Wildfires are a fact of life in Southern California. They are a constant threat to our way of life. There is no “fire season” in Los Angeles any more. It’s all year long, every year. That’s why it’s so important you understand wildfires, and know what to do if one threatens your home and family.
Fire needs three things to burn – fuel, heat and oxygen. This is called the Fire Triangle. Fire is a chemical reaction between oxygen in the atmosphere and some sort of fuel (wood or gasoline, for example). Of course, wood and gasoline don’t spontaneously catch on fire just because they’re surrounded by oxygen. For that to happen, you need heat. Heat, or ignition sources, can be many things, from a lightening strike, to sparks from a car’s exhaust.
Is fire a gas? A solid? A liquid? It’s none of these things. Fire can burn gas (like methane), or liquid (like gasoline), or even solids (like wood). But the fire itself isn’t any of these things. It’s not matter – it’s something matter can do.
A fire needs some kind of matter, or fuel, to burn. The fuel usually contains big molecules that have carbon atoms inside them. You can think of these molecules as little containers of energy. When they’re allowed to combine with oxygen, this energy is released as heat and light.
Fire is a rapid chemical reaction known as oxidation. All the heat and light of a fire comes from big, carbon-based molecules combining with oxygen.
So what is fire? It’s not the fuel or the oxygen or the heat or the light. Fire is what happens between all these things. It’s a chemical reaction.
“There is nothing like a wildfire,” says retired Los Angeles Fire Department Chief Deputy for Emergency Operations Joe Castro, who has fought hundreds of fires in his long career. “House fires are usually contained inside the four walls where they start. But wildfires are a whole other animal. Especially when they’re a wind-driven wildfire. They’re bigger, faster, less contained and more dangerous than any house fire.”
Wildfires can move as fast as a freight train. They create deafening noise as they burn everything around them. And the smoke and flying embers they produce make it impossible to see. If you’re caught in a wildfire, you can be nearly deaf and blind. Without your senses, escape is can be impossible.
There are several classes of wildfire.
How a wildfire behaves depends on what is burning, when it is burning, and where it is burning – in other words the fuel, the topography and the weather.
Fire spreads via a process called heat transfer. This is when the material immediately next to a fire is preheated to the point where it gets hot enough to ignite.
Fuel is one of the three elements in the fire triangle, so we know fire needs fuel. Firefighters are most concerned with fine fuels.
What does this mean?
In Los Angeles, if a hillside hasn’t burned in 20 years, it’s likely to be a wildfire risk. That’s because the brush has had a chance to grow for two decades and has shed plenty of dead vegetation over the years, creating a lot of fine fuels. On a hot day, these fuels can be superheated under the Southern California sun. The smallest of ignition source – a cigarette tossed away, a car backfiring – can set fire to this dense, bone-dry fuel.
This also means that once an area has burned, it isn’t likely to burn again for years. The fuel has burned away. Often, if there are no homes or people are risk, firefighters may allow an area to burn. Such a fire can eliminate dangerous fuels.
Heat rises. We call that convection. That means that fuel uphill from a fire gets very hot. Much hotter than fuel downhill from a fire. When that fuel gets hot enough, it ignites, and the wildfire climbs the slope. Rapidly. “If you find yourself on a slope above a wildfire,” says MySafe:LA Executive Officer David Barrett, “you need to evacuate as quickly as you can!”
Aspect is what direction a slope is facing – north, south, east or west. It’s also important to wildfire behavior, because it effects the type of vegetation and moisture on the slope. A west facing slope may be hotter and dryer and support more flammable vegetation. South facing slopes however, are usually cooler and wetter and support less flammable vegetation.
Weather influences fire behavior by creating conditions suitable for burning. Wildfires aren’t likely to ignite or burn when it’s raining, or even in on days when there is high humidity. But there aren’t a lot of rainy days or humid days in Southern California!
Hot temperatures will preheat fuel, and heat transfer will happen very quickly. But it doesn’t have to be hot for wildfires to burn. Days with low humidity means that fuels become bone dry, which is equally dangerous.
Wind plays a huge part of wildfires. Wind can dry out brush quickly. And once a fire starts, wind can speed up the process of heat transfer by pushing flames and heat sideways making the fire grow quickly. Wind can also change the direction of a fire and turn a fire flank (the side of a fire) into a fire front (the head of the fire). This can be VERY dangerous for firefighters who are trying to contain a wildfire.
Some wildfires can burn so hot and be so big, that they create their own weather. Vast amounts of super-heated air surrounding the fire can create wind tunnels as the air rises. The formation of pyrocumulus (provide link here) clouds can hang over large wildfires for days.