Walking on Fire: Part 1
You’re six days through a week-long thru-hike, making good time. It’s a brilliant, bluebird day. You’ve gone light on food, but it feels so good to be out in the mountains that you hardly notice. Your spouse is meeting you at the trailhead, only ten miles ahead. You crest the final pass and gaze downvalley.
Your chest tightens. The forest canopy is awash in pale gray smoke, rising upslope towards the ridgeline. Although no flame is visible from here, it’s clearly a big fire, at least a quarter-mile across. Vertically, it stretches from just above valley-bottom to timberline – and the trail goes right through the heart of it. As if on cue, an usually large puff of smoke and dust emerges from the near edge. A moment later, a muffled, clattering crash echoes through the valley: the sound of a big tree falling. Your heart rate jumps. You’ve got 2,100 calories of food left and no communications equipment. You’re in trail shoes. Your ride is on the other side of the burn.
You stop, breath deeply, and wash the jolt of adrenaline form your system. First, you realize, you need to assess the fire and terrain. Only then can you form a plan.
As light hikers, we cover large distances and travel more deeply into remote areas than many other wilderness users. As a result, we’re more likely than most wilderness users to encounter uncontrolled and unreported fires. Fortunately, the same factors which increase this likelihood – light loads, mobility, speed, and comfort in the backcountry – also make the light hiker unusually well-suited to respond to such a situation.
How do you assess the fire and anticipate its movement? What do you do?
The Nature of Flame
The Fire Triangle: Venerable icon of fire science, the fire triangle depicts the three critical components of fire.
Start With How We Cook
To understand wildfire behavior, we need to start with the fundamental nature of fire. Let’s examine a familiar tool: the liquid-fuel stove. In a stove, we combine an ignition source (heat) with oxygen and fuel. This starts a self-sustaining, exothermic (literally “heat out”) chain reaction called combustion. In a liquid stove, fuel flow-rate is usually used to control combustion intensity, but oxygen is used as the control mechanism for the physical location of the fire. The metal fuel-supply tube contains moving fuel and becomes very hot, but with the tube wall separating fuel from oxygen, no combustion occurs inside it. Combustion occurs only at the fuel aperture, where the fuel “daylights” and circulates with oxygen-rich air. A byproduct of this reaction, at the atomic level, is the release of photons, which we observe as flame.
A 100,000-acre forest fire burns under the same constraints as a stove. In fact, we can push the metaphor a lot further. Within the metal fuel tube, a process known as preheating occurs, in which the fuel is warmed. Since combustion only occurs above a certain temperature (which varies for different fuels), this heating prepares the fuel for combustion. At a certain point in the fuel tube, the liquid becomes a gas. It is these vapors – not liquid fuel- which actually combust with the oxidizer (oxygen).
A critical point: it is the vapor which combusts, not solid or liquid material. The carbohydrate and hydrocarbon fires we naturally encounter, from burning trees to burning cars, are flammable vapors combusting with atmospheric oxygen. Fuel is heated until it releases flammable vapors, the vapors mix with air, and the mixture combusts. Fire as we’re generally familiar with it is a reaction occurring in hot gases, and it behaves as such.
When you wrap an aluminum heat-shield around your stove, you create the “oven effect” of a heat shield, reflecting and/or capturing and re-radiating infrared heat. A pile of logs does the same thing – as does steep canyon walls. Dumping a cook pot of cold water on the stove absorbs the combustion heat, robbing the chain reaction of its “heat engine” and thereby extinguishing it, and may also momentarily occlude the oxygen supply.
When you blow out your stove – or a candle – you’re literally pushing the “cell” of hot gas away from the fuel source. The cell burns out or dissipates. You’ve literally blown the heat “off” the stove. If that object is hot enough (i.e., the metal on the stove), it will re-supply the heat-of-combustion, and the chain reaction will begin again. For this reason, it’s easy to blow out a candle or burning pine needle (except in very dry conditions), but a superheated log may endure even momentary immersion in a helicopter bucket drop: its mass forms a huge “heat battery.”
