I am not sure if this is the right forum for the question but failing to have any better location to ask it, i have come here.

In the UK we have a tradition of lighting bonfires on the 5th of November (or the weekend closest to it).

With numerous amounts of bonfires of a large size being created and knowing that forest fires in Australia can raise the temperature of the area there, is it feasible that the temperature of the UK slightly increases on bonfire night due to the amount of bonfires that have been lit or is the increase so infinitesimally small that it couldn't be measured let alone felt by the populace?

I have tried to find an answer to this but the best i can come up with is the current weather conditions for bonfire night which isn't even remotely close to what i am hoping to see.

As an addendum to this question....ironically it snowed in a lot of the UK only 5 days later........

  • $\begingroup$ "forest fires in Australia can raise the temperature of the area there" could you share the source? $\endgroup$ – pentane Nov 3 '16 at 13:28
  • $\begingroup$ I cannot give you a source, it was something i saw on TV a long time ago, basically the fires were raging through Australia and the firefighting process was being hampered due to the fact that the fire was so large it was causing the area the fire was in to experience an increase in temperature. This caused other fires to start more easily. This was however a few smaller fires that had merged to create a massive fire and not your normal "couple of trees". $\endgroup$ – user202944 Nov 3 '16 at 14:40
  • 1
    $\begingroup$ What will alter temperature is unlikely to be the fires but the smoke: dumping a lot of aerosol into the atmosphere can make a very significant (upward) difference to the surface temperature at night. See for instance this page (part-way down the page). $\endgroup$ – tfb Nov 3 '16 at 17:44

With numerous amounts of bonfires of a large size being created and knowing that forest fires in Australia can raise the temperature of the area there, is it feasible that the temperature of the UK slightly increases on bonfire night due to the amount of bonfires that have been lit or is the increase so infinitesimally small that it couldn't be measured let alone felt by the populace?

I doubt very much if the overall land area of the UK would be affected by the presence of an arbitrary number of bonfires. I have no idea of the number of bonfires, but let's do an estimation.

  1. The population of the UK is around 60 million people.

  2. Assume that the people most likely to light bonfires are in the 15 to 40 age group, UK Official Population Figures, produces a guessimate of 20 percent.

  3. So you have possibly 12 million potential firestarters, let's cut that down to 6 million actually available to light fires, and that 100 people attend the event.

  4. So you have 6000 large bonfires, (or a lot more smaller ones, it evens out) and say the area of each bonfire base is 10 square metres. So 6000 by 10 square metres is 60,000 square metres occupied by burning material. The total land area of the UK is 243.61 billion square metres. Actually that 6,000 is way too low but even 60,000 fires would still be ok in this rough estimation.

  5. Enough already, I think.

I am ignoring calculating the heat output of each fire. A better way to estimate, in my opinion, is to consider if an average 5 degrees Celsius of 243.61 billion square metres of air will be affected in any detectable way by 60,000 square metres at an average temperature of say 600 degrees Celsius, (the average temperature of burning wood).

No, it won't, even if my number for fires is to 60,000 fires, which looking at it now seems a more likely figure. No matter what the fire number is, it still has to contend with 244 billion square metres of 5 degree Celsius air.

In addition we may have to allow for 2 to 5 million hot car exhausts and 3 million plus domestic heating house fires, which would have far more effect than bonfires.

So locally, within an radius of 200 metres of each fire, the air temperature increase is detectable, beyond that, no chance of detection seems likely.

A better way to think about it, after the fires all go out about two in the morning, is to ask yourself, will you notice a distinct change in the weather for the next day? Will all this energy affect you?

You could find out yourself, by looking at the weather pattern over London during the 1940/3 blitz, for example.

I put this part as the real answer, because I believe that rather than saying it depends on various factors, research can be carried to establish if this idea is true, by checking on weather patterns shortly after the fires and looking for correlations.

More supporting, (but indirect) evidence is the deadly smog that regularly covered London due to domestic coal fires, but it's far too complicated as regards the number of variables to easily establish correlations between the bonfire and the weather.

The focus then moves from PhysicsSE to EarthSciencesSE, imo.

  • 1
    $\begingroup$ Your conclusion is probably correct, but your assumptions are wrong. Family bonfires for just a few people are common. 12 million potential fire starters could be 12 million bonfires, each for an average of five people (only one of whom is in your fire starter category). $\endgroup$ – Mike Scott Nov 3 '16 at 16:35
  • 1
    $\begingroup$ 3 million house fires?! That would mean about 1 in 8 houses is on fire. Do you mean domestic fireplaces instead? $\endgroup$ – Chris H Nov 4 '16 at 9:51
  • 1
    $\begingroup$ @ChrisH Oh, I should have researched the tradition a little more, Bad night to be a fireman ( or firewoman ? ) though. Anyway, consider it sorted out, thanks, BTW if you want to see a real bonfire, search google for images in northern Ireland, they compete to build the highest. wooden pallet fires. Thanks $\endgroup$ – user108787 Nov 4 '16 at 10:30
  • $\begingroup$ @MikeScott family bonfires sure as hell aren't that popular. There are 27 million households in the UK: I'd be surprised if the percentage who have their own bonfire hits double figures. In my street there's not a single bonfire ready for tomorrow, out of 150 homes (most of which contain families, and most of which have a large enough garden to host a bonfire) $\endgroup$ – Jon Story Nov 4 '16 at 22:44

Firewood produced 6190 BTU/lb. That's 14.4 MJ/kg. A 6" round log that is 2ft long should weigh about 10kg. That would give you 144MJ output, and probably lasts at least 4 hours, so let's say 36MJ per hour.

