Bushfires 1: Understanding bushfires
Terrestrial Ecohydrology Research Group
University of Technology, Sydney
Chief Executive Officer
Bushfire and Natural Hazards CRC
- Fire has three essential requirements: fuel, heat and oxygen. Deprive it of any one of these and it will go out.
- Different types of bushfire fuel (vegetation) burn differently: finer fuels like grasses burn more quickly, while more substantial, woodier fuels burn with greater intensity.
- A bushfire will occur when there is sufficient dry fuel to burn, weather conditions are hot, dry and windy, and there is a source of ignition, such as lightning.
- People living in fire-prone areas must prepare a bushfire survival plan and be ready to carry it out.
- Much of the Australian bush has been shaped by bushfires, with some plants requiring intermittent burning to complete their life cycles.
They can start with just a spark and can burn for months, affecting landscapes and lives for years.
Bushfires are a natural, essential and complex part of the Australian environment and have been for thousands of years. Our country’s first inhabitants lived seemingly hand in hand with fire, having developed complex fire management practices that complemented their deep understanding of the country and landscape in which they lived. Our ecosystems have evolved to be fire dependent, and require the periodic presence of fire. Herein lies the complexity: Australia is a land of fire where people have chosen to live.
Since European settlement, bushfires have been an event to be endured, a natural disaster with sometimes catastrophic effects. Research efforts have focused on how to predict and control bushfires in order to best protect lives and infrastructure, as well as understand the role that fire plays in the Australian landscape.
How does fire work?
What actually makes a fire burn?
We all know that if you gather up a bunch of dry twigs, grass and leaves and put a lit match to them, they’ll burn. Add some more sticks and bigger bits of wood and you’ve got a raging fire, ready for marshmallows. But why? How does fire actually work?
Fire is the result of applying enough heat to a fuel source, when you’ve got a whole lot of oxygen around. As the atoms in the fuel heat up, they begin to vibrate until they break free of the bonds holding them together and are released as volatile gases. These gases react with oxygen in the surrounding atmosphere. This chemical reaction causes a lot of heat, so much heat, in fact, that it can keep driving the reaction—as long as there’s enough fuel and oxygen still present, the reaction will become self sustaining. The actual flames of the fire are the release of some of the heat energy as light.
These components have led to the development of the ‘fire triangle’ of fuel, oxygen and heat. Remove any one of these and fire cannot sustain itself.
The fire triangle
For fire to continue burning, it needs oxygen, heat, and fuel—the three components of the "fire triangle". Remove one of these components to put out the fire.
- Select a button above to learn how that component can be removed from a fire.
- For very small fires (like campfires), oxygen can be removed by smothering, e.g. with soil or a blanket. It can be difficult to completely extinguish a fire this way.
- Heat can be removed by spraying the fire with water. The water heats up and turns to steam. This reaction requires a lot of energy, and it sucks away the heat energy of the fire.
- Fuel can be removed by collecting or raking away vegetation near the fire, or by setting up a fire-resistant barrier in the fire's path.
And why does water put it out?
The primary role water plays in putting out a bushfire is cooling it down so there’s no longer enough heat to sustain the fire. When you pour water onto a fire, the heat of the fire causes the water to heat up and turn into steam. This is a very energy-intensive reaction, and it sucks away the heat (which is a form of energy) of the fire. This leaves the fire without enough energy to keep burning.
Less significant is the role water can play in ‘smothering’ a fire, depriving it of the oxygen that it also needs to burn.
Predicting when and where bushfires are likely to occur is obviously an important and useful part of bushfire management. It will be particularly important across many parts of Australia where the number of days of extreme fire danger are projected to increase under climate change.
Current thinking around predicting bushfires incorporates four ‘switches’ which all need to be ‘on’ for a fire to occur.
Fuel to burn
The Australian bush, though it has a reputation for being dry and scrubby, varies greatly from place to place around the country. There are regions of open woodlands, grassland savannas, dense rainforest. A bushfire will burn anything that it finds in its path, but different types of vegetation burn differently. Generally, fuel is classified as being fine (grasses and twigs that are less than 6 millimetres in diameter) or heavy (branches, logs or stumps). Finer fuels burn more easily, feeding the spread of the fire, but heavier fuels burn with greater intensity, creating more heat and making the fire more difficult to put out.
Fuel loads accumulate in different types of vegetation at different rates. In Western Australia, jarrah (Eucalyptus marginata) forests build up fuel at a rate of around 1–2 tonnes per hectare per year, while karri (Eucalyptus diversicolor) forests accumulate around 3–4 tonnes of fuel per hectare per year.
