1 {photo of a large bushfire}
1 Caption: Bushfire

Creative Commons Attribution-Share Alike image by Bert Knottenbeld.

As the year draws to a close, the weather warms up in the southern hemisphere. It's a time for swimming at the beach, lazy picnics, and long holidays. It's also the time for bushfire.

Similar things are known elsewhere as forest fires or brush fires. Bushfire is fire in the Australian bush. ("Bush" is a word with several meanings. It can mean "wilderness" in general, including scrubland and even desert, or it can mean "forest" specifically. It can also mean "rural areas", as opposed to city areas, and so encompass farmland. Context is everything.)^[1]

Bushfires can be started by lightning strikes. Once started, they can grow rapidly and be near impossible to put out if the weather conditions are right. With human civilisation nearby, the number of ways a bushfire can start increases alarmingly. People drop cigarette butts. Car backfires can spark dried leaves in a gutter. Embers can escape from barbecues. Wind can blow down live power lines, creating sparks. People can, stupidly, light bushland on fire on purpose.

Fire needs three things. Fuel to burn. Heat, to sustain the chemical reaction. And oxygen to react with the fuel. Combustion is a chemical process in which elements in the fuel combine with oxygen, creating oxidised compounds. This releases chemical energy because the binding energy of the oxides is generally high compare to the binding energies of the various compounds in the fuel.

Creative Commons Attribution-Share Alike image by Bert Knottenbeld.

The fuel in wild fires is organic plant material made of hydrocarbons with traces of a few other elements. Hydrocarbons burn very well, decomposing and combining with oxygen to produce water and carbon dioxide. Some solid residue remains behind as ash, and some tiny solid particles are carried away as smoke. Forests where there has been a lot of growth in recent years provide plenty of fuel. This can be the case if there has been plenty of rainfall over past growing seasons, and if no fires have burnt in the area for several years.

Heat is supplied initially by the source of ignition, be it a lightning strike or a cigarette butt. Once a fire has begun, the reaction produces enough heat to ignite adjacent material, and the fire can spread. Ignition is easier if the fuel is dry and warm, rather than damp or cold. So hot, dry weather helps fires both in getting started and in spreading.

Oxygen comes from the air. Once a fire reaches a certain size, it is the amount of available oxygen that limits how fast and hot it can burn. The fire uses much of the oxygen in the air immediately around it and becomes reliant on incoming oxygen from air currents. If the weather is windy, this helps a large fire in two different ways. Firstly, it blows more oxygen on to the fire, allowing it to burn faster and ignite more fuel. Secondly, it blows heat and embers downwind, spreading the source of ignition to more fuel in that direction.

The combination of an abundance of dry plant material and dry, warm, windy weather provides the ideal conditions for spreading a bushfire like... well, like wildfire. Once started in such conditions, a fire can be almost impossible to put out, despite the best efforts of an army of firefighters armed with modern equipment.

Fire burning near properties. Creative Commons Attribution image by Flickr user thinboyfatter.

Once established, wind is an even bigger threat than high temperatures. It makes the fire more intense and pushes it on towards new fuel. The fire can spread in the direction of the wind faster than animals or people can flee from it. Wind also carries hot embers through the air ahead of the fire. These can land kilometres away from where the fire is burning. If they land on combustible material, they can ignite spot fires well in advance of the main fire front. These form a major threat not only to downwind forest but also to human buildings in the vicinity of the fire. A stray ember from a fire burning out of sight on the other side of a ridge, landing in a gutter full of dry leaves, can destroy someone's home.

To prevent buildings being threatened, people can clean out gutters, remove loose combustible material from around the premises, and hose everything down with water to make it damp and hard to ignite. Hot windy weather again makes this less effective, blowing debris around and drying up any moisture quickly.

