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Dublin: 7 °C Thursday 26 April, 2018

Explainer: How do wildfires happen and why do they spread?

With a fire burning over 200,000 acres of Californian forest, we look at what exactly these fires are, how they start and how they’re fought.

THIS WEEK A massive wildfire has raged through Yosemite  National Park in the US state of California.

The fire has reached the size of Chicago and has destroyed over 200,000 acres of land in the park. That is the equivalent of the entire county of Louth being incinerated in the space of a couple of days.

3,700 firefighters are attempting to tackle the blaze, but so far containment has barely reached 30 per cent.

Over 110 buildings have been lost and 4,500 more remain under threat, with at least 31 homes have been gutted by the fire.

With Yosemite about 200 miles from San Francisco, the threat to water and energy supplies to a major US city are real and the city of Reno, in nearby Nevada, has issued warnings over air quality.

To get a grasp on how big the fire is, the Yosemite National Park posted this time lapse of  the Rim Fire (as the fire is officially being called).
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What is a wildfire?

It is exactly what it sounds like. Also known as forest fires or bush fires, uncontrolled wildfires burn wildland areas and forests, as well as consuming homes and buildings.

The term is usually applied in the US to any unplanned or unwanted fire that starts in forest areas.

They tend to consume dry forests and brush areas that have abundant fuel in the form of dry leaves and trees.


How are they caused?

Though occasionally started by a lightning strike, the US Parks Service says that nine out of ten fires are caused by people.

In the US, many wildfires are caused by human carelessness, ranging machinery sparks, agricultural burning and cast-away cigarette butts.

However, heat waves, cyclical weather events and volcanic eruptions can also play a role in starting fires.


Why do they spread so quickly?

This year, over 2.8 million acres have been burned by wildfires in the US alone, as over 30 fires have raged across the dryer, more arid west of the country.

Some of the fires are intentional, or prescribed. They are set in fire-dependant ecosystems and burn away dead fuel that obscures sunlight for incoming vegetation, allowing regrowth.

However, most are unplanned and spread rapidly.


They do so for a number of reasons, primarily the abundance of fuel available in dry, warm forests.

Many aren’t even reported until they are out of control, such is the sheer scale of American forests and national parks. By the time fire fighters are alerted, containment, not extinguishing, is the name of the game.

How are they fought?

Using water. D’uh.

But not just that. Wildfire fire fighters are some of the best of the best and use an array of tools to fight the fires. These include state-of -the-art mapping systems, air tankers, helicopters and even bulldozers.


The state of California has leased two Bombardier CL-415 Super Scoopers from the Canadian province of Quebec to help fight the fire in Yosemite.

These planes can hold 6,137 litres of water and can refill water tanks on lakes. The sheer volume of water used by these planes and their inability to quell the Rim Fire should give scope to just how big a fire this is.

Fire fighting crews also include teams of “smokejumpers”, airborne fire fighters who parachute from planes to attack wildfires in remote areas.

Fighting these fires, of course, is not without its dangers.

In July, an entire crew of “hotshot” fire fighters died fighting a wildfire in Arizona.

In many cases, the fires are contained and allowed burn out.

How common are they?

As of now, there are 31 wildfires burning in the US.


Map courtesy of  US Forest Service

The basic thinking is that as the planet heats and, subsequently dries out, the added heat, drought and fuel (in the form of dried vegetation) will lead to bigger, more intense fires.

In a recent National Geographic piece, weather historian Christopher C Burt said that the coming months could be worse.

“I predict that this September and October will be horrific.

“We’ve received only 65 percent of normal precipitation in the burn areas. When the offshore wind flow develops in September and October, our summer wildfire season will really begin.”

All images: PA Wire/Jae C Hong

Read: California wildfire prompts air warnings 100 miles away

Read: Hotter planet will increase risk of wildfires

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