Dryness of fuel
Another key factor is fuel moisture, or how dry the bush is. The drier the fuel, the more easily it will burn. Fuel fits into two main categories: live and dead. Live fuels contain moisture that is regulated by the plants’ physiology as well as the amount of recent rainfall. Dead fuel’s moisture content tends to reflect the atmospheric moisture content—the relative humidity and level of moisture in the soil. These depend on recent rainfall and temperatures.
Researchers are developing methods of using remote sensing data from satellites or aircraft to detect the relative dryness of the bush. One way satellites can estimate fuel moisture is by picking up changes in the greenness of vegetation, an indicator of vegetation dryness. This is particularly important in grasslands. The key controls on the dryness level in fine fuels are relative humidity, temperature and recent rainfall.
Certain weather conditions fuel a fire and help it burn. Long-term seasonal weather patterns, such as periods of drought or rainfall, can affect the availability and moisture content of vegetation and the fuel available for a fire to burn. A fire is much more likely to ignite, and continue to burn, in hot, dry and windy weather.
Source of ignition
Lightning strikes can provide the initial spark that sets off a bushfire, and cause around half of Australia’s bushfires. Other causes can be faulty electrical wires, a cigarette carelessly tossed out of a car window, a hazard reduction burn gone wrong, arson, or accidental igniton.
The primary influences upon how bushfires move through the landscape are humidity, geography, wind and temperature.
Humidity and temperature
The effects of ambient temperature and humidity on a fire are pretty obvious. The hotter the air temperature, the closer any fuel is to its ignition point, and dry fuel will burn more easily. The lower the humidity, the drier the air is, again helping fuels burn as they release their moisture into the air more readily.
Slope of the land
The slope of the landscape is also important. Just consider a match and how much faster it burns when you hold it so that flame is burning up the stick (and towards your fingers!) than down. Similarly, fires burn much faster uphill than down. This is because the radiation and convection a fires creates preheat the fuel source, and this is much more readily done upslope than down. A 10-degree increase in slope usually results in a doubling of the speed of the fire. Fire will spread up a 20-degree slope four times as fast as it will along flat ground.
Fire moves faster uphill because there is less space between the flames and new fuel to burn. Also, the radiant heat caused by the fire pre-heats the fuel, making it easier to ignite.
The increased distance between flames and new fuel means fires spread more slowly when moving downhill (unless the slope of the land creates unusual air currents).
How wind spreads fire
Generally, as long as wind speeds are below 12–15 km/h, a fire will burn slowly. However, if wind speeds are even slightly higher than this, they will have a significant impact on the fire movement. A change in wind, often from a cold front, can activate the side of a fire, making it broader. In general, a wider fire will burn faster than a very narrow one.
Fires also create their own weather; the heat of a fire can result in whirlwinds and turbulent air currents. These can drive the fire sideways, broadening the fire front. Wind can also cause spotting—carrying pieces of burning fuel, like twigs, leaves or small embers, ahead of the fire, igniting more small fires.
Fire intensity is described in terms of the number of kilowatts of energy each metre of the fire front generates. A controlled hazard-reduction fire usually produces less than 500 kilowatts per metre of energy, while an extreme bushfire can generate more than 100,000 kilowatts per metre. Once a fire’s intensity exceeds around 2,000 kilowatts per metre it can no longer be safely managed by firefighters working on the ground.
A number of computer models have been developed to help predict the spread and shape of fires across the landscape. The most widely used of these is Phoenix Rapidfire, developed by the Bushfire CRC, the University of Melbourne and various fire agencies. Western Australian authorities used Aurora, a model developed by the University of Western Australia, Landgate and the Bushfire CRC, and CSIRO has developed a new model called Spark.
These models take into account all the factors discussed above that influence the behaviour and spread of the fire. They incorporate meteorological data as well as geographical landscape data like the slope of the land, vegetation type, and the presence of unburnable features like roads or water bodies. Phoenix Rapidfire also incorporates components that deal with spotting and fire suppression options.
When a fire becomes a firestorm
If a fire gains enough momentum, it generates so much heat that it creates its own wind currents, and becomes a raging inferno sometimes known as a firestorm. The fire’s heat creates an extremely strong updraught of air. The air at ground level around the fire is then drawn in strongly towards the fire’s centre to replace the rising air. The turbulence this creates can result in fire whirls which spiral and dart around, burning as they go.
