 7-1
The Fire Triangle.
Illustration by Sarah Lynch-Walker

Understanding the fire triangle (Figure 7.1) is how firefighters try to get control of a fire. Consider fire as a triangle. Remove any of the sides of the triangle and the fire ceases. Wildland fire specialists seek to manage one or more of the three elements in order to suppress an unwanted fire or guide a prescribed fire. When you listen to the news and hear about how much of the fire is contained, it generally means how much of a ring from which they removed the fuel has been put around the fire.

Fuel

Earlier in this book we talked about ways to minimize the fuel in your landscape. Later in this chapter we will discuss ways to arrange the landscape to further minimize the availability of fuel to an approaching fire.

In the fire triangle, our homes and landscapes are fuel. Fuel is held in both living and dead plant materials. Generally the moisture has to be driven off before the fuel in a plant can burn. However, very hot fires can dry out the fuel very quickly, almost instantaneously, causing that explosion of a tree into flames like you sometimes see in television coverage of a forest fire.

The types of plants you choose can have a good deal to do with the flammability and the amount of fuel as well as the availability of the fuel for ignition.

The quantity of fuel in a given area is known as the fuel load. Reducing fuel loading can reduce the severity of a fire. Our habit of trying to stop most wildfires as soon as possible over the past 100 years has resulted in a nationwide increase in fuel loading, which is partly responsible for the mega fires we are now seeing in the west.

How the fuels are arranged is also important. More than once Iíve looked out into my yard to see it covered with dry leaves, pine needles, and twigs all the way from the house to the woods. This is a continuous path of fuel for a potential fire. Or, thereís a pile of dried leaves near a bush that is next to a tree with low-hanging dead limbs. This is a fuel ladder, where, due to the placement of plants, fire on the ground can climb up shrubs and small trees to burn a house or to enable a surface burning fire to turn into a much more dangerous tree-top fire known as a crown fire.

Oxygen

Air contains about 16% oxygen, enough to sustain fire in most instances. It enables the burning and other chemical reactions that are part of a wildland fire. When a building burns, closed windows and doors can limit oxygen. In a wildfire itís tough to eliminate oxygen except on the smallest scale. Wind increases the supply of oxygen, pushes heat into adjacent fuels and can send embers ahead of the fire. More oxygen means more fire. Think of what happens when you use a bellows in your fireplace.

Heat

Heat is the third leg of the triangle. Heat refers to several different aspects of a wildland fire. Of course it is the initial spark that ignites the fire, but it is also necessary to maintain the fire and permit it to spread. Heat emanates from the fire and pre-heats surrounding fuel. This drives off moisture making the fuel more flammable.

Heat is transferred in three distinct ways during fire (Figure 7.2). Understanding these differences is critical to understanding the spread of a fire, ways to fight the fire, and ways to prepare your landscape and home to be fire resistant.

  • Convection: Heat rises. That process is known as convection. It is why chimneys work to draw smoke up and out of a stove or fireplace. It is also why surface fires can become crown fires and why fire seems to run up a slope.

  • Radiation: Radiation is the spread of heat by rays like from the sun or a flame. This radiant heat can heat fuels near a fire causing them to ignite without the flame actually touching the fuel. Radiant heat is why most fire codes require a non-flammable substance to surround a stove. Radiant heat is also why very flammable fuels like dry wood piles and propane tanks sometimes burst into flame from approaching wildfires.


    Figure 7-2
    Types of heat transfer during a wildland fire.
    Illustration by Sarah Lynch-Walker.


  • Conduction: Conduction is the flow of heat through a substance or a fuel. It is how a pan of water comes to a boil when placed on the burner of a stove. It can be the cause of fire within a home from fire on the outside of a home, but wood and plant materials are notoriously poor conductors of heat. Conduction is not a primary means of heat transfer in wildland fires.