8 ms·

I mean, cold itself (within Earthly limits) doesn't put out fires. In fact, it might make them burn better due to convection. That said, this reminded me of a

by bserge 6y ago

I mean, cold itself (within Earthly limits) doesn't put out fires. In fact, it might make them burn better due to convection. That said, this reminded me of a lake in Chelyabinsk that is so polluted you can set it on fire according to some locals (I don't remember the video, it was a few Russians filming a short tour of the city)

astrea 6y ago
> I mean, cold itself (within Earthly limits) doesn't put out fires. However, heat is very much a part of the fire triangle.
- bserge 6y ago
  But any fire is magnitudes hotter than -50. That cold is a blip on the fire's radar. Once started, and as long as there's fuel and oxygen, it won't stop because of negative Celsius temperatures.
  - foobarian 6y ago
    My uncle used to joke that his car ran better in the mountains because the air was cleaner and thus had more oxygen. He was not serious, but in case of cold air there would be more oxygen per unit volume due to density so that might improve the combustion.
    - munk-a 6y ago
      The air pressure is also higher at colder temperatures so if the fire is underground it'd have more forces working in its favor to cycle fresh air to it and the relative temperature difference between the freshly heated and exhausted air and the fresh air would be wider causing there to be more turbulence cycling the air around.
    - pletnes 6y ago
      Hence the invention of the intercooler diesel engine.
      - llampx 6y ago
        The turbocharger is what helps at high altitudes, not the intercooler which just cools the air. The turbo packs more air in to the engine, which helps at sea level but makes a big difference at high altitude. Superchargers do the same.
    - deleted 6y ago
      [deleted]
    - klodolph 6y ago
      Heat engines generate power from a difference in temperature. The larger the temperature difference, the more power. You can increase the temperature difference by making the heat source hotter or by making the heat sink colder.
    - mikestew 6y ago
      Ignoring, of course, the fact that "in the mountains" means an increase in altitude and thus a decrease in air density. My carbureted vehicles that were jetted for sea level always ran like shit in the mountains. And fuel injected vehicles run fine (because they can adjust for the altitude on the fly), but with noticeably less power. Apologies for pedantically deconstructing what your uncle meant as a smart-assed joke.
      - bigiain 6y ago
        Lower air density means old carburetted engines run leaner - which since most such engines are tuned to run slightly rich for reliability in favour of slightly lean, gives you more power up until the point they overheat and if they're a 2 stroke motor relying on the oil mixture in for lubrication the fuel, seize. One of the old traditional racetracks here in Australia is Mt Panorama at Bathurst. It's got about a 180m elevation change between the start/finish/pit straight and the top of the mountain. Back when they were allowed to race motorcycles there, old 2 stroke racebikes were notorious for seizing at the top of the hill if they'd been jetted down at the bottom and hadn't been set "rich enough" to allow for the altitude change. Many many riders crashed and said "It was running really strong, til it locked up on me and spat me down the road..." (No apologies at all for my off topic response to pedantry...)
        mikestew 6y ago
        Mmm, pretty sure sea level carbs run rich at altitude. Less air means a need for less fuel for the same ratio. But the fuel side didn’t change because we didn’t rejet, so now too much gets pumped in. If I’m guessing the bikes might have seized at the bottom of the hill instead? Or it just took until the top before the damage was done.
        bigiain 6y ago
        Ahhh, yeah - you're right. I've got it back to front there. In my defence it's been a long time singe they've been allowed to race motorcycles there. At least 20 years. And now you mention it I do recall bikes seizing at the end of pit straight, right down the bottom of the hill.
  - Retric 6y ago
    It can significantly slow down wild fires which need to heat a constant supply of fuel to ignition temperatures. Wood burns at 180 degrees Celsius, so the difference between 20c and -50c is +160C vs +230C or over 40% more heat. This is especially impactful when looking at how easily sparks propagate fires across gaps.
    - jameshart 6y ago
      A typical wildfire is not putting much of its overall energy output into bringing fuel up to combustion temperature, though. The combustion energy in a kilo of wood is measured in megajoules; the amount of energy needed to elevate the temperature of a kilo of wood by a degree is measured in kilojoules. So while you're right that the energy budget needed to combust new fuel goes up by 40% or so if the fuel is at -50°C, the energy budget going into that is only a percent or so fo the total energy the wildfire is putting out. The rest of the energy beyond what's needed to bring fuel up to combustion point is going into making massive walls of glowing flames, mile-high convection columns of air, sending smoke thousands of miles, etc., etc... That is, in many ways, precisely what makes it a wildfire, rather than a smouldering ember. In the limit - when a fire is dying out - then I guess, for sure, changing the fuel temperature can sap a larger part of the energy budget.
      - Retric 6y ago
