The Anatomy of a Wildland Fire
This may seem overly academic, but understanding how wildland fires start and progress from a spark and small flame to a firestorm will help you defend your home. As you read the following information, think of the terrain, growth and weather patterns in your area. Firestorms can be unpredictable; they will not always obey the rules of fire behavior. But knowledge of those general rules will help you make good decisions, both in precautions and in the event of an evacuation.
We already know fire requiresfuel, oxygen and heat to start. Three additional elements determine wildland fire behavior: fuel load, weather and topography.
Fuel load is measured by the amount of fuel — usually tons per acre. The fuel source is obvious: trees, brush, dried vegetation. The higher the fuel load, the hotter the fire burns and the faster it will spread. The heat and smoke generated from a brush fire evaporates moisture from adjacent fuel causing it to burn. This is how any fire spreads. The drier the vegetation, the quicker the approaching fire can ignite it.
Fuel type also plays a role. Flashy fuels, small materials like dry grass, leaves and pine needles burn more quickly than tree trunks, logs or stumps. The ratio of the fuel’s total surface area to its volume is the key. Twigs have little volume but relatively large surface area; they ignite and burn quickly like kindling in your fireplace.
Dense, closely packed brush takes longer to ignite than brush that is more spaced out because there is less oxygen available and clustered brush retains its moisture longer. But don’t assume you’re safe if your home is surrounded by dense growth.
Weather is a deadly factor in wildland fires. Given the right mix of temperature, wind and humidity levels, fires are a near certainty. The sun heats and dries fuel allowing it to burn and the fire to spread faster. Since that heat has a cumulative effect, afternoons are the most dangerous times for brush fires.
The biggest and least predictable factor is the wind. The hot, dry winds that swoop out of the western deserts stoke fires with oxygen, dry potential fuel and push the fire in their path. Heavy winds also set up spot fire situations. Embers are blown far in advance of the fire, igniting fuel often as much as a mile from the primary flames. Wind can push flames from ground level brush into trees causing an even faster spreading crown fire. Once a wildland fire gains strength, it can create its own weather patterns including winds called fire whirls, similar to a vertical tornado that can literally hurl flaming debris great distances.
Humidity can slow the progress and intensity of a fire or the lack of humidity can contribute to both the ignition and rapid spread. The most volatile conditions in Southern California are during a Santa Ana wind episode, high temperatures, dry, strong winds and single digit humidity.
Terrain, or more specifically the slope, is the final determining factor in the spread of a wildland fire. Fire travels faster uphill, pushed by winds that naturally flow up a slope. Depending on the steepness of the terrain, a fire can quickly pre-heat fuels in its path. Conversely, slope and natural air flow retard the speed of a fire down hill. There are exceptions to this rule, depending on the force and direction of the prevailing winds.
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