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Like many others, I have been following the sad development of the bush/forest fires in Australia recently. A claim that gets repeated is that one of the contributors to this blaze is the ongoing >45°C heatwave.

Now I am not really putting this in question here. This is probably a well established fact, but my immediate intuition of physics fails to grasp why. Here's my reasoning:

  • Obviously the vegetation has to be dry for this to happen, but surely - and in my experience - a +30°C temperature is more than enough to make sure everything is bone dry.

  • 45°C is not significantly closer than the aforementioned 30°C to the ignition temperature, which is several hundreds degrees. I am aware that some vegetation in Australia (eucalyptus probably) is remarkably easy to light, but still air temperatures are pretty far from that.

  • Heat conduction flux is also directly proportional to the difference in temperature (first order dependence) and not for example the square of it, so the few extra degrees shouldn't make grass that much easier to ignite.

So why does the heatwave matter?

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    $\begingroup$ It's not clear to me that the answer to this question is necessarily primarily based in physics as such. Might Earth Science be a better home for this question? $\endgroup$ – ACuriousMind Jan 4 '20 at 22:35
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This is more a question of chemistry and biology than physics.

Solid objects don't burn (try dropping a lit match on a piece of structural lumber sometime -- it'll just go out). Instead, they release or decompose into flammable gasses on heating, and it's those gasses that burn. Some plants (eg. pines) produce volatile resins that provide an easy ignition source, and the amount and speed of vaporization increases with temperature.

Eucalyptus is especially bad for this: eucalyptus oil is extremely volatile, extremely flammable, and produced in copious quantities. In hot weather, there's far more vaporized oil in the air than in more moderate temperatures, leading to easier fire starts and faster spreads. Eucalyptus has evolved not just to survive fires, but to encourage them, as a way to clear out competing plant species.

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    $\begingroup$ The Blue Mountains, just outside of Sydney, are so-called because the abundance of vaporized oil gives them a blue tinge when viewed from a distance... just an example of the abundance of flammable material in the atmosphere in some parts of Australia $\endgroup$ – Dancrumb Jan 6 '20 at 3:23
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    $\begingroup$ @Dancrumb Doesn't the atmosphere give everything a blue tinge from a distance? $\endgroup$ – user253751 Jan 6 '20 at 11:21
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    $\begingroup$ This is good but I think you need to include something about lack of moisture. Green leaves are much harder to set alight than dry ones. This is a well-known fact around Christmas tree fires. In a controlled temperature environment, a dry tree will erupt into flames much more easily and quickly than a 'wet' tree. $\endgroup$ – JimmyJames Jan 6 '20 at 18:13
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    $\begingroup$ tldr; some trees just want to watch the world burn $\endgroup$ – Cireo Jan 7 '20 at 0:25
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    $\begingroup$ @user253751 Have you been to the Blue Mountains? It is not Rayleigh scattering. There is a definite fog visible over the trees in the middle distance that has a blue hint to it. The effect is very subtle and difficult to photograph, but there are some shots on Google that show it. Also, the effect is absent in other, non-eucalyptus clothed mountain ranges. $\endgroup$ – Oscar Bravo Jan 7 '20 at 8:22
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Water evaporates faster at higher temperatures, so plants will lose water faster in a heat wave than in milder conditions. Drier plants will catch fire easier and burn faster and hotter than moist plants.

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    $\begingroup$ See transpiration. In one paper measured transpiration increased 40% when temperature was increased from $30^\circ$C to $40^\circ$C (at constant vapor pressure). $\endgroup$ – Eric Towers Jan 5 '20 at 5:03
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    $\begingroup$ This is probably even more of an issue for plants that are adapted to typical temperatures of 30°C as they will having coping mechanisms. These are likely to be overwhelmed by the extreme temperatures currently seen. Excessive night-time temperatures can also be a problem for some plants even though they're lower than daytime temperatures $\endgroup$ – Chris H Jan 6 '20 at 8:56
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EDIT – Due to the interest, while my previous answer was OK, I’ve summarised official Australian ABC (public broadcaster) Fact Checks on this subject to provide full and correct detail.

The three essentials on how bushfire behaves are: a) weather, b) fuel and c) topography.

