Unit 5: Fuel Moisture
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Natural Fuels :: Environmental Factors :: Timelag :: Dead Fuels :: Ignition & Combustion :: Exercises
Ignition Probability
Probability of ignition: A rating of probability that a firebrand (glowing) will cause a fire providing it lands on receptive fuels.
Fuel Moisture Effects on Fire Spread
- Fire spreads as a result of fuels ahead of the fire being preheated to their ignition point.
- Heat is required to drive moisture from fuels before they can support combustion.
- At some point fuel moisture content can slow combustion and the preheating of new fuels, and ignition temperature in new fuels is not reached.
- The intensity of the fire, then, determines whether moist fuels can be dried and preheated to their ignition temperatures.
Natural fuel complexes generally contain a combination of live and dead fuels. A fire passing through a fuels complex may or may not burn the live fuels.
- When live fuels are not consumed, there must be enough dead, dry fuels to support the fire.
- Some live fuels will burn although their moisture contents are 100% or higher, i.e., pine needles that have volatile substances.
Moisture of Extinction
Moisture of extinction: The fuel moisture content at which a fire will not spread, or spreads only sporadically, and in a nonpredictable manner.
The moisture of extinction varies by fuel situations:
- Moisture of extinction is dependent on various fuels characteristics, such as: fuel loading, fuel size, arrangement and chemical content.
- Moisture of extinction may be as low as 12% in certain fuel situations, but rarely higher than 30% in dead fuels.
| Fuel Model | Presence of Fuel Class | Moisture of Extinction (percent) | |||
| 1-H | 10-H | 100-H | Live | ||
| 1 Short Grass | X | 12 | |||
| 2 Timber and Grass | X | X | X | X | 15 |
| 3 Tall Grass | X | 25 | |||
| 4 Chaparral (6 feet) | X | X | X | X | 20 |
| 5 Brush (2 feet) | X | X | X | 20 | |
| 6 Intermediate Brush | X | X | X | 25 | |
| 7 Southern Rough | X | X | X | X | 40 |
| 8 Closed Timber Litter | X | X | X | 30 | |
| 9 Hardwood Litter | X | X | X | 25 | |
| 10 Timber with Litter | 25 | ||||
| 11 Light Logging Slash | X | X | X | 15 | |
| 12 Medium Logging Slash | X | X | X | 20 | |
| 13 Heavy Logging Slash | X | X | X | 25 | |
The above table gives the moisture of extinction for the 13 fire behavior fuel models. These can be used as a guide when predicting fire behavior activities. The presence or absence of fuel classes within the fuel models is also shown for your use as a reference.
