How Do Bushfires Start and Spread? The Science of Fire Weather in Australia

The short version

A bushfire needs three things: fuel, heat and oxygen. Take away any one of them and the fire goes out. Most bushfires in Australia are started by human activity or by lightning. After that, the fire stops being about how it started and becomes about the weather and the landscape, which decide whether it stays a small blaze or turns into a major fire event. Wind feeds it oxygen and carries embers ahead of it. Low humidity dries the fuel so it catches easily. High temperature pre-heats everything in its path. When those conditions line up, forecasters use a specific term for it: fire weather.

That is the whole picture in a paragraph. This article fills in the details, because understanding how fire behaves is the same knowledge the authorities use to warn communities, and it explains the orange skies and smoke haze that most Australians have seen at some point.

The fire triangle

Every fire, from a campfire to a bushfire, runs on the same three ingredients. Firefighters call it the fire triangle, and it is the simplest way to understand what is burning and how to stop it.

Fuel is anything that can burn. In the Australian bush that means grass, leaves, bark, twigs, dead wood and the fine material on the forest floor. Australian vegetation is exceptionally good at being fuel. Many of our native plants have evolved with fire and are loaded with volatile oils, the reason eucalyptus leaves burn so readily. A forest floor covered in leaf litter and bark is a fire waiting for a spark.

Heat is the ignition source, the spark or flame hot enough to start the fuel burning. It can be a lightning strike, a spark from machinery, a fallen powerline, a campfire or a deliberately lit match. The heat does not need to be dramatic. In hot, dry conditions, even a small heat source can ignite fine, dry fuel within seconds.

Oxygen comes from the air around the fire. Fire needs oxygen to keep burning, which is why wind matters so much. More oxygen reaching the fire means it burns hotter and faster, and why one of the first things firefighters do is try to starve a fire of the conditions it needs.

Remove any one side of the triangle and the fire cannot sustain itself. Firefighters use water and dirt to remove heat. They create firebreaks to remove fuel. Wind is the hardest to control, which is why the worst fire days are always windy days.

How most bushfires actually start

People start most bushfires. That is the uncomfortable, well-documented reality, and it is worth stating plainly. The human causes fall into a few categories.

Accidental ignitions are common. A spark from a mower or a grinder on a hot day, a chainsaw cutting through dry grass, a vehicle exhaust touching cured grass, a campfire left smouldering, a cigarette dropped from a car window. Powerlines also cause fires when they fall or swing together in high winds and throw sparks into dry fuel below. Each of these is a small event with the wrong timing, and on a bad day that is all it takes.

Deliberate lighting is a smaller share of the total but it happens every fire season, and it is the hardest for authorities to predict. When a fire is lit on purpose in dry, windy conditions, it can become dangerous very quickly.

Lightning is the other major cause, and it is the one that starts the largest fires in remote areas. Dry lightning, which is a thunderstorm that produces little or no rain, is the most dangerous kind. The strike hits dry bushland, and because there is no rain to dampen the fuel, the fire can smoulder and spread for days before anyone notices it. By the time it is seen, it may already cover a large area and be very hard to control.

The proportions shift from year to year and region to region, and the state fire authorities publish the causes of the fires they investigate. The honest generalisation is that in settled areas, human activity dominates, while in remote parts of the country, lightning is responsible for a large share of the area burned.

Why some fires spread slowly and others explode

This is where the weather takes over, and it is the section that explains why the same kind of spark produces a different outcome on different days.

The first ingredient is wind. Wind does two things. It feeds the fire extra oxygen, making it burn hotter, and it carries burning embers ahead of the fire front. Those embers can travel more than a kilometre in front of the main fire, starting spot fires that suddenly appear well ahead of the flames. A fire driven by strong wind is not a slow-moving wall of flame. It is a fast-moving front throwing embers in every direction.

The second ingredient is humidity, or rather the lack of it. Dry air pulls moisture out of the fuel. When the fuel is already dry, the fire catches easily and burns intensely. When the humidity is high, the fuel holds moisture and is much harder to ignite. This is why fire danger is higher on hot, dry days and why a cool change can take the edge off a fire.

The third ingredient is temperature. High temperatures pre-heat the fuel in front of the fire, driving out moisture and making everything more flammable. A 40 degree day does not just feel hot to people. It literally prepares the bush to burn.

Slope is the factor that surprises people, because it is not weather at all. Fire runs faster uphill than downhill or on flat ground. As the fire climbs a slope, the flames reach up the hill and pre-heat the fuel above them, pulling the fire forward. Firefighters talk about this constantly, because a fire on a steep slope in wind can move with terrifying speed.

The type of fuel matters just as much as the weather. Grass fires move fast but burn low and pass quickly. A grass fire can race across open country at a speed people cannot outrun, which is why grass fires are so dangerous in farming areas. Forest fires burn more slowly at the front but with far more intensity, because there is more fuel stacked up, and they keep burning in the ground and in the trunks long after the main front has passed. Understanding which kind of fire is possible in your area is part of understanding your risk.

