How Do Bushfires Start & Why Do They Keep Burning?

You may have heard the news: Australia has experienced one of its worst bushfire seasons. Besides the heat and infrastructure damage, the fires have caused both human and animal lives to be lost as well. But how do bushfires start, and how can they keep on burning for weeks and months? We find out today, right here.

A Little About Bushfires

Bushfires are a common occurrence through Australia, and are an intrinsic part of its environment. In fact, natural ecosystems have evolved with the fires, and Australia’s landscape and biological diversity have been shaped by the fires as well. Generally, bushfires are slow moving with a high heat output, and can smoulder for days.

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How Do Bushfires Start?

Just like any other fire, bushfires require three things: fuel, oxygen and ignition source. During certain seasons, fires are especially prone to happen due to low relative humidity and high ambient temperature. Some native plants are also very combustible, including eucalypt trees, whose natural oil promotes combustion. While we may think that bushfires are harmful and should be done away completely if possible, that isn’t the case. Bushfires, as a natural occurrence, are actually beneficial and several plants also depend on them to regenerate. Bushfires can start naturally (usually by lightning strikes), or by human activity, whether accidental or deliberate.

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Why Do They Keep Burning?

There are several contributing factors to a bushfire’s intensity and speed of spreading, and we’re breaking it down for easy understanding.

Ambient Temperature

Ambient temperature refers to the temperature of the surroundings. The higher the temperature, the more like a fire will start, or continue burning. Pre-heated fuel or fuel closer to ignition point at high temperatures also burn faster.

Relative Humidity

The dried the air, the more intense the fire. Plants are also more flammable at low humidity, because they release moisture more easily. Australia is generally classified as “humid”, but certain months and areas can bring about dry spells.

Fuel Load

Fuel load simply means: things the fire can burn and consume. Some fuel load in bushfires include fallen bark, leaf litter and small branches among other things. You probably already know, the more fuel load, the hotter and more intense the fire.

Fuel Moisture

Fuel moisture can make or break a fire. If fuel is wet or damp, it may not combust at all, while dry fuel will just spark and burn quickly. When assessing bushfire danger, the time since rainfall and the amount of rain is important.

Wind Speed

The wind feeds a bushfire by blowing flames into fresh fuel, bringing it to ignition point and providing a constant stream of oxygen. It also spreads fire by spotting, which occurs when the winds lob burning embers into the air and elsewhere. There’s a threshold wind speed of 12-15km/h, and fire burns slowly below this threshold. However, even a slight increase above this threshold can greatly increase fire intensity and spread.

Slope Angle

Generally, a fire accelerates travelling uphill, and decelerates travelling downhill. In fact, the speed of a fire front advancing doubles with every 10-degree increase in a slope.

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What About Climate Change?

Bushfires may be a natural phenomenon, but “man-made” global warming is now a significant contributor as well. With the rise of global warming, there is more CO2 in the air, raising temperatures and the chances of bushfires happening. With this finding, one can only hope that all world citizens do their part to change climate change, or risk being consumed by one natural disaster or another.

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For more information on bushfires, click here to visit the Australian Government website, where some information was obtained.