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Unit 1: The Changing Fire Environment

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Environment  ::   Heat Transfer   ::   Behavior   ::   Predictions   ::   Summary   ::   Exercises


Upon completion of this unit you will be expected to:

  • State how each of the three components making up the fire environment can vary over time and space to produce changes in the behavior of a wildfire.
  • List and give examples of the three methods of heat transfer, and give three methods of mass transport of firebrands on fires.
  • Give four primary environmental factors affecting each: ignition, fire intensity, and rate of spread of wildfires.
  • Discuss the relationship of fires of differing intensities to their environments.
  • Describe the behavior of fires in standard fire behavior terminology.
  • Give four necessary input values and five output values of a fire behavior prediction model.
  • Give five reasons why the results of a fire behavior prediction model may differ from the observed fire behavior.


In this unit you will be introduced to the most important variables that affect fire behavior. You will see how the interactions of fire with its environment must influence our assessments of fire behavior. This unit will also introduce you to mathematical fire models available to help us predict fire behavior.

The Changing Fire Environment

Fire Environment : The surrounding conditions, influences, and modifying forces that determine the behavior of a fire.

Environmental Component Examples
  • Moisture content
  • Vertical arrangement
  • Fuel loading
  • Compactness
  • Size and shape
  • Continuity
  • Chemistry
  • Wind speed and direction
  • Relative humidity
  • Precipitation
  • Temperature
  • Atmospheric stability
  • Steepness of slope
  • Position on slope
  • Aspect
  • Elevation
  • Shape of Country

The current state of each of the environmental components--fuels, topography, and weather or airmass--and their interactions with each other and with fire itself, determine the characteristics and behavior of a fire at any given moment. Changes in fire behavior in space and time occur in relation to changes in the environmental components.

Note the seven factors listed under fuels. At the head of the list is moisture content. One unit of this course will be devoted just to fuel moisture content. Another unit will be devoted to fuel models, which will help you to analyze the rest of the fuels factors and make some important estimations.

Under weather, wind speed and direction are our most critical factors. One unit has been devoted to winds and their effects on fire behavior. A large part of this course concentrates on fire weather, as this is the most variable and most difficult of the environmental components to predict.

Topography is the most constant of the three major components making up the fire environment. The most important factor under topography is steepness of slope, since changes in slope have very direct and profound effects on fire behavior. One unit of the course will discuss topography and how to measure slope.

Firefighters soon realize that fires seldom behave exactly the same way from time to time or place to place. This complexity of variable factors indeed offers a challenge to any fireman and to his ability to predict what a fire will do in the next 24 hours or after it has spread into new terrain.

Briefly note below how each of the three major components can change and influence fire behavior.

  1. Fuels

    • With time, fuel moisture changes on a short-time basis. Other changes occur due to man or nature.
    • With space, very significant changes occur due to region and site characteristics.

  2. Weather

    • With time, temperature, relative humidity, and wind change almost continuously.
    • In space, significant changes occur with terrain and general weather patterns.

  3. Topography

    • With time, no change. Terrain is generally constant.
    • Regarding space, changes can be considerable, especially in mountainous terrain.

Natural Processes or Features Related to Wildfire Which Concerns Firefighters

Now let's note some natural processes or features related to wildfire that concern firefighters. The first factor is combustion. This is defined as the rapid oxidation of combustible materials that produces heat energy. The second factor is ignition temperature. This is the point to which a combustible material must be heated to produce self-sustaining combustion. The third factor is fire intensity, which is the rate of heat energy released during combustion. The fourth factor is rate of spread. This is the relative activity of fire in extending its horizontal dimensions. Rate of spread is usually expressed in chains per hour of forward spread or chains per hour of perimeter increase. As the course progresses, you will see how we must analyze and tie all the many variables together to predict whether ignition of new fuels will occur, what the fire intensity will be, and how fast the fire will spread.

Ignition, Rate of Spread, and Field Conditions

Primary factors affecting the ignition of vegetative fuels

Our first concern should be: Will new fuels ignite? Note these factors on which the ignition of vegetative fuels depends. First, size and shape of fuels; second, fuel moisture content; third, compactness or arrangement of fuels; and fourth, fuel temperature. These are pretty much listed in order of importance, but all interact to determine whether ignition will occur if a firebrand is introduced into a natural fuel bed.

  1. Size and Shape of fuels
  2. Fuel moisture content
  3. Compactness or arrangement of fuels
  4. Fuel temperature

Rate of Spread


The relative activity of fire in extending its horizontal dimensions (expressed in chains per hour of forward spread or chains per hour of perimeter increase).

Primary factors affecting the rate of spread:

  • Fine fuel moisture
  • Steepness of slope
  • Occurrence of spotting
  • Fuel loading
  • Windspeed


Field Conditions

Conditions in the field that cause variations in the fire shapes

  • Heterogeneous fuel complexes that produce fingering
  • Barriers that stop or slow fire spread
  • Presence of slope
  • Spotting ahead of or down slope


Fire Intensity

Fire Intensity is dependent on these four factors:

  • Fuel loading
  • Compactness or arrangement of fuels
  • Moisture content of fuels
  • Atmospheric instability

The listing of these factors at this point in the course is intended primarily to acquaint you with the variables that must be analyzed and considered when making fire behavior predictions. Near the end of this unit, we will pull together the most important fire environmental factors and show what inputs are required to make fire behavior calculations and predictions.

Copyright 2008, Michael Jenkins. Cite/attribute Resource . admin. (2005, October 20). Unit 1: The Changing Fire Environment. 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