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When communicating about wildland fire management issues, it is important to include the basic scientific tenets of wildland fire, including an examination of the combustion process and the factors influencing wildland fire behavior. Depending on the expertise of the audience, a communicator may choose to limit the discussion to a simple overview of the fire triangle, or expand the discussion to include the technical details of flame structure and fuel chemistry for those who need that level of explanation. This section is intended to provide the communicator with an overview of the scientific processes occurring with wildland fires. An excellent resource for a more detailed understanding of these concepts is Introduction to Wildland Fire (1996) by Stephen Pyne (et al.).
The first step in teaching about wildland fire is to begin with the essentials as illustrated by the fire triangle and its three equal sides, representing heat, fuel, and oxygen; the interaction of the three are required for the creation and maintenance of any fire. When there is not enough heat generated to sustain the process, when the fuel is exhausted, removed, or isolated, or when the oxygen supply is limited, then a side of the triangle is broken and the fire is suppressed. The underlying theme is that wildland fire personnel seek to manage one or more of the three elements in order to suppress an unwanted fire or guide a prescribed fire.
Heat
Heat can refer to several aspects of wildland fire.
A heat source is responsible for the initial ignition of wildland fires, and heat is also needed to maintain the fire and permit it to spread. In addition, heat is constantly emanating from the fire, warming the surrounding air and preheating fuel in its path.
Heat transfer is a critical issue in the study of wildland fire. For a fire to grow and spread, heat must be transferred to the initial and surrounding fuel. Heat allows fire to spread by removing (evaporating) the moisture from the nearby fuel, enabling it to travel more easily. The mechanism and the speed of heat transfer play a great role in wildland fire behavior.
Three mechanisms of heat transfer exist: convection, radiation, and conduction. All three contribute in different ways to the combustion process, depending in part on the available fuel distribution, the wind speed at the fire site, and the slope of the terrain.
Convection is the transfer of heat through the flow of liquids or gases, such as when hot air rises through a chimney. Convection currents are often responsible for the preheating of the higher shrub layers and canopy, carrying the groundfire upwards into the canopy.
Radiation transmits heat by rays, such as from the sun or a flame. Radiation accounts for most of the preheating of fuels surrounding a fire. The temperature of these fuels can sometimes grow so high that the fuels ignite prior to contact with flames, spreading the fire.
Conduction moves heat from one fuel particle to the next, as when the stove burner heats a pan and its contents. Conduction allows the heat to be transferred inside and throughout the fuel, rather than only heating the surface. Because wood is a poor heat conductor, meaning heat does not pass through it easily, conduction is usually not the primary mechanism of heat transfer in a wildland fire.
Fuel
The fuel side of the fire triangle refers to both the external and internal properties of the fuel. External properties refer to the type and the characteristics of the fuel material. Internal properties of fuel address aspects of fuel chemistry. Types of fuel include living vegetation, dead vegetation, (duff, twigs, needles, standing dead snags, leaves, and moss), organic subsurface material (peat and coal), and human built structures. Fuel can be defined as any combustible material.
Fuel is characterized by its moisture content, size and shape, quantity, and the arrangement in which it is spread over the landscape. The moisture content of any fuel will determine how easily that fuel will burn. Live trees usually contain a great deal of moisture while dead logs contain very little. Before a wet fuel can burn, the moisture must be converted to vapor through the heat process. The greater the moisture content, the higher the heat temperatures required to dry the fuel. The presence of moist fuel can affect the rate and direction that a wildland fire spreads. High moisture content slows the burning process since heat from the fire must first expel moisture.
The size and shape of fuel in part determines its moisture content. Lighter fuels such as grasses, leaves, and needles quickly expel moisture, and therefore burn rapidly. Heavier fuels, such as tree branches, logs, and trunks, take longer to warm and ignite. In areas of light fuel, the temperature required for ignition is lower than in areas of heavier fuel. The oxygen surrounds lighter fuels and allows the fuel to burn with greater intensity, quickly exhausting the fuel supply.
The quantity of combustible fuel in a given area is known as fuel loading. These fuels may be arranged in a uniform pattern and distributed continuously across the ground, allowing a wildland fire to travel uninterrupted. Or, the fuel may be distributed unevenly in a patchy network, forcing the fire to travel over rocks and other barriers by wind-borne embers.
The vertical arrangement of fuel is also an important factor in wildland fires. Ground fuels are all of the combustible materials found below the ground surface, and include tree roots, duff, and organic material. Surface fuels are found at the ground level, including twigs, grass, needles, wood, and other vegetation. Aerial fuels are standing vegetation including tree crowns, branches, leaves, snags, and hanging moss. Crown fires are able to burn independently of surface fires, moving through the treetops.
Oxygen
The third side of the fire triangle represents oxygen. Air contains about 21% oxygen; most fires require air with at least 16% oxygen content to burn under most conditions. Oxygen supports the chemical processes that occur during a wildland fire. When fuel burns, it reacts with oxygen from the surrounding air, releasing heat and generating combustion products, e.g., gases, smoke, particles. The process is known as oxidation.