        The point is propagation speed. A burning tree surrounded by acres of burning trees is largely irrelevant at that point. It might happen to produce spark that travels miles, but that’s relatively rare. Which is why firebreaks can be useful.
  - why_Mr_Anderson 6y ago
    It's actually one way of putting out otherwise very persistent fires, especially the ones that like to only smoulder (really nasty chemicals, metal compounds, etc.) Removing heat from the reaction lowers the ignition temperate of the surrounding material to a point that fire dies out on its own. Side note: it doesn't work that well on burning plutonium.
- jonshariat 6y ago
  I think heat is meant to represent the ignition source. The reaction creates heat which then disapates into cold, right? If this were to happen in absolute 0, would it be possible to sustain a fire? From learning more (just now) about absolute 0 seems it wouldn't but for other reason. Can someone answer this, I find it a really fascinating question. How does cold effect fire if at all?
  - asdfasgasdgasdg 6y ago
    Cold affects the speed at which molecules are moving around. Fire is a phenomenon produced generally by exothermic chemical reactions between molecules. The part of the fire you see is, if I understand correctly, the photons emitted by excited electrons moving to a lower energy state after the reaction. All other things being equal, the colder it is, the less the candidate molecules are moving, and thus the less likely they are to encounter each other and react. IIUC almost all chemical reactions are less vigorous at lower temperatures, disallowing convection of course.
  - cozzyd 6y ago
    At absolute zero, ambient oxygen would solidify. Not sure what happens if you drop a match on a bunch of solid oxygen, but I imagine it's not good.
    - TylerE 6y ago
      Truly pure oxygen won't burn, it needs fuel. So, what would probably happen is it would burn super bright for a split second as the match itself was consumed, and then extinguish itself as it has no more fuel.
    - ashtonkem 6y ago
      It would go out. In a room cold enough for Oxygen to turn solid, there would be no gaseous oxygen to sustain the match’s combustion as it falls. The wood would have some latent heat, but not enough to re-ignite itself in a room that cold.
      - 6y ago
monadic3 6y ago
> I mean, cold itself (within Earthly limits) doesn't put out fires This is not a meaningful phrase when cold is the absence of heat. You need to describe the motion of the cold air against the warmer system to get a meaningful phrase. Being able to set a river on fire is not terribly unusual for a polluted river. I remember about ten years ago I posted up at the schuylkill, waited for an oily patch, and was able to set it on fire for about 5 seconds. These oily patches are uncommon. I have no clue where the oil came from but with the schuylkill it's anyone's guess. The patch itself is necessary for lighting the river on fire; you can't just set fire to an arbitrary section of river.
vaughnegut 6y ago
The river running through Cleveland used to be so polluted that it caught fire at least a dozen times.[1] [1] https://www.smithsonianmag.com/history/cuyahoga-river-caught-fire-least-dozen-times-no-one-cared-until-1969-180972444/ https://www.smithsonianmag.com/history/cuyahoga-river-caught...
- xxpor 6y ago
  And provided the spark that caused the Clean Water Act to pass. Impossible to imagine something similar today.
  - mdturnerphys 6y ago
    Maybe in the US, but I've seen some horribly dirty waterways in other countries.
  - sholladay 6y ago
    Unless you live in Flint. Then nothing is impossible to imagine. https://en.wikipedia.org/wiki/Flint_water_crisis https://en.wikipedia.org/wiki/Flint_water_crisis
    - xxpor 6y ago
      Sorry that was ambiguously worded. My point was that nothing like the CWA was even floated afterwards, since everyone knows the politics of today wouldn't allow it.
- lostlogin 6y ago
  I wonder if this is how Terry Pratchett was inspired, as the Ankh sometimes caught fire. https://discworld.fandom.com/wiki/Great_Fire_of_Ankh-Morpork https://discworld.fandom.com/wiki/Great_Fire_of_Ankh-Morpork
Zenst 6y ago
>I mean, cold itself (within Earthly limits) doesn't put out fires. In fact, it might make them burn better due to convection. Colder air will mean denser air and with that is more oxygen, so yes that is plausible.
- hansvm 6y ago
  Also less humid air
  - gpm 6y ago
    Does water already in vapor form really impact fires?
    - maksum 6y ago
      It appears humidity doesn't really impact an already burning wildfire, but lower humidity means drier fuel which makes the fire more likely to spread. Conversely, higher humidity can make a fire less likely to spread to moister fuel[1] [1]http://www.auburn.edu/academic/forestry_wildlife/fire/relative_humidity.htm http://www.auburn.edu/academic/forestry_wildlife/fire/relati...
      - dredmorbius 6y ago
        Peat itself is often fairly damp.
ivanhoe 6y ago
Fires are less likely to ignite in very cold weather as the environment is taking energy out of the reaction. Once it gets going cold doesn't matter much.
- ashtonkem 6y ago
  To a running fire, everything is cold.
  - number6 6y ago
    Poetic. I love it.
- cratermoon 6y ago
  https://americanenglish.state.gov/files/ae/resource_files/to-build-a-fire.pdf https://americanenglish.state.gov/files/ae/resource_files/to...
  - ivanhoe 6y ago
    Wow, this brings memories, I remember reading this story when I was around 9-10 y/o. Is it from the Tales of the North?
lmilcin 6y ago
Things that are deemed to be fuels emit way more energy than energy needed to heat air from ANY temperature. The fire would continue even if the air was liquid.

