In order to understand how fuel moisture, topography, and weather can change fire behavior in a particular fuel type, it is necessary to have a good understanding of the combustion process. The first stage of combustion is the heating and evaporating stage. Initially, heat is brought into contact with a piece of wood in the presence of air. Heat causes several reactions.  First, it raises the temperature of an area on the wood surface to some depth into the wood. As the wood's surface temperature approaches 212 ° F (100 ° C), the water in the wood begins to boil, then evaporates. As long as the water remains in the wood, its evaporation robs heat energy from the source, thereby keeping the wood cells from gaining more heat (Countryman 1976). Moisture must be driven off before combustion can begin. This is why wood with high moisture content is hard to ignite. 

Further heating initiates pyrolysis, the chemical process that splits woody fuels into organic vapors and charcoal. Unlike moisture, volatile gases are combustible. They burn and release heat. As the wood surface temperature rises beyond 212°F (100°C) to about 400°F (204°C), gases abundant in creosote are produced: carbon dioxide, carbon monoxide and acetic and formic acids. However, the gases generated in the first stage of combustion do not ignite until the moisture evaporates and the kindling temperature is hot enough.

The Second Stage of combustion occurs after moisture is driven from the wood and the heat raises the temperature of the wood above 800° to 900°F. (427° to 482° C.), the second stage of combustion takes place. This is the heat-producing stage. It occurs at two different temperature levels: primary and secondary combustion.

The process by which gases are released from wood and burned is called primary combustion. Primary combustion begins at about 540° F, continues toward 900° F and results in the release of a large amount of energy. Primary combustion also releases large amounts of unburned combustible gases, including methane and methanol as well as more acid, water vapor and carbon dioxides (NFES 2394, Fire Effects Guide 2001).

Forested areas in Rhode Island will likely be classified into NFFL models 8, 9 or 10 (table 1) based on the distribution of species. These three fuel models fall into the Timber category of the NFFL fuel model system. In the timber models, fine fuels (those belonging to the 1-hour time lag class) are the controlling factor in determining the spread of fire. It is interesting to note, that in these models, if the amount of 1-hour fuels is held constant and the amount of 10 or 100-hour fuels is increased , all fire behavior outputs will decrease . This is due to the fact that in a given area a certain amount of fuel is available. This available fuel has an absolute value of heat output, which it can release during combustion. In order for larger 10 and 100-hour fuels to ignite and sustain combustion, they must be heated to 300-400 degrees C. However, any remaining moisture in these fuels must be driven off before they will sustain combustion. In this regard, larger 10 and 100-hour fuels can actually retard the fire behavior that occurs at the perimeter of the fire.

The importance of 1-hour Fuel moisture.

The diagrams depict a plan view of how an ignition source passes it heat energy to adjacent fuel cells.

High 1-hour fuel moisture (wet) click to enlarge

Low 1-hour fuel moisture (dry) click to enlarge

Moisture must be removed from fuels before they can combust. The amount of fuel moisture can greatly influence fire behavior. As a result, the fuel moisture of 1-hour time lag fuels has a large influence on fire behavior.

Video of ambient fuel moisture fuels (40%) (on left) vs oven dried fuels (on right). Time lapse video shows how high fuel moisture can impede fire spread and heat transfer. Click to download (12 Mb)