Bushfires Explained

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Context: Wildfires started to rage through Australia in September. Thousands have had to evacuate to the coast for fear of the bushfires, and the military has been sent in to help as flames ravage the countryside in the worst bushfire season in the country’s history.

Prelims: General issues on Environmental Ecology, Bio-diversity, and Climate Change.

  • GS I- geographical features and their location- changes in critical geographical features (including water bodies and ice-caps) and in flora and fauna and the effects of such changes.
  • GS III-
    • Conservation, environmental pollution and degradation, environmental impact assessment.
    • Disaster and disaster management.


Fire is a natural part of our environment. It has shaped the landscape through natural ignition (lightning) and indigenous burning practices for thousands of years. Fire has shaped Australia’s natural vegetation and has had an impact on human societies, beginning with Indigenous Australians and continuing through European settlement to the present.

What is a bushfire?

  • One type of wildfire is known as a bushfire, an uncontrolled fire that burns through scrubland, which is common to Australia (and many places in Africa).
  • Different types of bushfire fuel (vegetation) burn differently: finer fuels like grasses burn more quickly, while more substantial, woodier fuels burn with greater intensity.
  • Bushfires tend to occur when light and heavy fuel loads in Eucalypt forests have dried out, usually following periods of low rainfall.
  • Bushfires are generally slower-moving but have a higher heat output. This means they pass in two to five minutes, but they can smoulder for days.
  • Fire in the crown of the tree canopy can move rapidly.

Bushfires into two types based on the shape and elevation of the landscape:

  1. Flat grassland bushfires:
    • These are generally fast-moving, fanned by winds blowing across flattish open landscapes, and burn through an area in 5–10 seconds and may smoulder for a few minutes.
    • They usually have low to medium intensity and can damage to crops, livestock, and buildings.
    • These fires are easy to map and fight due to relatively straightforward access.
  2. Mountainous bushfires:
    • These fires are slower-moving but much more intense, with higher temperatures.
    • As they usually occur in forested, mountainous areas, they also have more dead vegetation to burn and are harder to access and fight.
    • Fires in upper tree canopies move very fast.
    • Mountainous bushfires actually speed up as they burn up a slope (since they heat and dry out the vegetation and atmosphere in front of the fire, causing a runaway process of accelerating fire movement).

What factors create a favourable environment for bushfires to occur?

The basic factors which determine whether a bushfire will occur include the presence of fuel, oxygen and an ignition source.

The fire intensity and speed at which a bushfire spreads will depend on ambient temperature, fuel load, fuel moisture, wind speed, and slope angle.

  1. Fuel load: 
    • Fuel load describes the amount of fallen bark, leaf litter and small branches accumulating in the landscape.
    • Generally speaking, the greater the fuel load, the hotter and more intense the fire.
    • The fuel which is concentrated but loosely compacted will burn faster than heavily compacted or scattered fuel sources.
    • Smaller pieces of fuel such as twigs, leaf litter and branches burn quickly, particularly when they are dry and loosely arranged and will burn quickly in the fire front.
    • Larger fuels, such as tree trunks often burn later after the fire front has passed.
    • The natural oil within eucalypt trees promotes the combustion of fuel.
  2. Fuel moisture:
    • Dry fuel will burn quickly, but damp or wet fuel may not burn at all.
    • As a consequence, the time since rainfall and the amount of rain received is an important consideration in assessing bushfire danger.
    • Often a measure of the drought factor, or moisture deficit, will be used as an indicator of extreme bushfire weather conditions.
  3. Wind speed:
    • Wind acts to drive a fire by blowing the flames into fresh fuel, bringing it to the ignition point and providing a continuous supply of oxygen.
    • Wind also promotes the rapid spread of fire by spotting, which is the ignition of new fires by burning embers lofted into the air by wind.
    • Spotting can occur up to 30km downwind from the fire front.
    • There is a threshold wind speed of around 12 to 15km/h which makes a significant difference in the behaviour of bushfires in the open.
    • When wind speeds are below this threshold, fires with heavy fuel loads burn slowly.
    • However, even a slight increase in wind speed above this threshold results in a significant increase in fire behaviour and advancement.
    • The width of a fire front also has an influence on the rate of spread and a wind shift can immediately widen the forward edge of a fire.
  4. Ambient temperature:
    • The higher the temperature the more likely it is that a fire will start or continue to burn.
    • This is because the fuel is closer to its ignition point at high temperatures and pre-heated fuel loads burn faster.
  5. Relative humidity: 
    • Dry air promotes a greater intensity fire than moist air.
    • Plants become more flammable at low humidity because they release their moisture more easily.
  6. Slope angle: 
    • Fires pre-heat their fuel source through radiation and convection.
    • As a result, fires accelerate when travelling uphill and decelerate travelling downhill.
    • The steepness of the slope plays an important role in the rate of fire spread.
    • The speed of a fire front advancing will double with every 10-degree increase in a slope so that on a 20-degree slope, its speed of advance is four times greater than on the flat ground.
  7. Ignition Source:
    • Bushfires can originate from both human activity and natural causes with lightning the predominant natural source, accounting for about half of all ignitions in Australia.
    • Fires of human origin currently account for the remainder and are classified as accidental or deliberate.
    • Fires lit deliberately can be the result of arson or might be designed to achieve a beneficial outcome but conditions have changed, resulting in an uncontrollable spread.
    • Unfortunately deliberate and accidentally lit fires are more prevalent near populated areas and have a disproportionately higher risk of infrastructure impact.
    • Arsonists place people and property at serious and unnecessary risk, particularly when igniting fires on extreme fire weather days.

