The Burning Issue of Wildfires
Just be careful where you make your sparks, because a raging wildfire can consume dozens of lives, thousands of homes, and hundreds of thousands of acres, including whole towns. Huge blazes have charred areas the size of Rhode Island and greater Rome. They’re a burning issue. So let’s look at how they work.
Don’t Get Me Started
Fire is a chemical reaction that occurs whenever a combustible material (fuel) combines with oxygen at a sufficiently high temperature (around 617 degrees Fahrenheit, or 325 degrees Celsius, for wood) to release energy in the form of heat and light. Fire, from this perspective, is not the same thing as flame, which is simply a visible indicator of the chemical reaction.
Since every fire has these three “sides” – fuel, oxygen, and the heat needed for the chemical reaction to take place – firefighters and other experts often refer to “the fire triangle.” Reinforce any side of the fire triangle – that is, increase the amount of fuel, oxygen flow, or heat – and the fire will likely intensify. Take away any side of the fire triangle, and you will stop a fire in its tracks.
Caught in the Fire Triangle
Oxygen. Under normal conditions, wildfires have no trouble finding enough oxygen to keep “breathing.” The air around us contains about 21 percent oxygen; fires will generally burn wherever air contains 16 percent oxygen. Plus, intense fires can fan themselves by creating their own winds. Air heated by a fire rises, creating a vacuum into which more air rushes, in some cases producing hurricane-force blasts of up to 120 miles per hour (190 km/h).
Heat. Since there’s generally plenty of oxygen, a wildfire’s ability to spread is primarily determined by its ability to transfer heat to new fuel sources. Heat transfer occurs in three ways: convection, radiation, and conduction.
In convection, liquids or gases do the work of transferring heat, as when hot air rising from a ground fire heats leaves on the trees above (sometimes enabling a ground fire to climb to the treetops).
Radiation, as the name implies, is the transfer of heat by rays (think of lying on a beach, basking in the warmth of the sun’s rays). An intense fire can transfer enough heat through radiation to ignite new fuels even before its flames can reach them, enabling the fire to leap through space.
Conduction is the direct transfer of heat from one piece of fuel to another (think of the sand on the beach burning your feet with heat it has absorbed from the sun). In a wildfire, conduction is generally less important than convection and radiation, because wood is a relatively poor heat conductor – which hardly means, of course, that it’s not a good fuel.
Fuel. When a firefighter talks about fuel, don’t think “gasoline.” Think “anything that will burn.” Fire isn’t very choosy about what it consumes. And generally, the more it consumes, the hotter it gets and the faster it spreads.
Different fuels have different properties that affect when and how they will burn. From the fire’s perspective, the most important property is probably moisture content. Wet fuels won’t burn until they dry out – until heat vaporizes the moisture within. Size and shape matter, too. Thin, light fuels like grasses and leaves dry out and burn rapidly. Heavy fuels like trees and logs are slower to ignite but burn longer.
Fighting Fires (with Fires?)
Most firefighting techniques amount to attacks on one side (or more) of the fire triangle. Dousing a fire and its fuel with water reduces heat, forcing the fire to do more work if it wants to spread. Digging trenches or creating clearings around a fire removes fuel, depriving the fire of the “food” it needs to survive and thrive. Coating would-be fuels with fire-retardant chemicals prevents oxygen from reaching them, effectively choking the fire to death.
Perhaps the most interesting firefighting techniques, however, are those that involve fighting fire with fire. Firefighters sometimes intentionally set smaller fires, called “backfires,” that advance toward a raging wildfire. The purpose of such controlled fires is to burn up the potential fuel in an advancing wildfire’s path.
In other cases, fire experts set their own blazes even in the absence of existing wildfires. Called “prescribed burns,” such fires help to reduce the amount of fuel available in a given area – and so prevent more dangerous future fires. They also serve purposes that natural fires have always served, like preventing the grasses and forbs of prairie ecosystems from being overrun by shrubs and trees.
In the end, even wildfires have a purpose to serve – they play an important role in nearly every terrestrial ecosystem in the United States. But that’s no excuse for tossing a lit cigarette into the brush on your next hike.