In Australia, the Bureau of Meteorology produces ‘fire danger ratings’ in consultation with State authorities. While other risk factors are considered, the Forest Fire Danger Index (FFDI) of 1-100+, a formula developed by fire scientist Alan McArthur in the 60’s, is a major factor. This index combines measurements of air temperature (why heatwaves matter – indirectly via influence on fuel moisture content and local wind formation, plus the higher the temperature of the fuel, the more easily it will reach ignition temperature), relative humidity, wind speed and fuel conditions with a formula to account for the effects of drought, i.e. how long since it last rained/amount of last rainfall to assess how dry the soil is, and therefore how combustible the fuel is likely to be.

enter image description here

More re air temperature, worked example. If you download the NSW RFS Firefighter Pocketbook from the app store, you can see for yourself that all else being equal (say, 10% relative humidity, 15km/hr wind speed, lowest fuel load 1, drought factor 10, slope 0) an increase from 30 degrees to 45 degrees changes the FFDI from 34 (Very High) to 57 (Severe).

However, under extreme and catastrophic fire conditions, the McArthur model underpredicts the rate of spread and intensity. When you exceed a FFDI of around 50 you switch from a fuel-dominated to a weather-dominated fire. This is why there is a lot of focus/media reporting on the weather (air temperature, relative humidity and wind speed) in extreme and catastrophic fire danger rating periods. Basically, in those conditions (like we have been seeing) you only need the tiniest amount of fuel to produce a fire intensity that cannot be fought.

This is also why, with bushfires associated with an FFDI of >100, even in areas that have had hazard reduction burns less than five years prior, there has been proved to be no measurable effect on the intensity of the fires.

Prescribed burns, aka hazard-reduction fires, is therefore useful, necessary, and effective for lesser conditions, but even if you were to hazard burn everything to the max, its not going to give a benefit under ‘catastrophic’ fire weather conditions. Also, its not possible to hazard burn some of the areas now burning in NSW/VIC in cooler months, it is too wet.

But in anycase, the national parks have been hazard burnt. In the last 8 years in NSW, the NSW Parks & Wildlife carried out hazard reduction burns over 680,000ha – more than double that of the previous 5-year period. In QLD, in 2019 QLD Parks hazard burned 1.4M ha the largest area covered in the last 6 years. So there has been no ‘locking up’ of the national parks, they have been hazard burning more than ever, and yet the fires still burn – because of the weather, which of course, has been made worse by climate change.

https://www.abc.net.au/news/2019-11-14/fact-check-did-high-fire-danger-indices-come-earlier-this-year/11506082

https://mobile.abc.net.au/news/2019-12-20/hazard-reduction-burns-bushfires/11817336?pfmredir=sm

There has been 3 years of drought, and 2019 was the lowest rainfall on record in many parts of Australia. The humidity in the parts of NSW and Victoria on fire now has been very low <10%, and there has been strong winds. Then, it has been very very hot, temperature records have fallen, 2019 hottest on record, and also hot at night. All those together mean when the fires go, they burn like nothing seen before.

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Some reasons that I think that might be the cause:

  • Higher temperature leads to higher rate of evaporation of from sources of water causing them to become dry and easily ignitable (reason being that wet things require more heat to burn than dry ones as wet things have water in them)

  • Higher temperature lead to the loss in the efficiency of many vital enzymes in plant, which as obvious leads to the death of smaller plants, which further leads to the cause of drier vegetation.

  • Stronger winds are generated leads to the friction between twigs and other stuff (due to relative motion) and this causes temperature rise between these and hence leading more chances of fire.

  • In the absence of large nearby water body the humidity of the region falls.

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    $\begingroup$ I doubt that the friction between twigs makes any significant difference. The extra heat would be tiny and would immediately dissipate into the environment. $\endgroup$ – CJ Dennis Jan 6 '20 at 5:48
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    $\begingroup$ Starting a fire from friction between natural objects is very difficult to do. Those of us who also practice primitive fire starting actually try to make this happen, and many do not succeed. It's hard to do even if you have the perfect environment and the perfect setup. I'm not going to say it's impossible, but I would be surprised if it has happened naturally even once in the last few thousand years. $\endgroup$ – Aaron Jan 6 '20 at 19:52
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    $\begingroup$ I would add that in the less humid air the glowing shards can be carried further and set other regions on fire because they fade away slowly. $\endgroup$ – Crowley Jan 6 '20 at 23:18
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Adding heat when air drying corn drastically shortens drying times. While the temperature range discussed in the article is lower than in Australia's current weather the general rule is surely applicable, and most likely to other plant parts as well.