How do bushfires burn?

The three stages in the process of combustion- pre-heating, ignition, and flaming combustion. These stages operate together during the start of a bushfire.

  1. Pre-heating:
    • Before fuel will burn easily, moisture must be removed.
    • Hot, dry weather or the heat of an approaching fire will evaporate moisture.
    • The fine fuels such as small twigs, dry leaves, and grass lose that moisture more easily and provide the initial fuel for the fire to burn.
    • Plant matter contains cellulose, a carbon-based material.
    • The heat from a match, electrical spark or other source causes the cellulose particles to break down into smaller combustible products which are given off as a cloud of fuel gas.
  2. Ignition: 
    • The temperature of the cloud of fuel gas is raised by the heat source until ignition occurs.
    • The fuel gas reacts with oxygen from the air and becomes flame.
    • Heat energy is released.
  3. Flaming combustion:
    • The heat produced by the flames preheats the nearby fuel which also gives off fuel gas.
    • The fire spreads as this gas ignites.
    • The more fuel available, the more gas is produced.
    • As the heat energy increases, the fuel gas is given off from larger fuels such as branches, logs, and tree trunks.
    • As the amount of fuel gas given off is reduced, the flames die down until there is not enough fuel gas to support further flaming combustion.
    • Glowing combustion takes over, eventually leaving blackened charcoal and white ash.

How do bushfires spread?

A fire spreads when it has enough heat to keep it burning into unburnt areas. A spreading fire is really a transfer of energy. In a bushfire, there are three main ways in which this happens.

  1. Radiant Heat:
    • The vegetation ahead of the fire is heated by the approaching flames.
    • The radiant heat from the fire does not penetrate deeply but affects the surfaces exposed to heat.
    • The hotter the fire, the greater the amount of radiant heat and the faster the fire will spread as it dries out the fuel in its path.
    • Heat radiation decreases rapidly with increasing distance from the fire.
  2. Convection:
    • Convection is the movement of heated air.
    • Smoke and ash are carried upwards by convection currents.
    • As the fire grows, an increasing volume of air is heated and rises.
    • Strong winds also develop which can alter fire behavior.
    • The rising hot air lifts firebrands or embers which may be blown beyond the fire front igniting spot fires.
  3. Wind Speed:
    • 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 the 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
    • 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. 

In areas where there are uniform vegetation and no wind, the spread of the fire occurs in a slowly widening circle and can easily be extinguished. This pattern is common.
Factors such as fuel loads, wind speed and direction, and slope alter the rate of spread and the intensity of the fire. Many bushfires are irregular and elongated in shape.

Where do bushfires occur?

Wildfires can occur anywhere but are common in the forested areas of the United States and Canada. They are also susceptible to many places around the world, including much of the vegetated areas of Australia as well as in the Western Cape of South Africa.