The heat of the fire can cause thunderstorms or pyrocumulus clouds. These can produce lightning strikes, which can start new fires. With the right combination of atmospheric conditions, fire tornadoes can be created. These can have wind speeds of greater than 250 km/h and are extremely destructive.
Controlling a bushfire
A number of techniques are used to contain and extinguish bushfires. Smaller fires are fought directly, by firefighters applying water to the flames, either from the ground or the air. Fuel breaks can be created using hand tools such as rakes and hoes.
Another way to try and contain a fire is to deliberately burn sections of the fuel in its path, so that there’s no flammable material left to fuel it. Because these small fires are intended to ‘burn back’ to the fire front, this process is known as ‘backburning’. It’s used to try and stop a fire in its tracks, and also to protect particular areas, such as houses or other infrastructure, by creating a fuel break between them and the fire front.
Water bombing is another method used to control large fires. We’ve seen the footage on the news of helicopters flying over raging fire fronts, tipping what look like small buckets of water onto the flames. To the untrained eye, it doesn’t look all that effective. But the point of water bombing is not so much to douse the flames as to increase the humidity in the surrounding air. The water tipped out from the helicopter turns to steam, or water vapour, which makes it harder for the fire to continue burning.
Sometimes the water incorporates a gel which also helps put out the flames, or a chemical fire retardant, often dyed red so the areas where it’s been dropped are clearly visible. The retardant inhibits the fire’s ability to ignite fuels, and remains on the vegetation after water has evaporated.
Aeroplanes are also used to drop water or retardant on the fire. These can vary in size from relatively small to very large, such as the DC-10.
Warning and survival
Being able to predict bushfires means we can more effectively warn people, prevent loss of life and avoid catastrophic damage to property and infrastructure.
Australia has a national fire danger rating system that was originally developed in the 1960s. It is based on weather conditions and local fuel loads. The Australian Bureau of Meteorology provides weather forecasts and fire weather warnings to both media agencies and local fire authorities to help inform their decisions about issuing fire danger ratings. Meteorologists are also directly involved with decision making on the ground.
Following the Black Saturday bushfires in Victoria in 2009, the states and territories modified the warning levels to include a higher level: Catastrophic (referred to as Code Red in Victoria). Current efforts are updating the science underpinning the danger rating system and developing a standardised fire danger rating system that will apply across the country.
Surviving a fire
Recent research has revealed some interesting information about how people lose their lives in bushfires. Most people were killed when out in the open, with nearly three-quarters of these within 200 metres of their home. Around one-third of all bushfire deaths occurred within a house, and these mostly occurred on days with Catastrophic (Code Red) fire conditions. More than 90 per cent of these deaths occurred in rooms that did not have a door leading outside.
The common advice from the various authorities is to be prepared, and to either leave early, or stay and defend. Prepare, Act, Survive is the nationally accepted approach.
An understanding of the risks of staying to defend a home during a bushfire, or evacuating to a safe place, is essential. All agencies agree that if you choose to leave, you should leave early, usually the day before, or before 10 am on the day of risk. If you choose to stay, you must be both physically and mentally prepared. Look at the information from the various sources above to find out how best to prepare yourself or your property for the risk of bushfire.
In terms of protecting yourself, one of the most dangerous aspects of a bushfire is the radiant heat it releases. At times, this heat can be so intense it can kill people who are quite far from the fire. The most effective protection from radiant heat is distance, or a solid barrier, like a wall or an embankment. Next best is covering up—putting on protective clothing like long pants and a shirt, or overalls made from natural fibres, not sythetics. The suits firefighters wear are not simply a fashion statement. Even a blanket is better than no protection.
Other risks are dehydration, smoke inhalation and heat stroke, which can cause people to lose consciousness. The fumes from burning materials, particularly houses or cars, can be toxic, and breathing them in can lead to disorientation or death. In many cases, the fumes are heavier than air so they sink to the lowest part of the landscape or building. This is one of the reasons why sheltering in a cellar or ‘bunker’ under a house is not recommended.
The effects of fire
Clearly, all fires have an effect on the ecosystem, and research is ongoing to determine both the negative and positive consequences of fire. In the Australian Alps, wildfires have a greater impact on Alpine Ash forests than managed fires. However, it’s worth noting that a burn regime of less than 30 years could wipe out these forests. The Alpine Ash only produces seeds after it reaches maturity at an age of 30 years or more; fires occurring at a frequency of less than 30 years would therefore kill trees before they could produce seeds.