Fire on the Land
Basic Fire Behavior, Suppression, and Assessment
South Libby Fire, 2001
Our Hotshot crew marched through an ashy moonscape, skirting a burned and very unhappy little rattlesnake. Near midnight, we set to work cutting line up Libby Peak, following the fire edge and a flagline left by our scouts. Without solar heating and daytime winds, the fire had “laid down,” but was still burning actively in places. In fire lexicon, “humidity recovery was poor,” meaning the temperature stayed high and the humidity low all night. Tearing a swathe of cleared brush and barren soil up the mountain, we snaked along the very edge of the fire. The contingency plan was simple: if the fire started heating up ahead of us or spots fires started below our position, we’d step across the low-intensity edge behind us, and walk into the wilderness of ash behind it. There was nothing left to burn out there.
After sunrise, Tony, our superintendent, pulled us back into “the black” to await recon. We hiked up to a ridgeline and saw, for the first time, what he’d had us cutting around: an entire drainage running up the mountain, incinerated. The entire bowl, perhaps a half-mile or more long, had been sterilized. Black skeletons of trees and white ash were all that remained.
We sat down in the ash and held position as the fire wakened to a new day. A helicopter landed on the ridge and took Tony up on recon. Smoke activity increased, sending iron-gray plumes up from the mountain’s timbered flanks. Trees torched intermittently on a distant ridge, shooting banners of orange flame into the sky.
Miles to the north, a white cumulous cloud rose from an unseen valley, a series of wispy, lens-shaped cirrus clouds on its crown. Silent and alabaster, it bespoke – to the cognizant eye – violent and infernal events below.
It turned out this was the “column” from the Thirtymile fire. As we watched, it was burning four firefighters to death.
Technical Points of South Libby: South Libby demonstrates some basic and very practical fire science. In the following analysis, we’ll look at
- Parts of a Fire
- Topography: How Fire Interacts with Terrain
- Basic Fuel Types: What Burns and How
- Isolating the Fire and Mopping Up: The Basic Firefighting Acts
- Torching: Fuel Arrangement and Vertical Fire Behavior
- Nighttime Laydown
Parts of a Fire
Parts of a Fire: The three basic parts of a fire exist simultaneously across scales and can be identified on most flames.
Any given patch of fire has three basic parts – head, flanks, and heel or back – and five common ways of spreading – head fire, flanking fire, backing fire, spotting, and roll-out. In the graphic, we see the three spread types associated with the parts of a fire: head, flanking, and backing fire. These fire “types” occur on any given patch of fire, from a candle flame to the 175,000-acre Tripod fire.
At South Libby, we “anchored” at the vertical bottom of the fire, on the low-intensity heel. From there, we dug all night upslope on flanking and backing portions, where it was – in fire parlance – a “surface burn.” The head fire, on the other hand, had been so hot earlier in the evening that it had sterilized an entire drainage.
When interacting with any flame, practice differentiating the three basic parts of a fire. As you learn to differentiate them at a glance, you’ll be able to identify the hotter, faster-spreading parts of a fire almost instantly. The fatal Thirtymile Fire – which we watched “blow-up” – arguably resulted in four deaths because the trapped crew thought they were at the heel of the fire – the backing portion – when they were in fact near the head.
Light Hiker Tactic: Aside from actual flame angle measurements, nothing indicates head, flank, and backing fire as well as smoke. Activity may vary due to varying fuel types, but smoke almost always goes into the black at backing fire, along the fire front at flanking fire, and away from the fire and into unburned areas at head fire.
Topography: How Fire Interacts with Terrain
Basic Topographic Effects.
An idealized fire, on a continuous fuel bed, on a flat horizontal surface, and in the absence of wind, will spread in a more-or-less perfect circle. Very few of us, however, hike in an idealized windless version of Iowa. Usually, we’re in wind and rolling or mountain topography, which changes everything.
Wind, by displacing the hot gases via air movement and the radiant heat flux via flame-angle, will progressively attenuate the perfect circle of a flatland fire into an oval that gets progressively longer and thinner, as wind speed increases.
As terrain gets steeper and more complicated, terrain rapidly becomes a major influencer on the fire, eventually dominating it. This is partly through terrain’s place at the top of the food chain: mountain topography shapes local winds and controls water flow, creating the microclimates and vegetation patterns that further influence fire. Fortunately, fire interacts with topography in a reliable set of ways.
