Clive M. Countryman

THEFIRE ENVIRONMENTCONCEPT

PACIFIC SOUTHWESTForest and RangeExperiment Station

FOREST SERVICEU. S.DEPARTMENT OF AGRICULTUREP.O. BOX 245. BERKELEY. CALIFORNIA 947011972

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CLIVE M. COUNTRYMAN heads fire behavior studies at the PacificSouthwest Forest and Range Experiment Station, with headquarters atthe Forest Fire Laboratory, Riverside, Calif. He earned a bachelor'sdegree in forestry at the University of Washington in 1940, and joinedthe Forest Service the following year.

The FIRE ENVIRONMENT conceptExperience with wildland fires soon teaches that no two

are exactly alike. Fire behavior is not an independentphenomenon-it is the product of the environment in whichthe fire is burning.

Environment has been defined as "surrounding conditions,influences, and forces that influence or modify." In applyingthis definition to fire, we can then regard fire environment asthe conditions, influences, and modifying forces that controlthe fire behavior. Fire behavior must obey physical laws. Weconsider certain types of fire behavior unusual or unexpectedonly because we have failed to evaluate properly theconditions, influences, and forces that are in control. Topredict fire behavior, and to control and use fire effectivelyand safely, we must understand and use the interactions offire with its environment.

In the following discussion we will examine the fireenvironment-what it is, how it varies and why, and how fireitself alters the total picture.

COMPONENTS OF THE FIRE ENVIRONMENTThree influences, interacting, compose the fire environ­

ment. The first, topography includes such elements as slope,aspect, elevation, and configuration or "lay of the land." Inrelation to time, topography can be considered static, for theforces that change it generally work very slowly. In horizon­tal space, however, topography can change quickly, particu­larly in mountainous country. Variations can cause drasticchanges in fire behavior as a fire progresses over the terrain.

If we imagine a layer of vegetation added to thetopography, we have the second influence-the fuel. It is thesource of thermal energy and the driving force behind thephenomena of fire behavior. Thus, such characteristics as fuelloading, fuel bed porosity, fuel distribution and continuity,fuel sizes, moisture content, and chemical composition allaffect the way the fire behaves. Most of our fire controlmethods require treatment of the fuel-we scrape it away, orapply water and chemicals to stop the fire from spreadingand to extinguish it.

Most fuel characteristics change rather slowly with time.For example, fuel loading and the fuel distribution over theground surface may not change significantly for years. Somechanges in the fuel may take place seasonally, such asvariations in the moisture content of living material, thechemical composition, and the proportion of dead fuel. Ingeneral, changes in fuel over time take place so slowly thatwe can consider the fuel static for anyone fire. However, thepotential fire behavior can change dramatically in a particular

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fuel type from one part of the fire season to another, or fromyear to year.

An exception to the slow change of fuel is the moisturecontent of dead fuels. In a few minutes, rain can change theway the whole fuel complex bums. When there is noprecipitation, the moisture content of dead fuel is controlledlargely by relative humidity and temperature, and these arechanging almost continuously.

Like topography, wildland fuel can change quickly inhorizontal space. Large variations in fuel types are obvious, aswhen timber gives way to brush, or brush to grass. Butvariations within the same general fuel type can also be greatenough to alter fire behavior. For example, fuel loading andthe proportion of dead fuel can vary greatly over shortdistances. Fuel characteristics often change with elevation,and fuels on north slopes are often quite different-and bumdifferently-from fuels on south slopes.

An important source of variation is the tremendous humanimpact on wildland fuels. Timber harvesting, timber standimprovement, road construction, grazing use, watersheddevelopment, recreational use, wildlife management prac­tices-all may change the character and distribution of thefuel. In addition, the exclusion of periodic fire through fireprotection may change the amount and characteristics of thefuel. Man also changes the fuel-and complicates the firecontrol problem in the process-by building homes in thewildlands and living surrounded by flammable fuel. Thispractice has become so prevalent that the wildland fuel-build­ing mixture is now often considered a separate fuel type.

