ELi5 How does forest or grassland catch fire, does it always need a spark or can drying out cause spontaneous combustion?
In: 3
Spontaneous combustion is a thing, but most fires have an ignition source. Cars, smokers, lightning, power lines, and reflections from glass buildings are pretty common.
Drying out is a big problem, particularly for plants that aren’t heat tolerant. They are easier to ignite and burn much better once lit.
Fire needs an ignition source. Lightning strikes, embers from a campfire, or even a vehicle’s exhaust system can all ignite a fire in a suitable environment.
No, just the foliage or wood getting dry does not cause spontaneous combustion. The ignition point of dry leaves or wood is something like 300°C or higher. So you need a source of heat to reach that temperature.
“Spontaneous” combustion of organic material *can* happen due to fermentation. The fermentation reaction then provides the heat source. But, somewhat paradoxically, this actually *requires* moisture, since the microbes that do the fermenting need water to survive. Also, the only scenario that I’m aware of where this actually works is in hay piles. Hay has an incredibly low ignition point, around 55°C, which is a feasible temperature to reach with fermentation, under the right circumstances (compacted hay that is not too wet to ignite, but moist enough to support fermentation). This is why, to prevent fires, it is generally recommended not to bale and store hay until it is completely dry.
As far as I know, the conditions required for this type of fermentation-induced combustion are unlikely to happen “in the wild”, though. As I said, the organic material found in forests has a much higher ignition point. You’re never going to get up to 300°C through fermentation, as the microbes will be long dead at that point. Hay of course is basically dried grass, but you do need to store it quite compactly in order to get enough concentrated heat. A field of dry grass, if it ferments at all, will be too spread out for enough heat to build up.
Lightning can spark a fire, as can campfires, cigarette butts, sparks from power lines. Arson is also a cause way too often.
Just to add to the others. A peice of broken glass left from years or evan decades earlier can catch the sun just right to cause an ember. Enough to ignite dry grass underneath
Spontaneous combustion is a little odd. It requires a moist core of organic material. Bacterial activity creates heat, but will disapate unless the heat is contained. Bacterial will not occur without moisture.. The outer organic material acts as an insulator containing the heat.. A similar event happens with rags soaked in organic oils (vegetable oil, linseed oil and the like) and is insulated when they are piled up or put in a trash can.
As humidity goes down and organic material dries out the ease of ignition increases. Autoignition temperatures are the ambient temperatures for a material to start burning without a spark. Gasoline has an autoignition temperature of 536° F. However it will catch at -45° F with a flame or spark.
A few other factors are, how thick or thin the wildland material is. I.e. it takes a lot of energy to catch a block of wood on fire. It does not take much energy to catch a piece of paper on fire. The substance of the organic material such as turpines in pine trees become more flammable as they get dryer.
A haystack, mulch pile or chip pile have started wildfires. Its rare, but it has happened. Those are all man-made and are what we see starting fires from spontaneous combustion. Its an uncommon energy source. Other energy sources are much more common, arson, lightning, accidental events like catalytic converters, camp fires, broken glass (acting like a magnifying glass) gender reveals and the like.
In other words, low moisture levels , easily ignited material and an energy source in contact with the material. Occasionally the energy source will be a pile of organic material spontaneously combusting, but rarely.
Latest Answers