Basic Wildfire Topographic Effects:
- Fire sends heat, smoke, and flame uphill at a rapid rate in proportion to the steepness of the slope.
- Fire sends rolling and burning material downhill.
- Fire flows up gullies, draws, box canyons, and other features that funnel airflow. This is known as the “Chimney Effect.”
- Fire flows into saddles and passes.
- Fire typically loses intensity when striking a ridgeline, after running up slope.
- Deep gullies and enclosed terrain can have an oven effect, containing heat.
- Slope winds (diurnal updrafts, nocturnal downdrafts, and foehn or gravity winds) will drive fire.
Basic Fuel Types: What Burns and How
Spread Rates in Different Fuel Types: Three fires in otherwise similar conditions, at the same time, “t“, after ignition. All have spread from origins at roughly the same points (the white circles at middle and right, the small patch of black at left), with different spread rates (how fast they spread) and residence times (how long they burn on a patch of given ground). Residence time (coupled with intensity) affects the final temperature of the “black,” and hence the amount of time until it is inhabitable.
The nuances of fuel type are as varied as the biome itself and are further affected by humidity and arrangement. However, for a quick and intuitive ability to interpret fuel types, we can split them into three loose categories: Fine, Light, and Heavy. The cheat sheet on them:
Fine Fuels: Flashy Fast-Movers. Fine fuels are grass, pine needles, twigs, chaparral brush, and any other fuels that burn with a tinder-like intensity and speed.
- Burn hot, fast, and flashy.
- Can spread at very high rates in conducive arrangements (cured tall grass, standing), moving more quickly than most people can run.
- Cool quickly after burning.
- Absorb rain and atmospheric moisture rapidly.
- Can dry again in less than thirty minutes of sunlight and/or warm, dry wind.
Light Fuels: Sticks and Kindling. Light fuels, for this discussion, include kindling-weight twigs and a variety of sticks. Fine fuels will often readily ignite light fuels, and light fuels assist in igniting heavier fuels. Light fuels will usually equalize with ambient humidity over a period from thirty minutes to several hours.
Heavy Fuels: Environmental Equalizers. Heavy fuels include large branches, downed logs, and standing snags.
- Ignite and engulf relatively slowly.
- Generate tremendous heat when fully engulfed in flame, if dry.
- Burn for long periods, if dry throughout.
- Act as heat sinks for the fire around them, if wet.
- If dry, burning heavies can superheat the environment they burn in, making it hostile for some time after they are consumed.
- Absorb rain and atmospheric moisture very slowly. Rain squalls have little effect on them.
- Carry their moisture loads for days or years. Heavies reflect the environmental norms of drought or rainfall, not the last few days’ weather.
Light Hiker Tactic: Fuel Type Assessment on the Landscape. The categories of Fine, Light, and Heavy are metaphorical. In reality, fuels exist on spectrums of surface area, humidity capacity, thermal mass, and energy content, but these categories are useful tools.
Standing in any given place on the environment, break down every readily burnable thing you see into Fine, Light, and Heavy Fuels. Knowing how each burns (fines are flashy, heavies burn long and hot, and lights are in-between), what scene of fire intensity and duration does this create for you?
Isolating the Fire and Mopping Up: The Basic Firefighting Acts
Basic Suppression Sequence: Head fire is most difficult to contain. In this case, line is dug at “stand-off range.” Holding it may be difficult.
The basic wildfire containment technique is to isolate the fire by separating the burning fuel from unburned fuel. This done by creating a firebreak, a space which the fire won’t burn across. Isolation via firebreak is the primary tactic used on most remote fires in the western U.S. High-pressure water pumps, foam, helicopter water-drop, and other technical innovations are only accessories to the basic act of isolation.
Once the fire is contained, the primary extinguishing method is “dry mopping,” or churning any hot fuel in with damp soil. This isolates burning material at a fine scale (by putting soil between pieces of burning fuel) and uses the water in the soil to extinguish the fire.