All of these ingredients feed into the fire danger rating system, the scale from low to catastrophic that appears on signs and in weather forecasts. The rating tells you how dangerous a fire would be if one started, based on the weather that day. When the rating reaches severe or above, the safest response is to leave early, because that is the point where a fire can outrun any attempt to fight it.

When forecasters talk about fire weather, they mean these conditions lining up: strong wind, low humidity and high temperature. The Bureau of Meteorology issues fire weather warnings when the combination is dangerous, and state fire authorities use the same information to declare a Total Fire Ban. A Total Fire Ban means exactly what it sounds like. Lighting a fire in the open, including a campfire or a backyard fire pit, is banned for the day because the conditions make any fire too risky. If you see a Total Fire Ban declared, that is the day to check your bushfire plan and avoid any activity that could throw a spark.

Why the sky turns orange and red

Almost every Australian remembers the first time they saw a bushfire sky. The light goes strange, the sun turns into a red disc you can look at directly, and the whole sky takes on an orange or grey-orange glow. It looks apocalyptic. The science is actually straightforward, and it is the same physics that makes sunsets red.

Light from the sun is white because it contains all the colours of the spectrum. As that light travels through the atmosphere, it collides with particles in the air, and different colours get scattered by different amounts. The tiny molecules of the air itself scatter blue light far more than red light. That is why a clear sky is blue: the blue light is being scattered in every direction and reaches your eyes from all over the sky.

Smoke changes the equation by filling the air with particles that are much larger than air molecules. Those larger particles scatter the longer wavelengths, the red and orange end of the spectrum, much more effectively. So when smoke fills the air between the sun and your eyes, the blue light is scattered away and the red and orange light comes through, which is why the sky and the sun take on that fiery colour.

A sunset is the same trick without the smoke. When the sun is low on the horizon, its light travels through far more atmosphere to reach you, and the blue light gets scattered out along the way, leaving the reds and oranges. Bushfire smoke does the same thing during the day. The thicker the smoke and the lower the sun, the stronger the effect. That is why the worst orange skies tend to happen in the morning and late afternoon, when the sun is low and the smoke is thick.

What smoke haze actually is and where it goes

Smoke haze is what remains after the fire has passed and the smoke has drifted. It is made of fine particles, most of them far smaller than a human hair, carried downwind by the same weather systems that drive the fire. Those particles can travel for hundreds of kilometres. A fire in one region can put a brown-grey haze over a city on the other side of the state, which is why people far from any flames still smell smoke and see a milky sky.

The haze looks the way it does because of the same light physics described above. The fine particles scatter the light and wash out the colours, giving everything a flat, hazy quality. The smell is the particles themselves, and it lingers because the air is full of them.

The health advice around smoke haze is straightforward and worth repeating. The fine particles in bushfire smoke can irritate the lungs, and they are harder on people with asthma, other breathing conditions, heart conditions, older adults and young children. Health departments advise people in those groups to limit time outdoors when the smoke is thick, keep windows closed, and follow their doctor’s or the health department’s advice. Smoke haze is also connected to the wider question of what is in the air we breathe, the same subject as the science of how rain and pollen affect allergies, because both come down to tiny particles travelling through the air and into our lungs.

For most healthy people, a few hazy days are an inconvenience rather than a health crisis. But the haze is a useful reminder that a bushfire’s reach is much wider than its flames. The fire may be in one valley, while its effects are felt across an entire region.

Read the sky, check the warnings

The science in this article is the same knowledge that fire authorities use every day to keep communities safe, and it has a practical job to do.

When you understand the fire triangle, you understand why a Total Fire Ban matters. When you understand fire weather, you understand why one hot, windy day is dangerous while another is not. When you understand how smoke scatters light, an orange sky stops being a mystery and becomes information about what is in the air and where it came from. Understanding the science does not make a bushfire less serious. It makes the warnings make sense, and it lets you read the sky with a calmer, more informed eye.

If a fire does threaten your area, the state fire authority is the source for current warnings. The Bureau of Meteorology provides the fire weather forecasts. Both publish clear, up-to-date information, and both are worth checking before and during any high-risk period. The same is true of the climate that shapes these conditions, which is a subject of its own, and the science behind Australia's unpredictable weather is worth reading alongside this one.

Fire season is a fact of Australian life. The science does not remove the danger, but it removes the confusion. It explains the sights in the sky, the warnings on the news and the reason some days demand more care than others. That understanding is free, it applies everywhere in the country, and it is the calmest possible way to approach a subject that can otherwise feel overwhelming.

Sources:

• CSIRO, Bushfire behaviour and management
• Bureau of Meteorology, Fire weather and warnings
• Australasian Fire and Emergency Service Authorities Council (AFAC)