Helping the audience(s) understand the fire triangle concept is critical to helping them understand why certain actions are taken, e.g., backfires, prescribed burns. Without this understanding, especially in a suppression situation, firefighters' actions may be misunderstood.
All wildland fires begin with an ignition source. Lightning is a common ignition source of wildland fires, reportedly causing nearly 80 percent of the remote wildland fires in the United States. Nine out of ten fires, however, are started directly or indirectly by people, through discarded smoking products, sparks from equipment in operation, arced power-lines, campfires, arson, and other means.
Fire behavior describes the manner in which fuels ignite, flames develop, and fire spreads. The fundamental influences on the spread of wildland fire include fuel type and characteristics, weather conditions in the area, and terrain.
Fuel
Because of the complicated combustion process that occurs during the ignition and spread of a wildland fire, it may be useful to describe for your audience the difference between fire and flame. Fire is a chemical reaction, and flame is the visible indication of that chemical reaction. When a flame is visible, the combustion is termed "flaming combustion." With "glowing combustion" one will only see embers.
Fuels char at relatively low temperatures, but once charred can continue to burn by glowing combustion. As fire spreads, there is constant ignition of new fuels through one of the three heat transfer mechanisms described earlier, and the fire continues to advance.
Weather
Wildland fires are affected by wind, temperature, and humidity in the burn zone. Strong winds can affect fire behavior by pushing the flames toward new fuel sources. Wind is able to pick up and transfer burning embers, sparks, and other materials that are capable of starting "spot fires." Blowing wind can also serve as a fuel drying source in moist areas. Wildland fires are capable of generating their own wind. Air above the hot flames becomes heated, causing it to rise. This movement allows fresh air to fill the vacuum provided; this fresh air supplies the fire with a fresh supply of oxygen. In essence fires can generate their own winds, fanning their own flames.
During the day, sunlight heats the ground and the warm air rises, allowing air currents to travel up sloped landscapes. At nightfall, the process is reversed. The ground cools and the air currents now travel down the slopes. Often fires will burn upslope during the day and downslope at night.
Temperature acts upon the spread of wildland fires because the temperature of the fuel affects how quickly or slowly they will reach their ignition point and burn. Because fuels are also heated by solar radiation, fires in the shade will not burn as quickly as those in the direct path of sunlight.
Humidity is a measure of the amount of moisture in the air. This moisture dampens the fuel, 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.
The combination of wind, temperature, and humidity affects how fast wildland fires can spread. These combinations will change throughout the day and night, and the presence of fire will impact each factor, causing even greater variation.
Terrain
Topography of a landscape also affects the spread of wildland fire. Every wildland fire is different in the way that it behaves because of the changing combinations of so many factors, but terrain remains constant and therefore allows for more constant predictions of how fire will behave in a specific area.
An explanation of terrain includes the shape of the landscape, its elevation, the slope direction and its exposure to sunlight, and the slope steepness. The shape of the land determines how much sunlight or shade an area contains, affecting temperature and wind conditions. Certain fuels grow better under different conditions, and the amount of shade or sunlight, the temperature of an area, and moisture received by an area all determine the type of fuel available for wildland fires. In addition, if the landscape has barriers, including highways, boulders and rock slides, or bodies of water, the fire will not spread as quickly.
Elevation and slope direction affect the type and temperature of the fuel to the degree in which there are shaded and sunny areas. Elevation also impacts how much wind and moisture the area receives. Slope steepness is important in that it contributes to how quickly the fire will reach the crest of the land form. When a fire begins at the bottom of a slope, the fuels located uphill are preheated by the rising air, helping them to easily catch fire when they come in contact with flames. Fires that begin uphill may deposit burning material that rolls downward, allowing more fires to begin downhill.
While helping an audience understand the basic aforementioned concepts, it is critical to convey the complexity. The biogeophysical science behind wildland fire requires multidisciplinary knowledge of chemistry, physics, geology, meteorology, and ecology. That knowledge is then interpreted to help predict and explain fire behavior. Each situation is different in that fire does not function within the framework of a static model.
Wildland fire, as it moves, involves a changing situation. Fire itself changes its own environment, e.g., winds. In essence, in managing a fire the professionals are mixing a recipe in which the ingredients are known but the quantities going in and out of the recipe are constantly changing as is the heat. Such analogies may help your audience better understand why wildland fire management is a demanding art and a science.
Refer to the Wildfire–Feel the Heat Study Guide (Mullins, 1999) in this Communicator's Guide for further discussion of the science of wildland fires. For those requiring an in-depth scientific explanation of wildland fire see Pyne, Andrews, and Laven (1996).
Mullins, G.W. 1999. Wildfire–Feel the Heat Study Guide. Bethesda, MD: Discovery Pictures, Inc.
National Wildfire Coordinating Group. 1994. "Introduction to Wildland Fire" Behavior S-190, Student Workbook NFES 1860. Boise, ID: National Interagency Fire Center.
Pyne, S.J., P.L. Andrews, and R.D. Laven. 1996. Introduction to Wildland Fire, 2nd Edition. New York: John Wiley & Sons, Inc.
Author: Elizabeth Hall