As a salty old foreman of mine once said: “Separate the green stuff from the black stuff, put the brown stuff on the orange stuff, and stay out of the gray stuff. It’s not that complicated.”
Light Hiker Tactic: Quickie Fire-Isolation. With a very small fire, simply scrape the area around it down to mineral soil. In most cases, this will stop all spread. In more forgiving conditions, just sweeping back the most readily available fuel (pine needles atop a damp organic soil) is enough. Pouring or splashing water around the edge may also work, at least temporarily.
Light Hiker Tactic: Dry Mop-Up. Whether containing a small fire or putting out your own campfire, some simple and reliable dry-mop techniques are:
- Churn damp soil from under the fire into it. This is known as “potato patching.”
- Rub or “scrub” heavy fuels with soil, when they are cool enough.
- Check for heat hiding in cracks or fissures in logs.
- Chip off any exterior scab of charcoalized wood. This charcoalized layer can insulate heat beneath it. The burnt scabs are known as “gators” because they look somewhat like alligator hide.
- The dry mop is complete when you can dig your hands through all parts of it and handle all fuels without discomfort. This is known as “cold trailing.”
- After this, the fire-area may resemble a churned-up garden.
“Potato-patching” is a common hallmark of smokejumper tactics. You may have encountered aging areas of heavily worked ground, mixed with old burnt wood, in remote wilderness areas. Now you know: these are probably old smokejumper fires.
Torching: Fuel Arrangement and Vertical Fire Behavior
|Vertical vs. Horizontal Fuel Arrangement: At left, a “tipi” fire uses vertical fuel arrangement and inward angled fuels (which help channel air), creating an oven effect. A powerful central heat column drives efficient ventilation via a single large convection cell, and preheats the higher-placed fuels.
At right, fuels are spread horizontally. Ventilation is broken up into micro-cells. Heat escapes upward, failing to preheat other fuels, weakening the heat engine.
Fire spreads vertically as well as horizontally, and vertical spread – where fuel arrangement allows it – is more spectacular as a result of gas dynamics. From our perspective, the most important behavior is torching, the process by which fire climbs up and ignites the canopies of trees. Key points of torching:
- Often indicates increased fire activity. Increases in the torching rate often mean the fire is ‘heating up.’ A fire that is frequently torching should be given a lot of space. This fire may produce crown runs, in which the fire runs through the canopies of tightly spaced trees.
- Radiant heat potential. A torching tree or large mass of ground fire can produce enough infrared radiation to burn exposed skin. It’s a bit like being microwaved. Light-Hiker Tactic: If you must pass through an area with large flames and high radiant heat, pass through quickly. Consider wearing long sleeves, long pants, and a hat. A dry bandana can be used as a face-shroud in extreme situations.
- Spot fires and ember-cast. Torching often creates ember-cast, which may start spot fires. Spot fires are covered later, in Part III.
Fire is influenced by moisture, wind, latent temperature, and solar heating. Nighttime hours create a familiar drop in temperature, but also a rise in relative humidity (hence nocturnal dew), and frequently a lessening of solar-driven winds. In most situations, the fire will “lie down” (decrease in activity) overnight. Weather conditions and solar exposure being equal, wildfires burn hottest in mid-late afternoon (4:00 p.m. is a good landmark for the peak activity) and may continue to “roll” well into the darkness hours, easily to 8:00 or 9:00 p.m. in the right conditions. Fires are usually most quiescent during the hours before dawn (around 4:00 a.m.). This corresponds to the hottest and coolest portions of the day, which tend to rotate around the planet with a significant lag behind solar noon and midnight. This will be modified by changing weather (a cloud layer rolling in) or solar exposure (the fire might be on a very steep east aspect and lose its solar heating shortly after noon).
Light Hiker Tactic: Night Crossing. If you need to transect an area with an actively burning fire, wait until weather conditions and time-of-day cause the fire to “lie down.” Knowing that fire activity tends to lag behind solar time allows you to avoid the otherwise-easy mistake of attempting to transect a fire just as it enters its most intense burning period.
