The Oregon Wildland/Urban Human Fire Interface

Home/Abstract

How Fire Works

Wildlife-Wildfire Interface

Oregon's Vulnerability

Biscuit Fire

Urban-Wildfire Interface

Firefighting Techniques

Sources

GEOGRAPHY 361 HOMEPAGE

 

How Fire Works

The Basics

  Fire can only exist when three critical elements are combined: oxygen, fuel, and heat.   Fire will not be able to burn if one of these crucial elements is not present.  The Earth's  atmosphere is comprised of 21% oxygen, so there is usual no lack of this element.  Fuel is any combustible material.  Oregon has a lot of fuel: 41 million acres (>64,000 sq. miles) of forest and rangeland, significant agricultural production in the Willamette Valley and north central Oregon, and over 500 communities at risk in high fire prone areas.  Heat is primarily the result of weather, drought and heat waves, which dry out vegetation.  In the United States in the year 2000, 85% of US wildland fires were due to human causes, the remaining 15% were caused by lightning.  Arson made up 26% of the human caused fires, and smoking accounted for 4% of fires.  The fire triangle is an easy way to remember the relationship between these elements.  Remove one side of the triangle and there will be no fire. 

 

The Fire Triangle

The Stages of Fire
  Before a fire starts, a preheating stage occurs where water is expelled from plants and wood by nearby flames, drought, or a long hot summer day.  Wood must get very hot before it will combust.  At 615F the cellulose in wood breaks down and releases flammable gases.  The actual burning of a log involves many processes of energy release.  Heat moves inward through the wood by the process of conduction, it decomposes the lignin and cellulose of the wood into flammable gases which fuel the flames, this is called pyrolysis.  Heat is released as gases and hot air through convection, diffusion transfers particles to cooler areas, and radiation transfers heat as electromagnetic waves by flames. 
Text Box: Radiation
Text Box: Diffusion
Text Box: Conduction
Text Box: Convection

 

Text Box: Gases
Text Box: Wood

The Spread of Fire
  Fire spreads differently for varying reasons, but for the most part fuel, wind, topography, and behavior within the fire itself are the major factors which have the most effect as to the severity and extend a fire will spread.  Fuel, as was mentioned, is anything that will burn; in the wild fuel is mostly natural vegetation.  Chemical compositions and oils inside plants can have varying ranges of flammability, which can cause some fires to burn more severely.  Winds are a major factor in the spread of fire, and usually high winds accompany the fires that produce the most damage.  Winds bring fresh oxygen which push flames forward and preheat other areas where the fire can burn and spread to very easily.  Topography is a factor mostly due to slopes in hilly and mountainous areas; fire burns faster up slope.  Topography also supports little microclimates that can develop in valleys and other little places that contain different plant communities that can possibly burn with different intensities.  The fire itself will also have an affect where it will spread.  Fires produce unstable air and can change directions very fast. 
 

How Fire Travels

  A major factor affecting how travel fires is the presence and amount of "litter" or "slash" on the forest floor.  Litter or slash is organic matter on the ground beneath trees and forest left after logging or wind storms; it can also be the shrubs and grasses on the forest floor.  These smaller shrubs, trees, and grasses can also be called the understory.  When a fire is present the understory and litter will act as "ladder fuels," that will aid the fire to climb to the tops and crowns of the trees, created major disastrous fires.
 

Types of Fires

Ground Fire - Litter is usually scarce and fire will pass through the forest quickly, little harm is done to trees.

Surface Fire - Litter is thick fire will burn hot and slow and cause damage or death to trees.

Crown Fires - Litter is thick and there is a substantial understory.  Fire will move from ground to treetops because of these ladder fuels.  These are the most dangerous and destructive class of wildland fire.

      

                                               Ladder Fuels