I want to cover fire behavior, the public needs to know. I want them to be able to recognize land that is resistant to wildfire and how fire will travel through it, and what keeps it on the forest floor.
Wildfire Science & Behavior
(In simple layman’s terms)
A large wildfire is analogous to a simple campfire. By observing a campfire you can see a lot of the same phenomenon that goes on with a larger wildland fire. As combustion occurs, the heated air expands and becomes buoyant and rises. It’s the same way a hot air balloon gets it’s lift, except that the balloon captures the air to lift a load. With out that containment the air is free to rise, and become much like a dust devil or a tornado. I like to describe the effect like holding a vacuum cleaner nozzle to the ground. The surrounding air is sucked into the vortex, heated and sent on it’s way with the column of hot air. The effect of this is like blowing on a fire to get it going, or a blacksmith super heating coal using a bellows so that they can soften steel. I forces oxygen into the fire and we get super efficient burning and more BTUs of heat, which heats the air which rises and sucks in more air. So when they say; fire creates it’s own weather the bigger it gets; this is the engine that causes that.
Back to observing the campfire. The effect is very similar to a hailstorm from a thunderhead. The water droplets are drawn up into the column, and held there until the cooler upper atmosphere blocks their rise where they then get expelled to the outside of the column and fall back to earth. You know how hail is usually at the beginning and the end of a storm passing over. Well, watch how the hot embers on a campfire play off the column. Now add a little breeze to the campfire. The upwind side gets fresh air, while the downwind side gets recycled air that has been heated as it passes the flames. But air is being drawn into the fire from ALL sides. The column blocks the wind, but the air that makes it around gets sucked into the fire front along with the falling embers. It’s these embers that cause spotting ahead of the fire.
Weather and atmospheric conditions really play a big role as to how well this all works. You can have an inversion of air movements in the two layers of atmosphere that won’t let a column get started; or worse yet, collapse a monster column and make it expel all of the burning embers in all directions.
If you are going to fight wildfire, there are a lot of things that you can’t see that will get you, but if you understand these things and the indicators to watch for, you can generally avoid getting trapped. Observing the column of smoke tells us a lot. If you have smoke going over your head, you are probably in the line of fire.
Weather and Fire Fuels
The biggest factor affecting a wildfire is the weather, more specificity the moisture temperature and wind.
Moisture has a long term and short term influence on light flashy fuels like grass; and heavy fuels like green standing trees and dead & down logs. Light fuels are affected short term; and heavy fuels change on the long term scale. The fire service classifies these fuels as 1 hour to 1000 hour, depending on their diameter and how quick they dry out, or absorb moisture. Water boils at 212º F, and fire has to steam out the moisture in the fuel in order to begin the combustion process. The energy it takes to turn water to steam, makes moisture a major inhibitor to the spread of fire.
Wind will move the fire horizontal and transfer heat horizontally either by convection or conduction. Wind feeds oxygen to a fire making it burn hotter; resulting in heat transfer by radiation; the third method by which fire travels. The wind will also carry embers afar, starting new fires ahead of the main fire.
The suns radiant heat and the daytime heating of the air, warms fuels driving out moisture. Nighttime comes, and the cooler air further draws out moisture until the fuels cool. Morning usually sees a small recovery of fuel moisture in lighter fuels, as the cooler material absorbs moisture from the warming air.
Flashy Fuels: They include grass, light underbrush and the outer covering of heavy fuels, are the transportation for fire to find new sources of heavy fuel. If the flashy fuels are too wet to transport fire, you have a small chance of your fire going anywhere. They are called flashy, because of the time it takes to absorb humidity, or to loose it. Especially if they are dead and dry to begin with.
The influence of the sun’s heating during the day, and the cooler temperatures at night make this these fuels the most important factor for predicting extreme fire behavior. That’s why it is so important for firefighters to sample the temperature and Relative Humidity at regular intervals while on a fire. If the humidity drops below a certain point, you can expect your fire to start making runs; or torching single or multi, tree stands. These events are usually associated with the “burning period”, at the latter part of the evening when fires usually exhibit the most extreme behavior. However, in recent years, extreme behavior runs into the nighttime.
Flashy Fuels near the control line: The width of a control line is determined by the height of the fuel at that location. The greater the fuel (e.g. grass) height, the greater the flame height. Also think about grass stalks falling across the control line. A simple solution to flashy fuels like grass, is using a weed eater to lower fuel height, so the burning stays close to the ground where air flow is cut off and the ground absorbs the radiant heat. If people were to weed eat around their homes each season, that would be one of the most cost effective ways of reducing risk to your home.
Heavy Fuels: Large Diameter Green Trees and Dead Logs, usually loaded with medium size logs and debris (ladder fuel). Living Trees, have roots which draw available moisture from the ground, and are influenced mainly by rainfall. Dead material on the other hand is influenced by rainfall and Relative Humidity.