Light Hiker’s Tool: Basic Fire Behavior Screamsheet. My own experience in fire consists of three years Hotshotting and three years smokejumping – a very modest career, in light of all there is to know about fire. It’s therefore with real hesitancy that I list what I consider the most useful principles. That said, the key wildfire survival and avoidance principles, as I would summarize them, are:
- Fire follows the terrain, burning fast and hot uphill.
- Wind pushes fire.
- Watch the smoke. Smoke provides information about what the fire is doing.
- Fire spreads very quickly, but burns out very quickly, in dry grass and other light fuels.
- Fire spreads slowly, but burns hot and long in logs and other heavy fuels.
- Ravines, saddles, box canyons, and drainages funnel wind, and thus funnel intense fire. They are dangerous places if fire is below you.
- Higher temperatures + lower humidity = increased fire intensity.
- Lower temperatures + higher humidity = decreased fire intensity.
- Fire usually ‘lays down’ at night, in the rain, and as conditions cool.
- Fire mirrors water as it moves through the environment, in both time and space.
- It takes more than one rainstorm to put out most wildfires.
- Don’t ever assume a fire is out or “dead” (no longer capable of activity) unless it has been totally cold-trailed. One ember can ultimately generate a blow-up.
- A burned environment is radically destabilized.
- Nothing about fire is random.
Fire and Water
How Fire Moves in the Environment
Air flow, topography, weather, fuel type, and fuel arrangement are basic parts of fire behavior, but they don’t necessarily help us make more than cursory sense of what a given fire is doing. With these tools alone it’s very difficult to gauge what a fire will do. Anticipating fire requires an appreciation of its fundamental counterpart: water.
Chena Hot Springs, near Fairbanks, Alaska
Chena Hot Springs was a wet, cold, uncomfortable fire. The “grunge” was going around camp, which was itself a series of mud parking lots. In the zeal to create neat, disciplined rows of tents, my tent had been pitched atop a bush. Periodically, it rained. For the better part of a week, my leather boots and a conservative inventory of socks were somewhere between uncomfortably damp and sodden. We cold-trailed through the tundra along a complex, multi-fingered fire edge, ploughing through black spruce. Resinous spruce needles poured down our collars as we dug our bare hands through the damp moss and ash, feeling for hot embers.
Cold trailing in the tundra was interminable. Varying fuel moistures and burning intensities in the deep, vegetative layer had created a multi-level burn, hence multiple edges. The surface burn crept through lichen, cranberry, moose poop, and other material at the very top of the tundra. Incidentally, I have never seen much fire performance out of burning ungulate droppings. Their fuel arrangement is too compact, and they retain internal moisture too well. Usually, they are heat sinks. They are no substitute for cow pies or yak dung.
Below the surface was a rich, mossy mat, which the fire ate through more slowly, lagging behind the surface burn. Below that was deeper, moister organic material, through which the fire only burned in highly favorable conditions, but was triggered by a big heat source, like a log. The result was multiple fire tiers at ground level, each with a different edge, sometimes separated by tens or hundreds of feet, which combined with horizontal moisture variations from small-scale terrain and vegetation changes. The resulting fire edge was a maddening, three-dimensional mosaic of heat, ash, and unburned material. Under the right conditions, an ember almost anyplace in the entire Rubik’s Cube had the potential to creep beyond the furthest edge, and ignite a new flame front.
Below all this, sometimes less than twelve inches down, was permafrost. Mopping out smokes, we’d chop into the dirty ice, excavating chips of it to mix with the smoldering moss. As we worked atop one of the world’s largest continuous masses of ground ice, running from the Canadian Atlantic to the Bering Sea, almost none of it was available to influence the fire.
A thunderstorm rolled through. We spread out off the ridges and sheltered under space blankets as the light rain fell. Gusty winds raked their chill fingers through the boreal forest. The cell passed, then the sun broke through and heated the upper surface of the tundra. The fuels began to dry out again. Hidden heat re-emerged from sheltered nooks in the mossy edge, and the fire resumed its advance.
On hot portions of the fire, the black spruce were generating crown runs. Green and verdant, looking only a step removed from a temperate rainforest, they burned intensely in atmospheric humidities that would have laid down a fire in most Rocky Mountain biomes. Why? Hydro-botany, to coin a term. To survive the bitter interior winters, black spruce “drop” most of their water content. Although still green and wet-looking, come spring they are dry “resin bombs”: live masses of cellulose and resin, ready to burn hot.
