This Thing Called Fire

Fire. Whether it’s a tiny flame flickering on the end of a birthday cake candle, or a wall of flame 200 feet tall and a mile wide roaring through a forest, all fire is essentially the same. In simplest terms, fire is a chemical reaction.

Okay, here’s a slightly more technical explanation. Fire is the naturally occurring companion of energy release in the form of heat and light when oxygen combines with a combustible, or burnable, material at a suitably high temperature (about 617 degrees F, 325 degrees C for wood to burn).

Still with us? Good. Now, there’s a reason why "oxygen," "combustible" and "suitably high temperature" are in bold in the paragraph you just read. All three of those components are needed for fire. For simplicity’s sake, we’ll call combustible material "fuel," and suitably high temperature "heat." Oxygen we can leave the same.

Fuel, heat and oxygen are all needed in the right combination to produce fire. Combined, they’re called the "fire triangle." By nature, a triangle needs three sides. Take away one of the sides, and the triangle collapses. The same is true of fire. Take away any of the three components of fire – fuel, heat or oxygen – the fire collapses, meaning that it can’t burn. Firefighters try to do just that – remove one of the three essential components of fire. For example, when they dig a line around a fire, fuel is removed. When water is dropped on a fire, it reduces the heat. Retardant, a thick, soupy substance, coats fuels, blocking them from oxygen. If you think of fighting fire in terms of breaking the fire triangle, then it’s easier to understand the tactics of firefighters.

With a steady supply of oxygen (a fire needs air that contains at least 16% oxygen; the earth’s atmosphere is 21%), fuel and temperature become critical to sustaining a fire once it’s ignited. (By the way, most wildland fires are ignited by lightning; in a typical day, the earth receives about 8 million lightning strikes!) The general relationship between fuel and temperature is simple: the more fuel, the higher the heat. The more heat, the faster the fire spreads. When there is plenty of heat and fuel, fires pretty much take on a life of their own. In the words of one fire behavior expert, "Large fires live to feed themselves." Large fires can create their own winds and weather, increasing their flow of oxygen. A really large fire can generate hurricane-force winds, up to 120 miles an hour. The high temperatures "preheat" fuels in the fire’s path, preparing them to burn more readily. When fires reach this stage, there is little that firefighters can do. Nature is in charge. Some of the fires this year in Montana and Idaho fit this category.

Let’s talk about fuel in a little more depth. You’ve seen plenty of photos and videos showing trees or shrubs burning. Here’s a surprise. Technically, the trees and shrubs aren’t really on fire. It’s not the fuel that is burning. The fuel is actually being converted to gas. What you’re seeing burning is the gas produced by a fuel when heat is applied. Take a look at your fireplace the next time you have a log burning in it. If you watch closely, you can actually see a space between the surface of the log and the flame. What is burning is gas being given off from the fuel. There is very little oxygen on the immediate surface of the log. The gases produced by the chemical reaction when heat is applied to the fuel need to rise a little to mix with oxygen in order for them to burn. That also explains why "fire balls" of flame can explode tens of feet above a tree. Again, it’s not really the tree that is burning. It’s the rising gases being produced as a result of a chemical process that are on fire.

Eventually, this intriguing chemical reaction that produces fire breaks down. The wick of a birthday candle burns away, removing the fuel. A large wildland fire finally is circled by a line, taking away access to fuel, or the weather changes and rain or snow begins to fall, reducing the heat.

The key to fire is understanding its nature – what it takes to create fire, and more importantly, during difficult fire seasons such as this year – what it takes to control it.

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