by bserge 6y ago

astrea 6y ago

> I mean, cold itself (within Earthly limits) doesn't put out fires. However, heat is very much a part of the fire triangle.

bserge 6y ago
But any fire is magnitudes hotter than -50. That cold is a blip on the fire's radar. Once started, and as long as there's fuel and oxygen, it won't stop because of negative Celsius temperatures.
- foobarian 6y ago
  My uncle used to joke that his car ran better in the mountains because the air was cleaner and thus had more oxygen. He was not serious, but in case of cold air there would be more oxygen per unit volume due to density so that might improve the combustion.
  - munk-a 6y ago
    The air pressure is also higher at colder temperatures so if the fire is underground it'd have more forces working in its favor to cycle fresh air to it and the relative temperature difference between the freshly heated and exhausted air and the fresh air would be wider causing there to be more turbulence cycling the air around.
  - pletnes 6y ago
    Hence the invention of the intercooler diesel engine.
    - llampx 6y ago
      The turbocharger is what helps at high altitudes, not the intercooler which just cools the air. The turbo packs more air in to the engine, which helps at sea level but makes a big difference at high altitude. Superchargers do the same.
  - deleted 6y ago
    [deleted]
  - klodolph 6y ago
    Heat engines generate power from a difference in temperature. The larger the temperature difference, the more power. You can increase the temperature difference by making the heat source hotter or by making the heat sink colder.
  - mikestew 6y ago
    Ignoring, of course, the fact that "in the mountains" means an increase in altitude and thus a decrease in air density. My carbureted vehicles that were jetted for sea level always ran like shit in the mountains. And fuel injected vehicles run fine (because they can adjust for the altitude on the fly), but with noticeably less power. Apologies for pedantically deconstructing what your uncle meant as a smart-assed joke.
    - bigiain 6y ago
      Lower air density means old carburetted engines run leaner - which since most such engines are tuned to run slightly rich for reliability in favour of slightly lean, gives you more power up until the point they overheat and if they're a 2 stroke motor relying on the oil mixture in for lubrication the fuel, seize. One of the old traditional racetracks here in Australia is Mt Panorama at Bathurst. It's got about a 180m elevation change between the start/finish/pit straight and the top of the mountain. Back when they were allowed to race motorcycles there, old 2 stroke racebikes were notorious for seizing at the top of the hill if they'd been jetted down at the bottom and hadn't been set "rich enough" to allow for the altitude change. Many many riders crashed and said "It was running really strong, til it locked up on me and spat me down the road..." (No apologies at all for my off topic response to pedantry...)
      - mikestew 6y ago
        Mmm, pretty sure sea level carbs run rich at altitude. Less air means a need for less fuel for the same ratio. But the fuel side didn’t change because we didn’t rejet, so now too much gets pumped in. If I’m guessing the bikes might have seized at the bottom of the hill instead? Or it just took until the top before the damage was done.
        bigiain 6y ago
        Ahhh, yeah - you're right. I've got it back to front there. In my defence it's been a long time singe they've been allowed to race motorcycles there. At least 20 years. And now you mention it I do recall bikes seizing at the end of pit straight, right down the bottom of the hill.
