Fuel moisture

Wet fuels cannot burn because water prevents it from reaching combustion temperatures even though the heat for ignition may be present. Water must first be converted to steam and driven from the fuel before the fuel can reach temperatures high enough for ignition. This absorbs a great deal of heat. For example, lightning may supply enough heat to dry and ignite a portion of a snag, but if surrounding fuels are wet enough from accompanying rain to absorb the heat from combustion, the fire will go out.

Fuel moisture content is critically important to fire behavior, determining the overall flammability of fuels. It varies between fuel types and changes seasonally, daily and hourly as it is influenced by weather and topographic factors. The drier the fuel, the less energy required to bring it to the point of ignition. Because of their various sizes and characteristics, different fuels in the same area will have different moisture levels. Light fuels such as grass take on and lose moisture much faster than heavier fuels like wood. The amount of water in a fuel is expressed as a percentage of the oven-dry weight of that fuel.