Fire Behavior

Everything operational in this badge rests on this page. Suppression tactics, safety doctrine, prescribed fire prescriptions, and evacuation timing are all attempts to predict what a fire will do next. This requirement has three parts:

• 5a — how the fire environment (weather, topography, and fuel) drives fire behavior
• 5b — how removing heat, fuel, or oxygen suppresses a fire
• 5c — drawing a diagram of the parts of a wildfire

The diagram you make for 5c is not busywork, either — you will use it again in Req 7a to demonstrate suppression tactics, so make it good.

Requirement 5a: The Fire Environment

Two identical fires do not exist. Light a match in cured cheatgrass on a 20 percent slope at 3 p.m. with 15 percent humidity and a 20 mph wind, and you get a fire that outruns a person. Light the same match in the same grass at 5 a.m. in a valley bottom with 80 percent humidity and no wind, and it may barely creep. The fuel did not change. Everything around it did.

Fire behavior specialists call the three things around it the fire environment triangle: weather, topography, and fuel. Note that this is a different triangle from the fire triangle in 5b — one explains how a fire behaves across a landscape, the other explains what combustion needs to continue.

Weather

Weather is the most changeable leg of the triangle and the one that most often surprises people.

• Wind is the dominant factor. It pushes flames forward into unburned fuel, supplies oxygen, dries fuel ahead of the fire, and carries embers that start spot fires beyond the main front. Doubling wind speed can increase rate of spread several times over. Wind direction changes are what kill firefighters, because a fire’s long flank suddenly becomes a wide, fast-moving head.
• Relative humidity (RH) is how much water vapor the air holds compared to the maximum it could hold at that temperature. Fine dead fuels exchange moisture with the air within an hour, so RH drives their flammability almost directly. Low RH means dry fuel and easy ignition; a good overnight humidity recovery is what gives crews a working window at dawn.
• Temperature matters because hot fuel needs less added heat to reach ignition, and because it drives RH down. Fires typically build through the afternoon as temperature peaks and RH bottoms out, then lie down in the evening.
• Fuel moisture, though partly a fuel property, is set largely by weather over time. Fire scientists sort fuels by time lag — how long it takes them to respond to a change in humidity. One-hour fuels are grass, needles, and twigs under a quarter inch; 1,000-hour fuels are logs over three inches. A short dry spell dries out the fine fuels; a multi-year drought dries the big ones, and that is when very large fires become possible.
• Atmospheric stability decides whether a smoke column rises freely. An unstable atmosphere lets the column tower upward, pulling in air at the base and intensifying the fire. Watch for towering cumulus clouds and dust devils.
• Thunderstorms cut both ways. They start fires with lightning, and their downdrafts produce sudden, strong, erratic winds in any direction — one of the most dangerous conditions a fireline crew can face.
• Passing cold fronts bring an abrupt wind shift, often from southwest to northwest, plus gusts and dropping humidity. Front passages appear again and again in fatality reports.

Topography

Terrain is the one leg of the triangle that does not change during the fire, which makes it the most predictable — and the most useful for planning.

• Slope speeds fire uphill. Flames tilt closer to the fuel above them, preheating and drying it by radiation and convection, so the fire spreads faster with less effort. The reverse is also true: fire backing downhill is slow and much easier to work.
• Aspect is the compass direction a slope faces. South- and southwest-facing slopes in the Northern Hemisphere get more direct sun, so they are hotter, drier, less shaded, and often carry lighter, more flammable fuels. They burn first and burn hardest.
• Chimneys, chutes, and narrow canyons channel heat and air upward like a flue and can produce sudden, extreme runs. Never work above a fire in a chute.
• Narrow canyons can also allow fire to spread across to the opposite slope by radiation, and they generate strong up-canyon and down-canyon winds.
• Saddles and ridgetops concentrate wind where it squeezes through a gap.
• Elevation affects fuel type, snowmelt timing, and how long the fuels have been curing.
• Barriers — rivers, lakes, rock outcrops, talus fields, roads, and previously burned ground — are terrain features crews will try to use as control lines.
• Diurnal (daily) slope winds are worth memorizing: during the day, heating draws air upslope and up-canyon; at night, cooling air sinks downslope and down-canyon. The reversal happens near dawn and dusk, and a crew that forgets it can be caught by a fire that reverses direction.

Fuel

Fuel is the leg people can actually manage, which is why Req 3a and Req 4b matter so much.

• Fuel type. Grass ignites instantly and spreads fastest but burns out quickly. Brush burns hot with high flame lengths and holds heat. Timber litter spreads moderately but heavy dead-and-down material produces long-lasting heat and difficult mop-up. Slash — logging or thinning debris — is among the worst.
• Fuel load. Tons per acre of burnable material. More load means more heat released and longer burnout.
• Size and shape. Fine fuels have a high surface-area-to-volume ratio, dry fast, and ignite fast. Big logs take a long time to ignite and a long time to go out.
• Arrangement and continuity. Continuous fuel lets fire run; patchy fuel makes it stumble. Vertical arrangement matters just as much: ladder fuels carry a surface fire into the crowns.
• Compactness. Loosely piled needles hold air and burn readily; tightly compacted duff burns slowly but can smolder for weeks.
• Live-to-dead ratio. Cured (dead, brown) grass carries fire; green grass resists it. Late-summer curing is why fire season has a shape.
• Chemical content. Some plants carry volatile oils and resins — sagebrush, juniper, chamise, eucalyptus, palmetto — and burn far more intensely than their size suggests.