- Retric 6y ago
  It can significantly slow down wild fires which need to heat a constant supply of fuel to ignition temperatures. Wood burns at 180 degrees Celsius, so the difference between 20c and -50c is +160C vs +230C or over 40% more heat. This is especially impactful when looking at how easily sparks propagate fires across gaps.
  - jameshart 6y ago
    A typical wildfire is not putting much of its overall energy output into bringing fuel up to combustion temperature, though. The combustion energy in a kilo of wood is measured in megajoules; the amount of energy needed to elevate the temperature of a kilo of wood by a degree is measured in kilojoules. So while you're right that the energy budget needed to combust new fuel goes up by 40% or so if the fuel is at -50°C, the energy budget going into that is only a percent or so fo the total energy the wildfire is putting out. The rest of the energy beyond what's needed to bring fuel up to combustion point is going into making massive walls of glowing flames, mile-high convection columns of air, sending smoke thousands of miles, etc., etc... That is, in many ways, precisely what makes it a wildfire, rather than a smouldering ember. In the limit - when a fire is dying out - then I guess, for sure, changing the fuel temperature can sap a larger part of the energy budget.
    - Retric 6y ago
      The point is propagation speed. A burning tree surrounded by acres of burning trees is largely irrelevant at that point. It might happen to produce spark that travels miles, but that’s relatively rare. Which is why firebreaks can be useful.
- why_Mr_Anderson 6y ago
  It's actually one way of putting out otherwise very persistent fires, especially the ones that like to only smoulder (really nasty chemicals, metal compounds, etc.) Removing heat from the reaction lowers the ignition temperate of the surrounding material to a point that fire dies out on its own. Side note: it doesn't work that well on burning plutonium.
jonshariat 6y ago
I think heat is meant to represent the ignition source. The reaction creates heat which then disapates into cold, right? If this were to happen in absolute 0, would it be possible to sustain a fire? From learning more (just now) about absolute 0 seems it wouldn't but for other reason. Can someone answer this, I find it a really fascinating question. How does cold effect fire if at all?
- asdfasgasdgasdg 6y ago
  Cold affects the speed at which molecules are moving around. Fire is a phenomenon produced generally by exothermic chemical reactions between molecules. The part of the fire you see is, if I understand correctly, the photons emitted by excited electrons moving to a lower energy state after the reaction. All other things being equal, the colder it is, the less the candidate molecules are moving, and thus the less likely they are to encounter each other and react. IIUC almost all chemical reactions are less vigorous at lower temperatures, disallowing convection of course.
- cozzyd 6y ago
  At absolute zero, ambient oxygen would solidify. Not sure what happens if you drop a match on a bunch of solid oxygen, but I imagine it's not good.
  - TylerE 6y ago
    Truly pure oxygen won't burn, it needs fuel. So, what would probably happen is it would burn super bright for a split second as the match itself was consumed, and then extinguish itself as it has no more fuel.
  - ashtonkem 6y ago
    It would go out. In a room cold enough for Oxygen to turn solid, there would be no gaseous oxygen to sustain the match’s combustion as it falls. The wood would have some latent heat, but not enough to re-ignite itself in a room that cold.
    - 6y ago