Heavy fuels are the source of heavy torching and large black columns of smoke that can carry burning debris like pine cones and sticks, sometime miles ahead of the main fire. They make fighting fire very difficult and force firefighters to take an indirect approach to stopping it’s spread. Heavy fuels burn hot for a long time causing damage to surrounding trees and ground cover. Heavy fuels have more radiant heat transfer, causing surrounding fuels to dry out and catch fire. The closer these fuels are arranged to each other the more extreme the overall fire will be.
Snags (Dead Standing Trees): Snags that are close to a control line present a problem because they catch fire easily, and the sparks they give off travel with the wind for great distances. They tend to fall unexpectedly, and if they fall across the control line, they can really cause problems. In the interior you need to leave these snags as they are great wildlife habitat.
Terrain’s Effect on Fire
Let’s add a slope the picture, where the uphill terrain gets radiant heating of it’s fuels; picture what the air is doing in relation to the rising column of heated air. If we add wind pushing it into the hill, it will lay the flames right into the hillside causing rapid heating ahead of the flame front; leading to runs up the hill. Fire always wants to burn uphill; and the only thing that can stop it would be the wind blowing the opposite direction. I think of the Ski Run Road Fire, that started at night, on a chilly November. The winds were coming out of the NW, and the fire was making a run up a SE slope, with the slope blocking the fire from the wind. As the column rose above the ridge it was whisked back over the main fire; kind of bending the column back and allowing the air to ventilate through the flames below. All the way over to the Eagle Lakes Campground on Mescalero land, they were having spot fires start up. That’s more than a mile away from the main fire. So here we have a whole mix of variables to deal with; and I would say that experience fighting wildfire is the only thing that could possibly be referenced to predict where a fire is going to go. In the big picture the wind is what pushes a fire as a whole, but on a smaller scale, the shape of the terrain effects the local behavior, and the strategy used to deal with the fire.
The fatal situations that have caught firefighters unaware, took place in what’s called “chimney canyons”. Basically it’s a steep canyon slope, with steep sides that get preheated from radiation given off the fire; which is going up the middle of the canyon. Add upslope winds, both from the weather, and wind generated by the column, which is trapped in the effect of the box canyon, and you have a chimney that pulls a draft of fresh air through the fire, and pushes it down against the slope.
By thinning these chimney canyons, we can eliminate the threat of long range spotting, as this is where the most severe burning occurs. We don’t need to thin most of the area on the forested side of our control line, just the areas that will lead to long range spotting if a fire were to get blown through there. These canyons deserve special attention over the rest of the terrain.
Using Fire to Fight Fire
Also known as back burning, burning out, firing out or back firing. The idea being that once it burns it’s NOT going to burn again, unless it burned only part of the fuel, dried it out and made it ripe for that piece of ground to burn a second time. For our purposes, I will forget about the re-burn potential.
Building control line around a fire, you can either go direct, which means constructing the line as close to the fire as possible and letting the fire creep to the control line and go out. Or, if it’s too hot you go indirect and back off, usually to natural barriers where there will be less fire activity. In both cases, any unburned fuel inside your control line has the potential to flare up at a later date. It’s best to set fire to these fuels and get it over with. Firefighters refer to burned and unburned as “green and black”.
Attacking a fire directly, and following the burning edge building line, firefighters use the black as an Escape Route and Safety Zone. If anything goes wrong, they always leave time to use an Escape Route to get to a Safety Zone. They fall back to the “black” where they have already burned out and secured the line. In other words your Safety Zone follows with you as you progress.
Attacking fire indirectly, you have a lot of fuel between you and the fire. (not good) The trick is to burn back from your control line into the fire without it crossing the line; or the fire getting to the line before you get a chance to burn the fuel off. This takes a lot of planning and coordination before attempting. The weather has a lot to do with success, and the other half of the problem would be the fuels you are going to burn. Are you going to have torching sending embers afar, or heavy fuels on both sides of the line making it hard to keep it from crossing and getting away. These things can be controlled ahead of time, and waiting until the last minute to prepare, will give the fire and weather time to blow your best intentions.
This is my reason for pushing for a fuel break at the edge of town.
Prescribed Fire vs. Wildfire
We can wait for the right conditions to do a controlled burn, and have a good idea what the weather will be in the near future. Wildfire on the other hand dictates when, and usually chooses fire season to do it’s thing. They call it fire season for a good reason!
There is a whole imperfect science to finding the right moment to set fire to the woods, because of the many variable factors that can’t be measured. That’s why when all is said and done, a smart firing boss will always do a test burn before attempting to light the whole thing. The watch how it carries through the fuels and how intense it burns before chancing a major move. It seems there is a fine window to fuel moisture content, where it either burns too intense or it doesn’t burn at all. You want to burn in that window during favorable weather conditions.