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The Third Largest Wildfire In California History Explained

The third largest wildfire in California state history, the Thomas Fire, has raged for two weeks and continues to burn. The Thomas Fire has burned nearly a quarter million acres, destroyed over a thousands homes, and has forced tens of thousands to evacuate. What caused this fire to be so destructive and last for so long? Okay, let’s science.

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On Monday, the Thomas Fire not only became the largest California wildfire of 2017 but also the third largest in state history. It sits just behind both the Rush Fire that burned 271,000 acres in 2012 & Cedar Fire that burned 273,000 acres in 2003. As of December 18th, the Thomas Fire is also the 7th most destructive fire in California state history, burning 1,024 structures.

The Thomas Fire began on December fourth and spread rapidly. Two weeks later, the fire continues to burn. Over 8,000 personnel and nearly 1,000 fire engines have been tasked to contain and stop the fire from spreading. Their work is paying off, reaching 50% fire containment on Monday. Containment is a word you’ll hear often in reference to wildfires. It refers to the percentage of the fire’s perimeter that has been controlled or stopped from spreading.

For example, a fire that’s 25% contained has 25% of its perimeter under control, it’s the same for 50%, 75%, and you get the picture. When a fire is 100% contained, it doesn’t mean the fire isn’t still burning but that they have contained the burn to a set area and perimeter. Officials estimate the Thomas fire to be 100% contained by January 7th.

So what events led up to this fire and why did it spread so rapidly? To answer that question, let’s get to the basics of how wildfires form. Every big wildfire needs three ingredients: 1) fuel to burn, 2) ignition, 3) conditions to spread.

You’ve probably noticed when you’re starting a campfire, it burns better with drier wood. The same goes for wildfires. The drier the fuel, the easier it is to burn. Fuel can include bushes, shrubs, trees, dead sticks and leaves, the list goes on.

November and December mark the beginning of the rainy season in Southern California, racking up 1-3 inches on average each month. But thus far, November and December have been bone dry. June, July, and August are normally the driest months for the area and this year was no different with little to no precipitation falling from Santa Barbara to LA. The area has seen essentially no rain since June giving fuel ample time to dry out.

Fires are also more susceptible in areas that haven’t burned in a while because there’s more fuel for the fire to work with. Over time litter builds up on the forest floor just waiting to catch fire. One tactic used by the Forest Service designed to prevent this from happening is burning tree litter found on the forest floor in a controlled manner.

Next up, ignition. Stuff doesn’t just spontaneously combust, it needs something to trigger a flame. According to the National Park Service, 90% of wildfires can be blamed on us, humans. This can be power lines falling over, people throwing a cigarette out of your car window, or campers leaving a fire unattended. These actions cause 90% of wildfires, so please, please be aware and don’t be negligent. The other 10% of fires are triggered by lightning or lava. In the case of the Thomas Fire, the cause is still under investigation.

The final ingredient to a large wildfire is weather conditions conducive to rapid spreading. A crucial part to this is strong winds. In the case of the Thomas Fire, a high pressure system was well established across the Pacific Northwest and into the northern Rocky Mountains. Air rotates around a high pressure system clockwise, giving southern California a north-easterly wind commonly known as the Santa Ana Winds.

The direction of this wind is important. This wind goes up and over mountains and foothills and then descends back down, which warms and dries the air before reaching the greater Los Angeles area. As the air descends it compresses which causes the air molecules to move faster and the overall temperature to increase. Assuming little to no moisture is added or removed, an increasing temperature causes the relative humidity of the air to plummet.

These winds ramped up to over 70 miles per hour and set the stage for a disaster the night the Thomas Fire began. This general pattern has persisted for nearly two weeks, extending a nightmare for Southern California residents.

I’ve included several links in the description below if you’d like donate and help with the relief. One of which is the Thomas Fire fund which can be found at

So that’s the basics of the Thomas Fire and why it has continued to rage. Officials estimate 100% containment around January 4th when things can hopefully return to normal.

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