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Unit 5: Fuel Moisture

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Natural Fuels   ::  Environmental Factors  ::   Timelag   ::   Dead Fuels   ::   Ignition & Combustion   ::   Exercises

Environmental Factors Affecting Dead Fuel Moisture

Fuel Moisture Exchange With The Atmosphere

Fuels are constantly exchanging moisture with the surrounding air. During periods of high humidity and precipitation there is a net gain in fuel moisture. However, when the air is dry, with low humidity, fuels are giving up more moisture to the air than they receive. Several factors influence the rate of moisture exchange between fuels and the air.

If the moisture content in the atmosphere remained constant for a period of time, the fuels and the air would eventually achieve equal vapor pressures. This we call equilibrium moisture content, which occurs when there is no net gain or loss of moisture between fuels and the surrounding air. This can occur in small, fine fuels, but rarely occurs in larger fuels, as the time required to reach equilibrium in larger fuels is much longer.

Moisture Exchange Between Fuels and Air

The net rate of moisture exchange between fuels and air depends on:

  1. Difference in water vapor pressure between each.
  2. Presence or absence of wind.
  3. Size of fuels.
  4. Compactness of fuels.
  5. Proximity of fuels to damp soil.

Note that fuel moisture is directly influenced by temperature, relative humidity, and precipitation. Wind helps to speed up the exchange of moisture between the fuels and the air.

Other site factors of weather and topography influence atmospheric temperatures and relative humidity. Each of these site factors indirectly affects fuel moisture and must be considered in making estimations of fuel moisture content.

Equilibrium moisture content occurs when there is no net gain or loss of moisture between fuels and the surrounding air

Shade vs. Sunlight
Shade vs. sunlight effects on fuel temperatures

This figure illustrates how shade versus sunlight affects fuel temperatures. During sunny daylight hours, temperatures at the earth's surface can reach 160° F. That temperature decreases very rapidly a few feet above the surface where air is mixing. At 5 feet above the surface, the air temperature may be 85° as observed in a weather instrument shelter. Relative humidity is much lower where temperatures reach 160°; thus, in this example, fine fuel moisture at the surface will be considerably lower--3 percent in the open, exposed area, as opposed to 8 percent in a shaded area.

Comparison of Fuel Moistures on Various Aspects


  1. Fuel moisture is generally lower on south aspect slopes.
  2. East aspects reach their lowest moisture contents by late morning.
  3. Southwest aspects have the lowest afternoon fuel moisture contents.

Wind Speeds Up Both the Drying and Wetting Processes in Fuels

Wind speeds up the evaporation process

During calm air conditions, the air near the fuels tends to become saturated with water vapor, decreasing the evaporation rate of moisture from the fuel. Wind removes this saturated air, continually replaces it with drier air, and thus speeds up the evaporation process.

Wind can speed up the absorption process

Moist air blowing over dry fuels provides a continuous supply of moisture for fuel moisture increase.

Size of Fuels Affects Their Ability to Absorb Precipitation

Precipitation can raise dead fuel moisture more rapidly than any other factor. Both the amount and duration of the precipitation are considerations when predicting fuel moisture increases in various size fuels.

Fine, Dead Fuels :
React very rapidly to precipitation and reach their saturation points quickly. Additional rainfall has little effect on the fuels.

Heavy, Dead Fuels :
React slower to precipitation as much of the rain may run off the fuel. Fuels continue to absorb moisture throughout the duration, thus duration is more important than the amount.

Precipitation and Moisture
Duration of precipitation and fuel moisture

The above figure illustrates the effects of duration of precipitation on fuels of three size classes. The horizontal axis represents hours of continuous precipitation, while the vertical axis is fuel moisture content in percents. The dashed line representing 1-hour time lag fuels starts at 5 percent, rises rapidly, and reaches 30 percent moisture content within the first hour. The broken diagonal line representing 10-hour time lag fuels starts at 8 percent and increases at a slower rate, but reaches 30 percent moisture content after 6 hours. The solid line which represents 100-hour fuels starts at 12 percent and only reaches 20 percent after 16 hours of continuous precipitation. The data used to prepare this chart represent average western fuel situations with standing and down, dead fuels.

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