Requirement 5b: Removing Heat, Fuel, or Oxygen

Combustion is a chemical reaction that needs three ingredients at once, and the fire triangle is the shorthand: heat, fuel, and oxygen. Remove any one side and combustion stops. Every suppression action ever taken is one of those three moves, and being able to sort tactics by which side they attack is exactly what this requirement is testing.

Removing heat (cooling)

Heat is what keeps the reaction self-sustaining: burning fuel releases enough energy to bring the next fuel up to ignition temperature. Break that chain and the fire goes out.

• Water is the best cooling agent available. It absorbs an enormous amount of energy warming to boiling and far more converting to steam, which is why a small amount of water applied to the right place does so much work.
• Hose lays and engine attack put water directly on the flaming edge and on hot spots.
• Aerial water drops from helicopters and scoopers cool and knock down flame length so ground crews can move in.
• Wetting agents (“wet water” or Class A foam) reduce water’s surface tension so it penetrates duff, moss, and rotten logs instead of beading off.
• Cold trailing in mop-up means feeling with a bare hand for residual heat, then digging out and cooling anything still warm.

Note what water usually does not do: extinguish a running crown fire outright. Air tankers and helicopters support ground crews; they very rarely put a fire out by themselves.

Removing fuel

Fuel removal is the core of wildland firefighting, and it is what makes wildland fire so different from structure fire.

• Fireline (also called a control line or handline). Crews scrape a strip down to bare mineral soil so there is nothing left to burn across. Width depends on flame length and fuel; a grass fire may need two feet, heavy timber many times that.
• Dozer line and plow line. Machines do the same job much faster on suitable ground.
• Burnout and backfiring. Firefighters deliberately burn the fuel between their line and the advancing fire, so when the main fire arrives there is nothing left. This is a high-skill tactic that requires authorization.
• Reducing fuel in advance. Prescribed burning and thinning are fuel removal too — done years early.
• Removing snags and hazard trees takes away sources of embers and firebrands.
• Retardant lines. Long-term retardant dropped from aircraft chemically alters fuel so it will not ignite, effectively taking it out of play even though it is still physically there.

Removing oxygen (smothering)

Fire needs roughly 16 percent oxygen to keep burning; normal air is about 21 percent. Cutting off air supply is the least-used of the three at landscape scale but very common on small fires and in mop-up.

• Throwing dirt on flames with a shovel smothers them, and it is the classic tool when water is not available.
• Swatting and beating with a flapper or spade separates flame from its oxygen supply and knocks it down.
• Covering smoldering material with soil after it has been cooled deprives it of air.
• Foam and gels blanket fuel and block both oxygen and heat transfer.
• Stirring and separating burning material actually works the opposite way for a reason worth knowing: pulling a pile apart during mop-up increases air but removes the mutual radiant heating that keeps the pile burning.

Requirement 5c: Draw a Diagram of the Parts of a Wildfire

Every wildland firefighter uses the same vocabulary to describe a fire’s shape, because on a radio there is no time for “the part over on the side near the road.” Learn these terms and your diagram will practically draw itself.

• Point (or area) of origin — where the fire started. Investigators protect this spot carefully; it holds the evidence of cause.
• Head — the part spreading fastest, usually driven by wind or running upslope. The most intense and dangerous part of the fire.
• Rear (also heel or base) — the slowest-spreading part, usually opposite the head and often near the origin. The lowest-intensity place to begin an attack.
• Flanks — the sides between the head and the rear. Named right flank and left flank as viewed standing at the rear looking toward the head. Getting this convention right is exactly the kind of detail a counselor notices.
• Perimeter — the entire outer edge of the burned area.
• Fingers — long narrow strips of fire extending out from the main body, usually where fuel or terrain favors spread. Fingers can create dangerous pockets.
• Pockets — unburned indentations in the perimeter, typically between fingers.
• Islands — patches of unburned fuel inside the perimeter. They can ignite later and threaten crews working the interior.
• Spot fires — new fires started outside the perimeter by embers carried ahead of the fire. Frequent spotting is a sign the fire is about to become uncontrollable.
• Black — the burned area behind the fire. Once cooled, the black is a place with no remaining fuel, which is why it can serve as a safety zone.
• Green — the unburned vegetation ahead of and around the fire.
• Slopover (or breakover) — fire that has crossed a control line but is still small enough to be caught.
• Smoldering / creeping / running / torching / crowning / spotting — standard descriptions of how the fire is behaving, listed roughly from calmest to most extreme. Torching is a single tree or clump burning from bottom to top; crowning is fire moving through the canopy.

You can now read the fire environment, explain how suppression works chemically, and describe a fire’s anatomy in the language crews actually use. Before anyone gets near that fire, though, there is a subject that outranks every tactic in this badge: keeping firefighters alive.