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Unit 1: The Changing Fire Environment

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

Determining a Fire's Potential Behavior

Why do some fires remain small while others get large very rapidly? What happens when a fire gets large in size and intensity? How does fire interact with its environment?
Let's first consider the extent of the fire's environment. For a very small fire, the fire environment is limited to a few feet horizontally and vertically. As a fire grows in size, so does the extent of the environment affected. In a large fire, the fire environment may extend many miles horizontally and thousands of feet vertically. High intensity wildfires, whether large or small in size, usually have considerable effect on the atmosphere vertically. This is evidenced by their convection columns. There are generally three factors that determine the extent of vertical development of a fire's convection or smoke column. These are the heat energy generated from the fire, the instability of the lower atmosphere, and the winds aloft. Stable air and/or strong winds tend to discourage vertical development of convection columns.

High Intensity Fire

A high intensity fire will create much stronger indrafts that can help feed convection columns to many thousands of feet into the atmosphere. This is sometimes called a three-dimensional fire.

  • Fire can control the environment
  • Sphere of influence becomes greater
  • Can significantly modify weather elements near or adjacent to the fire

Low Intensity Fires

Low intensity fires will create weak indrafts at the fire's edge that will feed a low, weak smoke or convection column over the fire. This we sometimes refer to as a two-dimensional fire.

  • Environment controls the fire
  • Sphere of influence is very small
  • Only slight modification of weather elements in immediate proximity of fire

Open vs. Closed Environment

These images illustrate open and closed fire environments. On the left, we see a fire burning through all levels of the vegetation and exposed to various winds and other weather elements. It will be readily affected by any atmospheric changes, and fire behavior can change drastically as a result of wind shifts, etc.
On the right, the fire is burning under a forest canopy. This is somewhat similar to a structural fire burning inside a building. Conditions outside the building have relatively little effect on the fire inside. Such fires usually remain low in intensity. However, once the fire breaks out of the building or out through the forest canopy, fire intensity and spread can increase drastically as outside atmospheric conditions then influence the fire.

Open Fire

Remember that any wildfire is a heat source that can and will interact with its natural environment. The size of that sphere of influence will depend on the size and intensity or heat energy output of the fire. The physical location of the fire, and the sheltering effect from surrounding terrain and vegetation is often a contributing factor to the potential behavior of that fire.
Let's once again compare low intensity fires to high intensity fires. We can generalize by saying that with low intensity fires, the environment mostly controls the fire. The sphere of influence is very small, and the fire causes only slight modification of weather elements in the immediate proximity of the fire.
On the other hand, high intensity fires can control the environment to a marked extent. The sphere of influence becomes much greater, and high intensity fires can significantly modify weather elements near and adjacent to the fire.

Low Intensity Fire

  • Environment controls the fire
  • Sphere of influence is very small
  • Only slight modification of weather elements in immediate proximity of fire

High Intensity Fire

  • Fire can control the environment
  • Sphere of influence becomes greater
  • Can significantly modify weather elements near or adjacent to the fire
Copyright 2008, by the Contributing Authors. 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: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_1__The_Changing_Fire_Environment_9.html. This work is licensed under a Creative Commons License Creative Commons License