Fire Mirrors Water
The Chena Hot Springs fire was a tour of water in the environment and how it influences flame. It’s no coincidence that humans across cultures place fire and water at opposite ends of the elemental spectrum, and they’re often associated with opposite-but-associated polarities: Yin and Yang, Sun and Moon, Male and Female, Chocolate and Peanut Butter.
Water is not combustible, in any state. Water has tremendous heat capacity – perhaps 200 times that of air – and the evaporation of water consumes even larger amounts of heat energy. For combustion to occur in typical forest fuels, the fuel must be heated sufficiently to vaporize the fuel’s water content, driving it out as steam. Only then, once at least the surface layer of the fuel is dry, will the fuel become available for burning. This is the basic way water inhibits burning.
To understand the movement of fire in the environment, look to water. Everything in our environment exists in a constant equilibrium with atmospheric humidity. If the air is more humid than the fuel, water transfers into the fuel. If the air is drier, water liberates from the fuel into the air. This process occurs rapidly for fine or light fuels with their high surface-area-to-volume ratio, which go from soaking wet to tinder-dry in less than half an hour, but slowly in heavy logs, which carry moisture loads in their heartwood from one year to the next. Incidentally, this also explains why in a hot summer in a climate with a decent annual moisture average, often only the outer inch or less of a log will burn: as the fire burns inwards, it encounters a trend of moisture increasing towards the annual average. Eventually, it sputters out.
On a large scale, the different water content of our environment is easily observable as changes in vegetation. On a smaller scale, microclimates have a major affect on fire behavior. Moisture tends to collect preferentially in draws and bottoms, where to some extent it may counteract the chimney effect of these areas. Shade elevates relative humidity. Proximity to water sources (the proverbial riverine habitat) increases the amount of water available for evaporation.
Vegetation itself creates microclimates, since living plants are – in effect – biological water pumps. Large masses of living vegetation, like a timber stand, will increase humidity of the area, through a combination of shading, cellular respiration, and the their own standing water mass. Concentrations of dead and downed trees in timber stand make more dry fuel available, and may also make the whole area drier by action as massive moisture-sinks.
Subtly, humidity even varies with distance from the ground. Dry tinder held near damp ground dampens more quickly than it does at eye-level. Fire, in turn, will change its behavior through these microclimates.
At the risk of sound romantic, fire and water dance with one another. It’s a bit like the tango: intricate and flashy, but based on a clear underlying relationship and rules. Water leads, less obtrusive but very communicative, in a dark suit. Fire follows, athletic and spectacular, a red satin dress.
Light Hiker Tactic: Quickie Microclimate Assessment. Humidity is a quick indicator for how intensely the fire is likely to behave in any particular area. If you come upon an uncontrolled wildfire, you want to be able to calibrate specifically how fuels in the local area are burning. Look to see if there are areas where fire isn’t spreading or burning well. This will help you calibrate your sense of local conditions.
To find areas that won’t burn, seek green grass, water-loving plants, and stands of living trees like willows and cottonwoods. Dead and downed wood that is heavy in your hands has higher water content, and therefore will ‘burn cool’ or even act as heat-sink, impeding the fire.
As a general principle, these types of terrain or microclimates have a certain humidity character:
- Riparian / Riverine Areas: More humid than the general terrain.
- Riverine Whitewater: Extremely humid. The air carries an increased load of moisture due to aeration and suspension of the whitewater.
- Swales and Depressions: Often collect humidity. Look at the plant character.
- Ravines and Draws: May or may not be more humid than the general area. Avoid them unless these areas exhibit clear evidence (ex: green water-loving plants, whitewater, etc.) of high humidity and are not threatened from below. Otherwise, they are terrain-channels for fire and smoke.
- Ridges: Drier than surrounding areas. Although fire is often less intense on ridges than in draws and terrain concavities, this may be counterbalanced by lower humidity. Again, look to the plant type and health for clues.
Next week: Part II
- Escape & Evasion
- Using the Tools You’ve Got