Bushfires in Australia
  • Bushfires are an intrinsic part of Australia's environment.
  • Natural ecosystems have evolved with fire, and the landscape, along with its biological diversity, has been shaped by both historic and recent fires.
  • Many of Australia's native plants are fire-prone and very combustible, while numerous species depend on fire to regenerate. 
  • For thousands of years, Indigenous Australians have used fire to clear grasslands for hunting and to clear tracks through dense vegetation.
  • However, this was only in periods of high rainfall and in very small grassland zones bordering desert.
  • The Australian climate is generally hot, dry and prone to drought. At any time of the year, some parts of Australia are prone to bushfires.
    • For long periods, the vast desert regions of Australia, which cover more than 50% of the continent, have little or no vegetation available to burn.
    • The central and mid-north-west of Australia are grassland regions where farming of crops and cattle occurs.
    • These areas of grassland, open scrub and woodland experience bushfire from time to time.
    • The remaining 4% of the continent’s vegetation is forest and can easily burn when dry. 
  • The widely varied fire seasons are reflected in the continent's different weather patterns.
    • Areas that experience a temperate climate tend to have the most bushfires.
    • The conditions associated with a temperate climate- moderate winter rainfall and warm to hot conditions in summer- occur throughout the southeastern and south-western regions of Australia.
    • The mild winter temperatures and plentiful rainfall encourage vegetation growth.
    • This is cured by the high summer temperatures and the amount of fine fuel necessary for a severe bushfire becomes available.
    • Strong northerly winds associated with high-pressure systems blow across the desert region of central Australia in summer, bringing hot, dry air to south-eastern Australia. 
    • These winds reduce the moisture content of the grasslands and forests and create the conditions for fires to burn. 
    • At the same time, a cold front approaching south-eastern Australia from the west brings a mass of cool, moist air.
    • The heavier, cooler air is able to force its way under the hot dry air ahead of it.
    • The cold front causes rapid changes in wind direction, temperature, and humidity.
    • The cool air is often associated with strong, gusty, south-westerly winds that can change the direction of the fire, turning its eastern side into a new front.
    • These strong, gusty winds also create rapid fire spread.
  • For most of southern Australia, the danger period is summer and autumn.
  • For New South Wales and southern Queensland, the peak risk usually occurs in spring and early summer.
  • The Northern Territory experiences most of its fires in winter and spring.

Climate change and bushfire risk:

  • To be clear, the current bushfires are not specifically triggered by climate change.
  • But Climate change is increasing bushfire risk in Australia by lengthening the fire season, decreasing precipitation and increasing temperature.
  • Bushfire risk is highest in warm to hot, dry conditions with low humidity, low soil and fuel load moisture (and are usually worse during El Niño situations)– all factors that climate change in Australia affects.
  • Thus, Climate change is increasing the risk of more frequent and intense bushfires.
  • Widespread drought conditions, very low humidity, higher than average temperatures, Heatwaves, El Niño, and regional weather patterns such as high-pressure ridges can increase the risk and alter the behavior of wildfires dramatically. 
  • These factors, as well as strong westerly winds driven by a negative Southern Annular Mode, have collided right now over large areas of the eastern seaboard.
  • This has triggered extremely unusual bushfire conditions– certainly catching many communities unawares before the start of the official bushfire season.
  • The unprecedented fires, which have killed at least 25 people and destroyed 2,000 homes, have been burning since September.
  • In all, about 15 million acres have burned across the country, an area roughly the size of West Virginia.
  • While climate change might not ignite the fires, it is giving them the chance to turn into catastrophic blazes by creating warmer temperatures, increasing the amount of fuel (dried vegetation) available and reducing water availability because of higher evaporation
  • Different regions of Australia have traditionally experienced peak bushfire weather at different times.
  • This has meant that individual households, communities, and emergency services have had specific periods of the year to prepare.
  • These patterns now seem to be breaking down, and bushfires are happening outside these regular places and times.
  • Years of precipitation followed by warm periods can encourage more widespread fires and longer fire seasons.
  • Since the mid-1980s, earlier snowmelt and associated warming have also been associated with an increase in length and severity of the wildfire season, or the most fire-prone time of the year, in the Western United States.
  • Global warming may increase the intensity and frequency of droughts in many areas, creating more intense and frequent wildfires.
  • A 2015 study indicates that the increase in fire risk in California may be attributable to human-induced climate change.
  • A study of alluvial sediment deposits going back over 8,000 years found warmer climate periods experienced severe droughts and stand-replacing fires 
    • This concluded climate was such a powerful influence on the wildfire that trying to recreate the presettlement forest structure is likely impossible in a warmer future.

