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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

  1. Fire spreads as a result of fuels ahead of the fire being preheated to their ignition point.
  2. Heat is required to drive moisture from fuels before they can support combustion.
  3. At some point fuel moisture content can slow combustion and the preheating of new fuels, and ignition temperature in new fuels is not reached.
  4. 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.

  1. When live fuels are not consumed, there must be enough dead, dry fuels to support the fire.
  2. 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:

  1. Moisture of extinction is dependent on various fuels characteristics, such as: fuel loading, fuel size, arrangement and chemical content.
  2. 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.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 07). Unit 5: Fuel Moisture. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License