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Unit 2: Fuels Classification

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Distribution & Behavior   ::  Characteristics  ::   Timelag & Life Cycle   ::   Availability   ::   Models   ::   Exercises

Fuel Characteristics

Our analysis of fuel complexes and their potential to support combustion and spread fire requires a more detailed study of individual fuel components. Here we introduce the principal characteristics of fuel components that can give us an indication of potential fire behavior within a fuels complex.

The principal fuel characteristics which affect fire behavior are:

  1. Loading
  2. Size and Shape
  3. Compactness
  4. Horizontal Continuity
  5. Vertical Arrangement
  6. Moisture Content
  7. Chemical Properties

Fuel characteristics chart

Each of these seven characteristics contributes to one or more fire behavior processes. In the figure below we have diagramed the primary relationships. Let's take a few minutes to study these relationships. First, we're concerned with whether ignition will result in a sustaining fire. There are five fuel characteristics that most affect ignition. These are compactness, loading, chemical content, size and shape, and moisture content.

Fuel Characteristics Chart

Our next concern is how fast the fire will spread. Here six primary characteristics are involved. How hot or intense will the fire be? What are the possibilities of spotting, torching, or crowning? We can relate individual fuel characteristics to each of these. Since we have not given you the definition for each of the seven characteristics yet, we will move on at this point but return later to study the diagram in more detail.

Fuel Loading

The first principal characteristic is fuel loading. Loading is defined as the oven dry weight of fuels in a given area, usually expressed in tons per acre. Natural fuel loadings vary greatly by vegetative or fuel types. The pictures below give you some examples of total fuel loadings. Grassland areas may produce fuel loadings of 1 to 5 tons per acre. Brush species such as chaparral, may produce 20 to 40 tons per acre; logging slash, 30 to 200 tons per acre; and timber, 100 to 600 tons per acre. These are all typical ranges but will not fit every fuels complex. Often fuel loading refers only to surface fuels that are less than 3 inches in diameter. If this is the case, the loading for the timber stand in the above example would be 4 to 12 tons per acre.

Two very different fuel types with different fuel loadings

You can see that fuel loadings involve different size classes of fuel particles, various fuel arrangements, and particle distribution over a specific area. Fuel loading descriptions may not only state the total weight or mass per acre, but give weights by fuel size classes and describe their distribution vertically and horizontally. For example, it is important to know the amount of fuels in various size classes, whether the fuels are standing or lying on the ground, and whether the fuels are scattered or in piles.

Size and Shape


Size and Shape affect the surface area to volume ratio of fuels. Small fuels and flat fuels have a greater surface area to volume ratio than larger fuels.

Major Size Classes of Fuels

We've been using the terms large fuels versus small fuels in a relative sense. To be more specific for fuels analysis purposes, we normally break dead fuels into four size classes.

The Major Size Classes of Fuels Are:

  1. Grass, Litter, duff: less than 1/4 inch diameter
  2. Twigs and small stems: 1.4 inch to 1 inch diameter
  3. Branches: 1 to 3 inch diameter
  4. Large stems and branches: greater than 3 inch diameter



Compactness is the spacing between fuel particles and affects the rate of combustion.
The next principal fuel characteristic we need to discuss is compactness. Compactness affects the rate of combustion. The images below illustrate how the closeness and physical arrangement of the fuel particles affects both ignition and combustion. Those that are closely compacted have less surface area exposed and less air circulation between particles, thus requiring more heat or time for ignition.

Horizontal Continuity


Now let us look at horizontal continuity as a principal fuels characteristic. This characteristic influences where a fire will spread, how fast it will spread, and whether the fire travels through surface fuels, aerial fuels, or both.
Horizontal Continuity is the extent of horizontal distribution of fuels at various levels or planes.

Vertical Arrangement


We've discussed some aspects of surface fires versus torching out and crown fires. A very important fuels characteristic involved here is the vertical arrangement of fuels.
Vertical Arrangement : The relative heights of fuels above the ground and their verical continuity. This influences fire reaching various fuel levels or strata.

Fuel Moisture Content

A very important fuels characteristic is fuel moisture content. It can vary in different fuel levels and thus influence whether those levels become involved with fire. In nature, dead-fuel moisture very seldom gets below 3 or 4 percent. Dead fuel moisture fluctuates considerably over time due to several environmental factors. Live fuel moistures run much higher, perhaps 300 percent or more, but they change less rapidly than dead fuels. This is an interesting area of study that we will resume in Unit 5 of this course, which is entitled "Fuel Moisture."
Fuel moisture content is the amount of water in fuels expressed as a percent of the oven dry weight of that fuel.

Chemical Properties

Here we have another principal fuel characteristic for discussion, the chemical properties of fuels. There are certain fuels having rather high amounts of these volatile substances that can contribute to rapid rates of spread and high fire intensities. On the other hand, certain fuels may be high in mineral content, which can reduce fire spread and intensity. A firefighter is primarily concerned with the volatile substances that make his job more difficult.
Chemical properties include the presence of volatile substances such as oils, resins, wax, and pitch in the fuels, which affects rate of combustion.

Volatile Substances

Some well known fuels in which volatile substances contribute greatly to fire intensity and fire spread are:

  1. Chaparral in the Southwest
  2. Gallberry bushes in the Southeast
  3. Sand Pine during varnish stage in Southeast
  4. Fountain Grass in Hawaii
  5. Pitchy stumps from some conifers

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, October 20). Unit 2: Fuels Classification. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License