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 The Fire Triangle

The first step in teaching students 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.                                              

1.    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 allow 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 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 tree canopy, carrying the groundfire upwards into the tops of trees. It also allows fires to spread quickly up steep slopes.

                                                                                

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 even more quickly. This is especially true when the fuels contain volatile compounds like resins, as is the case with chaparral habitats.

Conduction involves the transfer of heat between two objects that are in direct physical contact, 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 an effective mechanism of heat transfer in a wildland fire.

2.    Fuel 

Fuel can be defined as any combustible material. 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 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 and removed through the heat process. The greater the moisture content prior to ignition, the higher the temperature 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.

                 2000 Karen Watenmaker

Wildland fires can move quickly if weather conditions have pre-dried the fuel*

             

           © 2000 Karen Watenmaker                               Man-made structures ignite and burn

Ponderosa Pines (aerial fuel) burst into flames      during the 2003 Southern California fires*                              

The size and shape of fuel in part determines its moisture content. Lighter, thinner fuels such as grasses, leaves, and needles quickly lose moisture, and therefore burn rapidly. Heavier, denser and thicker fuels, such as tree branches, logs, and trunks, take longer to warm and ignite. Thus, in areas of light fuel, the temperature required for ignition is lower than in areas of heavier fuel.

 

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 patches, 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 treetops (e.g. tree crowns), branches, leaves, snags, and shrubs. Crown fires are able to burn independently of surface fires, moving through the canopy or treetops.

             Surface Fire   © 2002 Eric Knapp                 Crown Fire © 2000 Karen Watenmak                                                                               

3.     Oxygen 

The third side of the fire triangle is oxygen. Most fires require air with at least 16% oxygen content to burn under most conditions; air contains about 21% oxygen, on average. 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 byproducts (e.g., gases, smoke, particles).

Fire Behavior 

All wildland fires begin with an ignition source. Lightning is the most common, natural ignition source of wildland fires, reportedly causing nearly 80% of the remote wildland fires in the United States. However, 90% of all wildland fires are started directly or indirectly by people through discarded smoking products, sparks from equipment in operation, downed 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 - 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 in this way the fire continues to advance.

 

Chaparral fire lines can be miles long making containment impossible *

© 2000 Karen Watenmaker 

Blown by heat-generated wind, hot embers ignite "spot fires"

Weather - Wildland fires are affected by wind, temperature, and humidity. 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. This is why many of the most catastrophic, large-scale wildfires in Southern California occur during Santa Ana wind conditions. Wildland fires are even 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. By generating their own winds, fires can fan their own flames.

                                                The Cedar fire created a huge smoke

                                                                 updraft*

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 as great a distance.

Terrain - Topography, which refers to the shape of the land (e.g., rolling hills, valleys, steep slopes), 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 accurate predictions of how fire will behave in a specific area.

An explanation of terrain includes the physical features of a 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 spread of the fire may be hampered.

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.

             Many of San Diego's newer housing developments sit on ridges flanked by steep canyons,

                 potentially dangerous fire environments at certain times of the year*

 

Teaching Tip

When helping students understand the basic concept of fire, it is critical to convey the complexity of the fire process. The science behind wildland fire requires an understanding of chemistry, physics, geology, meteorology, and ecology. That knowledge is then interpreted to help predict and explain fire behavior. Each wildland fire is different in that fire does not function within the framework of a static model.  In the wild, fire is a dynamic process.

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 students better understand why wildland fire management is a demanding profession, a combination of art and science.

 

Source:

Adapted from the National Interagency Fire Center. (2005). Wildland fire communicator's guide. Retrieved January 12, 2005 from http://www.nifc.gov/preved/comm_guide/wildfire/index2.html

Photo Credits:

Watenmaker, K. (2000). Photos of Clear Creek (Salmom, Idaho) and Sula, Montana. Retrieved January 21, 2005 from National Interagency Fire Center (Image Portal) at http://firepix.blm.gov/nifc/main.jsp?screenwidth=1152&ip2=0/

*California Department of Forestry and Fire Protection (2004). California fire siege 2003: The story. Retrieved January 18, 2005 from http://www.fire.ca.gov/php/fire_er_2003siege-video.php

*San Diego Fire Recovery Network. (2005). Speakers Bureau (CD ver. 01). San Diego, CA: Author.

Knapp, E, (2002). Photo of Sequoia-Kings Canyon fire treatment. Retrieved March 16, 2005 from USGS Western Ecological Research Center at http://www.werc.usgs.gov/fire/seki/ffs/photos.htm

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Copyright 2004 San Diego State University Foundation