Sunlight produces about 1kw per square meter, at the UK's latitude that is about 600w per square meter. That's about 2.2 MJ per square meter in direct sunlight.

So if you placed a 4 meter by 4 meter grid across the entire UK and burned one of those logs in each grid square it would produce about as much heat energy as sunlight during the day. The UK is approximately 244 billion square meters so that would be about 15 billion burning logs.

With a population of 64 million, that means every man, woman, and child in the UK would have to burn 234 logs to produce that rise in temperature, say 10 degrees celsius difference between daily high/low temperature.

Say that 1/2 the population is involved, 20 people per bonfire, 20 logs per bonfire. That's 0.5 log-persons so maybe 1/400th the rise produced by sunlight in a day, or 0.025 degrees.

There can certainly be localized changes in temprature. Here in the US they use burn barrels or smudgepots in areas where the temperature rarely goes below freezing but a frost could ruin a crop.

  • $\begingroup$ And this doesn't even factor in what these people would be doing otherwise, if they would not light a bonfire? Maybe turn their heating up at home, if they were indoors. Or do other activities which would produce heat. So the difference the bonfires actually make to the activities of a normal evening is even smaller... $\endgroup$ – Falco Nov 4 '16 at 11:42
  • $\begingroup$ I like this reply and will up vote as you have taken the work value of wood and the energy requirement of the UK to raise temperature. So while the increase that you would feel is almost 0 there is a possibility that at least on some level, no matter how small, there is an increase in temperature. $\endgroup$ – user202944 Nov 4 '16 at 13:01
  • $\begingroup$ @user202944 You fart and there is on some level, no matter how small, an increase in tempurature. $\endgroup$ – Yakk Nov 4 '16 at 18:13
  • $\begingroup$ @Yakk you must know me personally! ;) $\endgroup$ – user202944 Nov 7 '16 at 12:15

I think the main effect is not the direct heat from bonfires, it is the release of particulates into the atmosphere, which then, unless it is windy, results in the condensation of a dense fog. My, perhaps mistaken, recollection is that there is almost always a fog on bonfire night unless the wind is blowing.

A fog will tend to keep the temperature at ground level higher than it would be under clear skies, since infrared radiation from the ground does not directly escape into space and radiation from low cloud heats the ground. Whether it feels warmer is a different matter, since fog increases the thermal conductivity of the air, making it feel colder.


Fires can affect temperature. Orange groves used to take advantage of this to keep fruit from being damaged on frosty nights. Pollution regulations prevent this now.

Cold air collects in low spots. So a grove in a low spot is at risk.

Smoke, being hot, tends to rise. But a smokey fire has lots of fine particles in it that increases the density. So people would light smudge pots. This prevents cold air from filling the grove. It doesn't even take a large fire.

It doesn't warm the surrounding countryside. It protects a local area from being cooled.

A comment after reading tfb's comment.

I live in Southern California. We have always had fires here, though they have only been newsworthy for the past 15 years or so.

A fire near Los Angeles can drift over San Diego. It looks like clouds, but browner. They can block sunlight. As tfb notes, they are warmer than 2.7K of space. They prevent infrared radiation from warm objects from reaching space. They radiate in the infrared and warm the ground more than space does. All of these can affect the temperature.

An effect I once found important is that smoke is full of charged particles. In the 1990's we had a large fire that left the sky full of smoke for a week. The weather was dry. For that week, static electricity was much more prevalent than usual. I was continually getting shocked just walking around. Then it rained and washed the smoke away.

At the time, I worked at a company that had computer components in a warehouse. The danger of static electricity to electronic components was not so well understood at the time. We lost 6 motherboards that week.

  • $\begingroup$ Now that is clever, so there could be a false sense of warming, not through heating of the air but by negation of the cold air coming in? $\endgroup$ – user202944 Nov 3 '16 at 14:35
  • $\begingroup$ I suspect the effect might be more subtle. Evaporating water has a significant cooling effect. Condensing water on the other hand releases heat. The smoke particles may be forming condensation nuclei. $\endgroup$ – MSalters Nov 3 '16 at 17:18
  • $\begingroup$ The most important thing smoke often does is prevent you seeing the sky. Because on a clear night you are looking at something which is at around 2.7K. That's why frosts happen on clear nights. $\endgroup$ – tfb Nov 3 '16 at 17:42
  • $\begingroup$ @MSalters - Good thought. Still, frost often only forms in the low spots where the coldest air collects. Those spots are still full of air that is much colder than at slightly higher elevation. I understand that a blizzard is measurably warmer because of condensing snow. It might have an effect in frost and smudge pots. But I still suspect it isn't a large effect. $\endgroup$ – mmesser314 Nov 3 '16 at 19:08
  • $\begingroup$ "Smoke, being hot, tends to rise." Not really, no. Smoke is a suspension of solid and liquid particles and the particles themselves move with the air mass. It's the hot gaseous combustion products (carbon dioxide, water vapour and other things) that are convecting, taking the smoke with them. $\endgroup$ – David Richerby Nov 3 '16 at 23:00

protected by Qmechanic Nov 3 '16 at 19:59

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.