Impacts of bushfires on forest Ecosystem and People:

  1. Vegetation: 
    • The frequency of fire affects the growth cycle of plants.
    • If fires do not occur for a long period, seed from some species will lie on forest floors unable to germinate.
    • If fires occur too frequently, some plants will not be mature enough to produce and distribute seeds.
    • If they are destroyed by fire before producing seed, the area may be taken over by other species better adapted to frequent fires.
    • The vegetation balance would then be altered.
  2. Animals:
    • Fire also plays an important role in maintaining populations of certain animal species.
    • Fire affects the age, structure, and composition of vegetation, creating habitats that suit different animal species. 
    • Most animal species are reduced in number during or immediately after a fire.
    • Many individual animals may be killed through burning or suffocation.
    • Others may survive the fire but die shortly afterward due to predation by other species or food shortages. 
    • Many insects and spiders are also killed, especially in high-intensity fires that destroy the bark and litter layer in which they live.
    • Flying insects have a higher chance of survival if they can move away from the fire.
    • The loss of habitat has a major impact on arboreal (tree-dwelling) species.
    • The hollows in tree trunks, that are their main habitat, are either burnt, destroyed or exposed to further decay.
    • Many of these hollows can take over one hundred years to form.
  3. Soil and water:
    • Bushfires can have several effects on soils, depending on how intense the fire is and how hot the soil becomes.
    • Usually, only the top few centimeters are affected as they are subjected to the highest temperatures.
    • Low-intensity fires can sterilize the soil (cause the death of living organisms within the soil).
    • Higher soil temperatures (greater than 100ºC) may change the amounts and availability of nutrients such as nitrogen and phosphorus. 
    • Fire may cause changes in the soil’s ability to absorb water.
    • If vegetation is destroyed during a fire, the soil is subject to the action of wind and water, making it very susceptible to erosion.
    • Very heavy rainfall immediately after a fire may cause massive erosion or mudslides.
    • Fire can affect stream water quality and may also influence the amount of water produced by a forest that becomes available to streams. Erosion may cause soil, ash, and nutrients to be transported into streams.
    • This increases the sediment load and the turbidity (cloudiness) of the water or may even fill the stream channel.
    • The quantity of water produced by an area that has been burnt may initially increase, as there is little vegetation to trap the rainwater.
    • After a period of time though, the new vegetation uses a large amount of water to grow and therefore can reduce the water available from a catchment.
  4. People:
    • The loss of life is the biggest impact of bushfires on human life.
    • Radiant heat may cause heatstroke – this occurs when the body’s cooling the system fails and, unless treated, will cause death.
    • During a bushfire, a person might lose two liters of water per hour.
      • If this is not replaced, the person will become dehydrated and their body will not function properly.
      • Their kidneys and liver may fail and death may occur.
    • If a person inhales hot smoke, lung tissue may burn and be unable to get sufficient oxygen to the rest of the body. 
    • The trend of semi-rural lifestyles and the ongoing development of the rural-urban fringe means more lives and assets are at risk from bushfires.
    • The value of assets such as houses and buildings has also increased dramatically with rising property values. These assets, during the events of bushfires, can be destroyed.

Controlling Bushfires:

  • 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 ‘turn back’ to the fire front, this process is known as ‘back burning’.
    • 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 vapor, 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 the water has evaporated. 
    • Airplanes are also used to drop water or retardant on the fire. 


Historically, bushfires have caused a loss of life and significant damage to property. While naturally occurring bushfires cannot be averted, their consequences can be minimized by implementing mitigation strategies and reducing the potential impact on areas that are most vulnerable. 

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