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The third component of the fire environment is the airmass. It represents such weather elements as temperature,wind, relative humidity, cloud cover, precipitation, and airstability. The air mass is probably the most variable of all thefire environment components. It can vary greatly over shortdistances, often in accord with changes in topography andfuel. It is nearly always changing in time-day by day, hourby hour, and frequently minute by minute.

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INTERACTION IN THE FIRE ENVIRONMENTIn fire control and fire use operations the air mass

component frequently receives more attention than any ofthe others, possibly because its effect on fire is so oftenreadily apparent and it is so variable. Nevertheless, effectiveevaluation of the fire behavior potential, requires that allthree components be considered. In symbolic form, they arethe sides of the fire environment triangle. This symbolimplies constant interaction, which takes place primarilythrough solar energy. The sun's heat evaporates water fromoceans, lakes, streams, and other moist surfaces, and thusprovides the atmosphere with moisture that is returned to theearth as precipitation-the moisture that is essential to plantgrowth, and through erosion helps shape the topography.Heat from the sun also provides the energy for photo­synthesis, the basic process in plant growth. Finally, the sunis responsible for differential heating, through which topog­raphy and vegetation affect the air mass.

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Differential heating

We have seen that the air mass is not static, bu t varieswidely and often quickly, in both space and time. The forcebehind most of the changes is differential heating. Energyfrom the sun reaches the earth as electromagnetic radiation.Because of its high temperature, the sun emits short waveradiation, which passes through the earth's atmospherewithout warming it much. But when these short waves strikea solid object, such as the earth's surface or something on it,part of their energy is converted to heat, warming the object.The heated object radiates energy too, but because of itsrelatively low temperature, the radiation is in the form oflong waves. This radiation can be absorbed by the watervapor and carbon dioxide in the air, thus raising thetemperature of the air. The heated air near the earth's surfaceis distributed through deeper layers by convective processes.

All materials are not warmed equally by the sun'sradiation. Water surfaces are heated much less than landsurfaces, and land covered with vegetation is heated less thanbare ground. In general, dark or nonreflective materials areheated more than light-colored or reflective materials. Theamount of heating is also strongly dependent on the angle ofthe sun's rays; the more nearly perpendicular to the surfacethey are, the more intense the heat. Thus, the amount ofheating varies with the steepness and aspect of the groundsurface, the time of day, the season of the year, and thelatitude of an area-its distance from the equator. At nightthe heating process is reversed, and the earth loses heat tospace. Generally, the kinds of materials that absorb heat mostreadily during the day tend to lose it most rapidly at night.

The differential heating brought about by differences inlatitude and between water and land masses sets up circula­tion by convective and other means and creates large-scaleweather and climatic patterns. But this differential heatingoccurs on a small scale, too, and along with the mechanical

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effect of the topography on airflow it can help set up localweather and climate patterns. Even objects in the landscapecan be heated unequally-a boulder or a tree can be heatedmore on one side than another. These small-scale heat sourcesand the airflow circulations associated with them help tocreate the microclimate. The fire environment is thus anintegration of the effects of all of its components-very smallscale environmental patterns interact with larger ones, andthese with still larger patterns, to form the over-all pattern.

Chain effectsBecause the fire environment components are interrelated,

changes in one component can set off a chain of reactionsthat will cause changes in others. For example, cutting atimber stand will change the radiation receiving surface sothat it will be heated more strongly, raising the air tempera­ture in a cleared area. Because rela tive humidity varies withtemperature, the relative humidity will be lowered. Themoisture content of the dead fuel is dependent on bothtemperature and humidity; hence the moisture content willalso be lowered. Removal of the trees that restrict theairflow, and the more active convection resulting from thehigher temperature, can alter the airflow pattern. Thus, thefire environment-and the fire behavior-in the cutover areawill probably be much altered, not only because the fuelcharacteristics are changed, but because the air mass charac­teristics are also changed.

FIRE IN THE ENVIRONMENTWhere does fire itself fit into the environmental picture?

As we have seen, in an environment without fire, radiantenergy from the sun is almost the only source of heat, andlocal variation in heating creates variability in local weatherand fuel conditions.

