Moisture of Extinction is the Fuel Moisture Above Which Fire Stops Spreading
Moisture of extinction is the dead fuel moisture content at which a fire can no longer sustain spread through a given fuel bed, because the heat needed to evaporate the water exceeds the heat the flame can deliver to the fuel ahead. It is a property of the fuel bed, not of the weather: it is a parameter of a fuel model rather than a universal number, typically around \( 12\% \) for short grass, \( 25\% \) for compact litter and \( 30\%\)–\( 40\% \) for heavy slash. Below the threshold spread rate falls smoothly as moisture rises; at the threshold it goes to zero, which is why spread predictions become unstable near it. The confusion this resolves is imagining one universal 'too wet to burn' figure — a moisture that extinguishes fire in pine litter can still carry fire in cured grass. After this Concept you can state why the same rainfall stops a fire in one fuel type and leaves another fully capable of running.
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Moisture of extinction is the dead fuel moisture content at which a fire can no longer sustain spread through a given fuel bed, because the heat needed to evaporate the water exceeds the heat the flame can deliver to the fuel ahead. It is a property of the fuel bed, not of the weather: it is a parameter of a fuel model rather than a universal number, typically around \( 12\% \) for short grass, \( 25\% \) for compact litter and \( 30\%\)–\( 40\% \) for heavy slash. Below the threshold spread rate falls smoothly as moisture rises; at the threshold it goes to zero, which is why spread predictions become unstable near it. The confusion this resolves is imagining one universal 'too wet to burn' figure — a moisture that extinguishes fire in pine litter can still carry fire in cured grass. After this Concept you can state why the same rainfall stops a fire in one fuel type and leaves another fully capable of running.
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