Not only is the terminal humidity of the plant material lower in hotter air, it is also reached earlier, which may explain why the fires are so bad relatively early in the season. While in cooler years a plant may bridge dry spells between precipitation events it may not be able to do that at 45℃.1 And as others have noted, the hot air also dries soil and surface so that the plants find less water to replenish. Heat stress with insufficient water supply may have killed more plants than usual at this time of year, adding more dry fuel than usual.


1 I would think that this temperature is also generally not far from the absolute maximum temperature for all but the most specialized plants, as it is for animals, including humans.

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Overall there isn't a simple answer to your question, prolonged periods of elevated temperature effect the chance of ignition, rate of fire spread, and what types of fuels become involved in the fire. These of course are also strongly effected by other factors like the moisture content of the fuel, and the strength of the wind. For instance if you have a heat wave and high wind, the fire danger is usually very severe.

The poster and several of the responses have correctly noted that elevated temperature dry the fuel more, however there is a dependence on time that is not addressed. The key thing here is that in general, the larger the piece of fuel, the harder it is to evaporate the moisture contained in it. So a prolonged heat wave would allow for larger fuels, wetter fuels, or fuels out of direct sunlight, such as dead trees, timber elements in buildings, piles of leaf litter, and living/green vegetation to become bone try as well, increasing the intensity of the fire, and allowing them to spread faster. For this reason, wildland fuels are often characterized as 1 hr, 10 hr, 100 hr, and 1000 hr fuels based on the characteristic time it would take for them to dry. If say you had a fire after a prolonged heat wave (longer than 1000 hrs), both larger and smaller fuels would be involved while if the heat wave had only been for ~10 hours, less of the fuels would be involved, reducing the intensity of the fire.

Higher temperatures also make ignition more likely. Consider a carelessly discarded cigarette igniting a pile of leaves. Before the leaves are ignited, they heated, and some of that heat is being lost to the environment based on the temperature difference between the leaf and the environment. If the heat losses are too great, ignition will not occur, but if the heat losses are too small, ignition will occur.

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You are correct, it is not the heatwave itself, but its cause, drought.

There are mainly three causes of the fires:

  1. wind

  2. heat

  3. drought (lack of moisture)

It is not the heatwave itself, but the heatwave, when it lasts, like in your case, brings drought. Fire needs fuels, and we call fuels, in the case of brushfires, the small sticks, trees, and undersbrush receive radiant heat from the Sun, and if the heatwave lasts, the drought will bring these fuels to a level where they can ignite faster.

A lasting heatwave usually means no rain, so drought.

So basically the heatwave itself just helps with the ignition, but the fuels need to be as you say bone dry (because of drought) to get ignited.

Once the fuels get ignited, it is the wind that makes the fire get oxygen.

https://science.howstuffworks.com/nature/natural-disasters/wildfire2.htm

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  • $\begingroup$ This answer is incorrect on many levels, and demonstrates no real understanding of the current disaster. Three-year droughts are not caused by 10-day heatwaves. The mega-fires are the result of very hot air masses from Central Australia arriving in the drought-stricken eucalyptus forests of S.E. Aust, and ignition (typically by dry lightning) rapidly evolving into an unstoppable crown fire fuelled by eucalyptus oil vapour. Ground fuels play only a small part in such fires, acting mostly as an initial accelerant. $\endgroup$ – Chappo Hasn't Forgotten Monica Jan 13 '20 at 3:15
  • $\begingroup$ @ChappoSaysSEDuddedMonica I live in CA, and I might be mistaken, but I thought the question was generally about brushfires and heatwaves. The process I wrote down perfectly describes the process here for the brushfires in CA and the heatwaves. If it just about the specifics for Australia that you describe here in detail (and you must be correct), then I think the question should be edited so that it includes your details too. $\endgroup$ – Árpád Szendrei Jan 13 '20 at 3:28
  • $\begingroup$ The question clearly refers to the current Australian fires. For a U.S. comparison, you might be interested in this short video about the Las Conchas megafire, or see the full one-hour show on the science behind megafires, which explains the current disaster - very much worth watching. And this report gives the science behind why 2019 was Australia's hottest AND driest on record. FYI :-) $\endgroup$ – Chappo Hasn't Forgotten Monica Jan 14 '20 at 0:54

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