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We might consider fire as just another localized heatsource. As such, it reacts with its surroundings in the sameway sun-created heat sources do, interacting with the air massto bring about changes in local weather, and with the fuel tomodify fuel moisture and temperature. There is an importantdifference, however. Ground and fuel surfaces heated by thesun do not become extremely hot; surface temperatures over200° F. seldom occur, and are usually much lower. Tempera­ture differences between adjacent land areas are also notgreat-a surface temperature difference greater than 100° F. isnot likely to occur. But temperatures in a fire can be veryhigh, often exceeding 2,0000F., and the temperature differ­ence between the fire and the adjacent environment can becorrespondingly large. Therefore, the reaction of a fire heatsource with the environment can be much more violent thanthat of a sun-created heat source, particularly in its impact onthe air mass. A large and intense fire can overwhelm all otherheat sources and dominate the local environmental pattern­in effect, the fire creates its own weather.

A wildland fire determines the extent of the environmentof concern to its own behavior. For a very small fire the fireenvironment is limited to a few feet horizontally andvertically. But as the fire grows in size, so does the extent ofthe environment that will affect it. In a large fire, the fireenvironment may extend many miles horizontally andthousands of feet vertically. For an intensely burning fire thefire environment usually extends to a much greater distancevertically than in fires of low intensity.

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Most of the changes in fire behavior result from changes inenvironmental patterns as the fire moves over the terrain andas time passes. But sometimes behavior changes can takeplace very quickly, and almost without horizontal movement.For example, a surface fire burning under a timber stand isburning in an environment different from that existing abovethe stand. Within the timber, the temperature is usually lowerand the humidity and fuel moisture higher during the day,than they are outside the stand. At night the temperature ishigher and fuel moisture lower. Wind speeds are almostalways lower within the stand than outside or above it.

A fire burning under a stand is much like a fire burninginside a building. In the building the air mass, and hence thefuel moisture, is controlled largely by the heating or coolingsystems, and the conditions outside the building haverelatively little effect. But once the fire breaks out of thebuilding, outside conditions can influence its behavior, andthe fire can spread to other fuel and grow in size andintensity. So it is with our fire. Once it increases in intensityenough to crown, it then comes under the influence of theconditions outside of the stand and the fire behavior canchange drastically.

The same situation can exist in other fuel types too, suchas brush and grass. But because of the lesser vertical extent ofsuch fuels, the environmental difference is not so pro­nounced, and fires seldom remain long in the more temperateenvironment.

Earlier it was pointed out that the fire environment is apattern phenomenon; the effects of small heat sourcesinteract with each other to form larger patterns, which in

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turn interact to form still larger patterns. The fire heat sourceis a pattern phenomenon too. Wildland fires seldom burnuniformly. The variations in topography, fuel, and the airmass may cause the fire to build up in one area while it isdying out in another. Thus, there can be hot spots within thefire area, analogous to those in the sun-heated environment.Because of these temperature differences, the behavior of onepart of the fire can affect the behavior of another part. Theconsequences may be disastrous to fire control operations,but the principle can also be used to advantage. For example,it is possible to purposely build up an intense fire in one partof the fire in order to affect the fire favorably for controlaction at a more dangerous point. Similarly, in slash burningan area of intense heat can be built up near the center of theburn area to draw the fire in from the edges, and in this waylessen the chance of fire escape.

All these observations lead to a single conclusion. Thecurrent state of each of the environmental components­topography, fuel, and air mass-and their interactions, onewith another and with the fire itself, determine the behaviorof a fire at any moment. By adding fire to the environmenttriangle, we can represent this relationship symbolically withthe fire behavior triangle.

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PREDICTING FIRE BEHAVIORThe foregoing discussion suggests that the behavior of a

wildland fire is a very highly complicated phenomenon, andso it is. Principles of aerodynamics, chemistry, thermo­dynamics, and combustion physics-operate in a relation socomplex that despite years of work researchers have not beenable to develop a satisfactory theoretical or mathematicalmodel. It has been said that we know more about theprinciples of atomic explosions than we know about theprinciples governing wildland fire behavior. But the lack of afire model and the gaps in our knowledge do not mean thatwe cannot predict fire behavior, at least to an extentadequate for most fire control and fire use operations. Overthe years, many wildland firefighters have developed aremarkable skill in predicting the probable behavior of a fire.In particular, this skill is shown by firefighters who haveworked in an area long enough to become familiar with theexisting fire environment patterns and the fire behaviorassociated with them.