But there are also cases in which fires actually help plants regenerate. The survival of the Mountain Ash depends on the occurrence of fires. Mountain Ash can only regenerate from seeds, and fire is essential to provide the right conditions for their germination. However, if fires occur before the young trees reach maturity and produce seed (at 50 years old), the species may be completely wiped out. It may then be replaced by other species better adapted to that fire regime.
Some plant species, such as eucalypts, regenerate from lignotubers, which are stimulated by fire to develop shoots. Also, smoke from plant material promotes seed germination in more than 400 species of Australian plants.
It’s not just wildlife and plant species that can be affected by fire. Researchers at the University of Melbourne are exploring the effects of bushfires on water supply. Bushfires can have important consequences for water supply, by increasing erosion and dirtying water, making it unsafe to drink. Following some bushfires, townships have been forced to boil water to make it safe to drink. Along with a degradation of water quality, in some forests, particularly the Mountain Ash forests of Victoria, bushfires can reduce the amount of water available for drinking. This is because the forests use more water as they grow back after fire.
Australia’s most devastating fires
Between 1900 and 2011, 866 people in Australia were killed by bushfires. The most destructive of these occurred fairly recently.
The Ash Wednesday bushfires of 16 February 1983 that swept through southern Victoria and South Australia were devastating. Seventy-five people lost their lives—47 in Victoria and 28 in South Australia. More than 300,000 hectares of land were burnt and 3,000 houses and other buildings were lost.
Ten months of dry weather and a hot summer produced a dry landscape, primed for fire. Temperatures were in the high 30s to mid-40s, the air was dry and there were gale-force winds.
There were already 104 fires burning that Wednesday, but by the afternoon several more were burning throughout southern Victoria. It’s not known exactly how all these fires started, but some were caused by powerlines clashing with each other and with trees. Some were deliberately lit.
The Ash Wednesday fires were the deadliest on record in Australia until the Black Saturday fires of February 2009.
Once again, there were hot, dry conditions, with extremely high temperatures and strong winds. On 7 February 2009, around 400 fires started in Victoria. Temperatures in the region reached the mid-40s and winds were stronger than 100 km/h. Around 12.30 pm, powerlines fell in Kilmore East, sparking a fire that went on to become a firestorm that devastated the communities of Kinglake, Strathewen and St Andrews.
Most fire activity occurred during the afternoon and early evening of Black Saturday. Although evening brought a cool change to the region, it was accompanied by winds stronger than 120 km/h. Burning a total area of around 445,000 hectares of land, the fires moved at an average speed of 12 km/h, with some fires travelling up to around 72 km/h. One hundred and seventy-three people were killed in the Black Saturday fires. More than 400 people were injured and 2,100 homes were lost.
The amount of energy released by the fires was estimated to be equivalent to around 1,500 Hiroshima atomic bombs. In some situations, the fires produced sufficient radiant heat to kill people 400 metres away.
A Royal Commission was established to examine the causes and consequences of the Black Saturday fires. Its final and interim reports are a valuable source of information on the impact of bushfires in Australia.
Bushfires and climate change
As described above, four things need to be in place for a bushfire to occur: fuel must be available; the fuel must be dry; weather conditions need to be hot, dry and windy; and there needs to be a source of ignition. So, is climate change likely to affect any of these?
As plants use carbon dioxide (CO2) from the atmosphere to generate their energy, it’s possible that higher atmospheric CO2 concentrations will lead to enhanced plant growth in some areas. This is known as the CO2 fertilisation effect, and it will ultimately create more fuel for a potential fire. However, this effect only improves plant growth when there are sufficient amounts of other nutrients and water. Warmer temperatures at night and during winters will also increase the growing season of many plants.
The drier the fuel, the more likely it is to burn. Increased average temperatures caused by climate change will contribute to fuel dryness. Rainfall will also influence how dry (or wet) the fuel is. While the predictions for changes in rainfall are not as definitive as those for temperature, it is expected that there will be less rainfall in much of Australia in the future, particularly in the south-eastern and south-western regions.
The weather conditions are part of the criteria used to assess the degree of fire risk on any given day. An increase in warmer and drier weather means a higher number of days of high or extreme fire risk. Indeed, we’ve already seen an increase in high and extreme risk days in Australia over the past four decades. Along with more days of higher risk, climate change will also bring an extension of the fire season.
It is inevitable that Australia will always have large fires but, with better land management and improved understanding of bushfire behaviour, hopefully the catastrophic loss of life that has occurred in the past can be avoided.
More on bushfires: Managing landscapes