The steepness of slopes is a factor in the amount of

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The steepness of slopes is a factor in the amount of solar radiation received on various aspects, and this affects the fuel moisture content of fuels on various slopes. (See figure 11.) You should note that surfaces perpendicular to incoming radiation receive considerably more heating than slopes that are almost parallel to these heat rays. The angle at which solar radiation hits various surfaces changes throughout the day and with the time of year. The steepness or percent slope on north aspects is particularly important, as there may be times of the year when such slopes receive no direct solar heating at all. Next, we recognize wind as a factor influencing fuel moisture from the standpoint of helping fuels to reach equilibrium moisture content with the atmosphere at a faster rate. Here's how wind speeds up the drying or the evaporation process: During calm air conditions, the air next to the fuels tends to become saturated with water vapor, decreasing the evaporation rate of moisture from the fuel. Wind removes this saturated air, continually replacing it with drier air and thus speeding up the evaporation process.
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Wind can also speed up the wetting or absorption process. Moist air moving over dry fuels provides a continuous supply of moisture for fuel moisture increase. Precipitation can raise dead fuel moisture more rapidly than any other factor. (See page 12.) 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. However, there are some feelings that more rainfall can be responsible for wetting the soils in contact with fuels, thus keeping those fuels damper for a longer period and prolonging the effects of the rainfall. Heavy, dead fuels react more slowly to precipitation, since much of the rain may run off the fuel. Fuels continue to absorb moisture throughout the duration of precipitation; thus duration is more important than amount. Figure 12 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.
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  • Spring '04
  • MIchealJenkins
  • fuel moisture

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