Firefighters have various tactics for dealing with the advance of a bushfire. Water is a major one. It can put out flames and make fuel too damp to burn easily. The problem is delivering enough of it to be effective. Hoses can only deal with fire up to a certain size, so are restricted to the fringe of a fire front and putting out spot fires before they become established. Planes and helicopters can drop loads of water over a fire, but even this amount often only serves to extinguish patches within a large fire.

Another important tactic is to remove fuel from the path of the advancing fire, by back-burning. This is deliberately setting fire to fuel ahead of the fire front, hopefully in a more controlled way, to create a region with very little left that can burn. If the fire reaches this firebreak, it is more difficult for it to keep advancing - though not impossible, as embers can ignite spot fires across even fairly wide firebreaks. Roads form firebreaks, and often this is where back-burning begins, to widen the break that already exists. When a fire crosses a road, it can be a bad blow for firefighters, as the fire has new swathes of forest to burn through.

Remains of houses and car after a bushfire. Creative Commons Attribution-NonCommercial-ShareAlike image by Stuart Murdoch.

Adding to the danger, fires can be unpredictable. They advance fastest in the direction of the wind, and the wind can change without warning. If a fire changes direction, prepared firebreaks can become useless, and forest and properties previously thought safe come under threat. Changes in direction can close loops in the fire front and cut off people on the ground, surrounding them with fire, or cutting off the only easily traversable ways to safety. This is when fires get really dangerous.

Heat is the immediate danger from a fire. A large fire gives out prodigious amounts of heat, and it is difficult to get close to it without insulated protective gear. But people will generally not get dangerously close to a fire if they can help it because the heat will drive them back. A greater danger comes from smoke, which is harder to avoid and which can overwhelm someone suddenly, choking their lungs and making it extremely difficult to breathe. If overcome by smoke, a person can pass out, which can be deadly if the fire is heading towards them. Even if rescued, smoke damage to the lungs can leave someone needing a respirator for days afterwards.

The danger to property can also be catastrophic. Surrounded by a bushfire, or in the direct path of an advancing front, a house has little chance. Anything combustible will burn with a ferocity that also melts metal and collapses brick and concrete, often leaving little more than a pile of rubble and ash. Firefighters try to save properties, but their emphasis has to be on saving lives, getting people out of the danger zone, so sometimes buildings have to take the brunt of the flames.

Banksia seed cone after a fire. Creative Commons Attribution-ShareAlike image by Doug Beckers.

Besides the threats to humans and property, bushfires also pose a major threat to wildlife. Slow moving animals in the path of a fire have no escape unless they can burrow far enough underground to wait out the fire without getting roasted. Faster animals may flee, and may be lucky. But many will be lost. Trees and other plants suffer too. Immobile, they form most of the fuel that sustains the fire.

But all is not lost. Fire has a renewing property after it has wrought its destruction. Ash is rich in elemental nutrients that new plants need to grow. And many plants have evolved methods to deal with periodic fires sweeping through their habitat. Australian plants in particular have a variety of means of regenerating quickly after a fire. Eucalyptus trees, banksias, and others have hardy lignotubers, protected by a thick layer of woody bark. After a fire, new shoots quickly emerge from these. Banksias also have formidable seed cones which protect the seeds within. The heat of a fire merely serves to weaken the cones enough for the seeds to fall out and sow the ground around the burnt parent tree. In fact some species cannot release their seeds until the cones have been burnt.

And so, within a few years after a major fire, new growth has appeared quickly and the forest begins to re-establish itself.

We humans like to think that we have tamed fire. Various cultures have legends about how the first people domesticated fire. Even the words "tamed" and "domesticated" make fire sound like an animal, rather than a force of nature. We may be able to use it as a tool, and a very useful tool it is when under control. But when out of our control, fire is a rampaging monster that can devastate enormous tracts of land, wiping out plants, animals, and human civilisation. Never underestimate it.

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Title image is the Mount Archer bushfire in Queensland, 2010. Photo by Chris Ison, licensed under Creative Commons Attribution.

[1] You can see all the bushfires currently burning in my state of New South Wales on this map, which is updated live.