A bushfire is one of the most terrifying natural phenomena that anyone is likely to experience in Australia. To be caught in a bushfire is to witness a true hell on earth — conditions hot enough to melt metal, heat fluxes that literally vaporise vegetation, and smoke plumes so dense they turn day into night.
The climate, topography and vegetation of the continent is such that at some time during the year bushfires will occur somewhere in the country. Most bushfires are relatively mild and often burn themselves out when they run out of fuel, or the weather changes, particularly in the north of Australia. However, when the outbreak of a bushfire coincides with hot strong winds and previous weather conditions that have left the vegetation across the landscape continuously dry and flammable (such as after an extended period of drought or when grasslands are cured off), bushfires have the potential to become widespread conflagrations that are essentially uncontrollable until the weather moderates.
Despite the widespread and endemic nature of bushfires, however, the average Australian is unlucky to experience one first hand, even once in their lifetime.
The seasons for fire
Bushfires across Australia are seasonal, following the advance of dry, warm conditions as the year progresses. In the north, the bushfire season coincides with the dry season, running from late autumn to late spring. The bushfire season commences later in the year the further south one travels. In south-eastern and south-western Australia the bushfire season runs from early summer to early autumn.
The onset of extreme bushfire weather is generally associated with synoptic weather patterns that bring the hot dry air from the centre of the continent toward the coast. In eastern Australia this is predominantly a high pressure system located in the Tasman Sea with a cold front or trough approaching from the south-west out of the Southern Ocean that directs very strong, hot and dry winds from the north-west. In the west it is a high pressure system approaching from the Southern Ocean or present in the Great Australian Bight.
Bushfire conditions are dangerous even without the fire. The conditions associated with extreme high intensity bushfires are often the same ones associated with life-threatening heatwaves. These conditions, which include very high air temperatures, extremely dry air and very strong winds, are such that the health of many people, particularly the elderly, infirm or very young, can be affected.
Fuel for the flames
The primary fuel for bushfires is the fine dead vegetation on the forest floor. This is comprised of fallen leaves, twigs and bark, as well as cured grasses. On bad bushfire days this vegetation becomes highly combustible across the whole landscape.
Bushfire risk has a lot to do with fuel moisture. When bushfire fuel contains more than 30% moisture by weight it is impossible to ignite; when it is less than 20%, it can be readily ignited; when it is less than 10%, combustion is rapid and fires can spread easily; and when it is less than 5%, fire behaviour is highly erratic and fire spread is rapid. In Victoria on Black Saturday, 7 February 2009, the entire landscape for much of the afternoon had a moisture content of less than 5%. Under these conditions the slightest spark can ignite the fuel and fires will spread very rapidly.
Inside the fire zone
The peak conditions for high intensity bushfire behaviour generally occur mid to late afternoon. In south-eastern Australia, the prevailing winds on a bad fire day will blow the dense smoke generated by a bushfire to the south-east, blocking out the sun and throwing everything in the path of the bushfire into almost absolute darkness. The sun, when it is glimpsed through the smoke, is a hellish red giving the impression of approaching doom.
The noise of an approaching fire, generated by the rapid cellular decomposition of vegetative matter and shockwaves associated with the gas phase combustion of the released volatiles, can be most frightening, particularly when the fire itself cannot be seen due to smoke and topography. It is often compared to the sound of a steam train at full tilt or the roar of a jet engine.
If a cold front arrives during a bushfire, it generally brings with it gusty winds that change the direction of the prevailing winds and can turn the long downwind flank of a bushfire into a raging head fire. In most cases, the area burnt by a bushfire after the arrival of a cold front is much greater than the area burnt before the wind change.
Inside the turbulent diffusion flames of a bushfire, the temperature of the reaction zone, where the volatile gases released from the thermally degrading vegetation mix with oxygen in the air and combust, can be in the order of 1600°C. The temperature of the flames themselves, however, is less than this adiabatic value, with the maximum temperature at the base of tall flames reaching approximately 1100°C due to mixing with ambient temperature air. The tips of flames are around 600°C.
The radiant heat flux from a thick bushfire flame can reach 100 kW/m2. By comparison, the average radiant heat flux from the sun at midday on a summer’s day is about 1 kW/m2. The pain threshold for most people is about 2 kW/m2 and at this rate bare skin will undergo a partial thickness (2nd degree) burn in about 40 seconds. In the midst of a high-intensity head fire, radiant heat fluxes in excess of 150 kW/m2 have been measured.
The convective energy released by a major bushfire provides enough buoyancy to lift the smoke many thousands of metres above the fire (sometimes greater than 10 km), often breaking through the tropopause and into the stratosphere, carrying the smoke around the world. The condensation of water from the combustion products can form pyro-cumulus clouds that can form rain and lightning, starting new fires downwind of the main fire.
The combustion of the fine dead litter and grass generates the bulk of the energy of a bushfire. As a fire increases in intensity, it will burn and release the energy of other vegetative fuel such as the live leaves in shrubs and ultimately the crowns of trees. In most eucalypt forests, the tree crowns are not dense enough to support an independent crown fire. Instead, fires burning through the surface fuels can ignite the tree crowns in what is called torching of individual trees or a dependent crown fire of a stand of trees.
Bushfires are often described as being chaotic and capricious. This is generally a result of the highly turbulent nature of the winds that drive fires on bad days, combined with the highly complex nature of the fuels and their combustion. The primary constituent of biomass is cellulose. When heated, cellulose will undergo one of two reaction pathways, one that produces volatile gases and flame and one that produces char and glowing combustion. The thermal and chemical feedbacks between these pathways means that a fire burning in vegetation will rapidly switch between these very different reactions, producing behaviour that, when combined with the great variations in the structure of the fuel as well as the wind, appears to not follow any rules. However, bushfires do follow the laws of physics and at larger scales and longer timeframes their behaviour can be predicted adequately enough to allow for suppression planning and the issuing of warnings.
A kilogram of dry vegetation contains enough energy to power a 100 Watt light globe for 50 hours. In a bushfire that energy is released in a few seconds. That garden bed outside your family room window may look nice but the water-saving organic mulch contains enough energy when it is burnt, in the form of radiant and convective heat, to break the window and ignite the curtains in less time than it takes you to click through to the next article.
Managing the fuel consumed by a bushfire, not just in the bush but around our homes and critical infrastructure, is the only practical measure we have to influence the behaviour of bushfires, particularly when fires grow beyond the point that direct or indirect suppression is feasible. Research has shown that halving the level of hazard presented by fuel will roughly double the chances of firefighters successfully attacking and suppressing a fire on first attack. The more fires that are successfully suppressed on first attack will drastically reduce the number of fires that grow out of control to threaten lives and property.