monadic3 6y ago

> I mean, cold itself (within Earthly limits) doesn't put out fires This is not a meaningful phrase when cold is the absence of heat. You need to describe the motion of the cold air against the warmer system to get a meaningful phrase. Being able to set a river on fire is not terribly unusual for a polluted river. I remember about ten years ago I posted up at the schuylkill, waited for an oily patch, and was able to set it on fire for about 5 seconds. These oily patches are uncommon. I have no clue where the oil came from but with the schuylkill it's anyone's guess. The patch itself is necessary for lighting the river on fire; you can't just set fire to an arbitrary section of river.

vaughnegut 6y ago

The river running through Cleveland used to be so polluted that it caught fire at least a dozen times.[1] [1] https://www.smithsonianmag.com/history/cuyahoga-river-caught-fire-least-dozen-times-no-one-cared-until-1969-180972444/ https://www.smithsonianmag.com/history/cuyahoga-river-caught...

xxpor 6y ago
And provided the spark that caused the Clean Water Act to pass. Impossible to imagine something similar today.
- mdturnerphys 6y ago
  Maybe in the US, but I've seen some horribly dirty waterways in other countries.
- sholladay 6y ago
  Unless you live in Flint. Then nothing is impossible to imagine. https://en.wikipedia.org/wiki/Flint_water_crisis https://en.wikipedia.org/wiki/Flint_water_crisis
  - xxpor 6y ago
    Sorry that was ambiguously worded. My point was that nothing like the CWA was even floated afterwards, since everyone knows the politics of today wouldn't allow it.
lostlogin 6y ago
I wonder if this is how Terry Pratchett was inspired, as the Ankh sometimes caught fire. https://discworld.fandom.com/wiki/Great_Fire_of_Ankh-Morpork https://discworld.fandom.com/wiki/Great_Fire_of_Ankh-Morpork

Zenst 6y ago

>I mean, cold itself (within Earthly limits) doesn't put out fires. In fact, it might make them burn better due to convection. Colder air will mean denser air and with that is more oxygen, so yes that is plausible.

hansvm 6y ago
Also less humid air
- gpm 6y ago
  Does water already in vapor form really impact fires?
  - maksum 6y ago
    It appears humidity doesn't really impact an already burning wildfire, but lower humidity means drier fuel which makes the fire more likely to spread. Conversely, higher humidity can make a fire less likely to spread to moister fuel[1] [1]http://www.auburn.edu/academic/forestry_wildlife/fire/relative_humidity.htm http://www.auburn.edu/academic/forestry_wildlife/fire/relati...
    - dredmorbius 6y ago
      Peat itself is often fairly damp.

ivanhoe 6y ago

Fires are less likely to ignite in very cold weather as the environment is taking energy out of the reaction. Once it gets going cold doesn't matter much.

ashtonkem 6y ago
To a running fire, everything is cold.
- number6 6y ago
  Poetic. I love it.
cratermoon 6y ago
https://americanenglish.state.gov/files/ae/resource_files/to-build-a-fire.pdf https://americanenglish.state.gov/files/ae/resource_files/to...
- ivanhoe 6y ago
  Wow, this brings memories, I remember reading this story when I was around 9-10 y/o. Is it from the Tales of the North?

lmilcin 6y ago

Things that are deemed to be fuels emit way more energy than energy needed to heat air from ANY temperature. The fire would continue even if the air was liquid.