The ability to predict fire behavior, whether for an entirefire or only a segment of it, required awareness that firebehavior is entirely the result of the interaction of the firewith its environment. To become skillful, we must recognizevariations in the fire environment and understand theirprobable effect on the fire.

In some respects, this is not a difficult task. The beginningfirefighter quickly becomes aware that a fire burning upslopebehaves differently from one burning downslope under thesame weather and fuel conditions, that wind speed anddirection can quickly affect the rate and direction of firespread, and that a fire in logging slash differs from a grassfire.

But many of the variations in the fire environment andtheir effects on fire are not so obvious, and skill inrecognizing them must come from experience and training. Afirefighter's ability is often equated with his years ofexperience with fire. But his real ability to control or use firesafely and effectively is best measured by his skill inpredicting fire behavior under prevailing conditions. Thisability may represent the difference between ten years ofwildland fire experience and one year of experience repeatedten times.

We have been concerned here only with the general natureof the fire environment and its variations. Other publicationswill describe in more detail the ways of recognizing variationsin the fire environment and their effects on the fire, as well asthe ways in which the fire itself influences its environment.

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SUMMARYFire environment is the surrounding conditions, influences,

and modifying forces that determine the behavior of a fire.Prediction of fire behavior for safe and effective control anduse of fire requires understanding of the interactions of firewith its environment.

The fire environment consists of three major components­topography, fuel, and air mass. From a fire standpoint,topography does not vary significantly with time, but doesvary greatly in horizontal space. The fuel component varies inspace and also in time; however, fuel characteristics, exceptfor the moisture content of dead fuel, change slowly enoughto be considered static for anyone fire. The air mass isusually the most variable component, changing rapidly inboth space and time.

The thermal energy responsible for most environmentalinteraction comes from the sun. Because the earth's surface isnot heated uniformly, temperature and air circulation pat­terns are set up that create large scale, local, and microscaleclimatic and weather patterns. The interaction of thesepatterns with other conditions determines the fire environ­ment for a particular area.

Fire can be considered as a local heat source. As such, itinfluences and modifies the fire environment. Because a firecreates high temperatures, it can dominate sun-caused heatsources.

The extent of the environment of concern to fire behaviordepends primarily on the size and characteristics of the fire.It ranges from a few feet to many miles horizontally andthousands of feet vertically. The vertical extent of theenvironment varies with fire intensity.

Most changes in fire behavior occur as the fire moves overthe terrain and as time passes. But abrupt changes can occurwhen a fire moves vertically from one kind of environment toanother, as when a surface fire in timber crowns.

Fire behavior is the interactions of the environmentalcomponents with each other and with the fire. The currentstate of each of these influences and their interactionsdetermine the behavior of a fire at any moment.

Fire behavior is the result of complex interrelationships ofaerodynamics, chemistry, thermodynamics, and combustionphysics. Nevertheless, it is possible for firefighters to acquiresufficient skill in predicting fire behavior to allow safe andefficient control and use of fire. Development of this skillmust come from experience, and from training in thefundamentals of fire behavior and fire environment.

* GPO 791- 976

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The Forest Service of the U.S. Department of Agriculture· .. Conducts forest and range research at more than 75 locations from Puerto Rico to

Alaska and Hawaii.· .. Participates with all State forestry agencies in cooperative programs to protect and im­

prove the Nation's 395 million acres of State, local, and private forest lands.· .. Manages and protects the 187-million-acre National Forest System for sustained yield

of its many products and services.

The Pacific Southwest Forest and Range Experiment Stationrepresents the research branch of the Forest Service in California and Hawaii.

14: u.s. Government Printing Office: 1972·794-236/4208