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Dead Fuel Moisture

10 Hour Dead Fuel Moistures
1000 Hour Dead Fuel Moistures

The fuel moisture index is a tool that is widely used to understand the fire potential for locations across the country. Fuel moisture is a measure of the amount of water in a fuel (vegetation) available to a fire, and is expressed as a percent of the dry weight of that specific fuel. For example, if a fuel were totally dry, the fuel moisture content would be zero percent. Fuel moisture is dependent upon both environmental conditions (such as weather, local topography, and length of day) and vegetation characteristics. When fuel moisture content is high, fires do not ignite readily, or at all, because heat energy has to be used to evaporate and drive water from the plant before it can burn. When the fuel moisture content is low, fires start easily and will spread rapidly - all of the heat energy goes directly into the burning flame itself. When the fuel moisture content is less than 30 percent, that fuel is essentially considered to be dead. Dead fuels respond solely to current environmental conditions and are critical in determining fire potential. The dead fuel moisture threshold (10–hour, 100–hour, or 1,000–hour), called a time lag, is based upon how long it would take for 2/3 of the dead fuel to respond to atmospheric moisture. Small fuels (less than 1/4 inch in diameter), such as grass, leaves, and mulch respond more quickly to changes in the atmospheric moisture content, and take 10 hours to adjust to moist/dry conditions. Larger fuels lose or gain moisture less rapidly through time. Fuels that are 3 inches to 8 inches in diameter, such as dead fallen trees and brush piles can take up to 1,000 hours to adjust to moist conditions, and are represented by the 1,000–hour dead fuel moisture index. 1,000+ hour fuels do not burn easily, but if they do burn, they will generate extreme heat often causing dangerous fire behavior conditions.

Definition of the Dead Fuel Moisture Time Lag Classes
Time Lag Fuel Size Determination
10–hour 0.25 to 1 inch diameter Computed from observation time temperature, humidity, and cloudiness. Can also be an observed value, from a standard set of fuel sticks that are weighed as part of the fire weather observation.
100–hour 1 to 3 inches diameter Computed from 24–hour average conditions composed of day length, hours of rain, and daily temperature/humidity ranges.
1000–hour 3 to 8 inches diameter Computed from a 7–day average conditions composed of day length, hours of rain, and daily temperature/humidity ranges.

Check out the Wildand Fire Assessment System for additional information.