Figure 15 shows the daily relationship of relative

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Figure 15 shows the daily relationship of relative humidity to fine, dead fuel moisture. With no major airmass changes, relative humidity typically rises during the night with lowering temperatures until it reaches the highest humidity just about sunrise. Relative humidity then usually starts to drop with rising temperature until the lowest humidity is reached during midafternoon. The fine, dead fuel moisture curve follows the relative humidity curve with a short time lag of about 1 hour. Before you have finished this unit, you will be able to estimate fine, dead fuel moisture content for various times of day or night, given atmospheric conditions and other site factors. Now do question 8; mark your choice or choices. You should have marked choices 1, 3, and 4. These are valid reasons for slower reaction times in surface litter. We have covered natural fuel complexes and their ranges of fuel moistures, environmental factors affecting fuel moisture, and the fuels time lag concept. We're now ready to discuss methods of determining dead fuel moistures and lead you through the steps to making your own calculations. See page 17. Here are some ways in which fuel moisture contents can be determined for each of the time lag categories. Note the following under item C: One-hour time lag fuels--fuel moisture charts, National Fire Danger Rating System nomograms, and drying oven and scales, for 10-hour time lag fuels--fuel moisture sticks, NFDR or National Fire Danger Rating System nomograms, and drying oven and scales; for 100-hour time lag fuels--NFDR System nomograms; and for 1000- hour time lag fuels--NFDR System nomograms. Determining fuel moisture percents for 100-hour and 1,000-hour time lag fuels gives managers an indication of drought conditions and overall severity of a fire season. 10-hour fuels are much more important in making fire behavior predictions than 100-hour fuels, but not nearly as
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important as 1-hour fuels. One-hour fuels are the primary carrier of the fire. We will there-fore work primarily with 1-hour time lag fuels in this course. On the following pages, you will be introduced to tables which can give you acceptable estimates of 1-hour time lag dead fuel moistures under a variety of conditions. Before leaving page 17, we want to discuss briefly how the tables are to be used. First of all, you will determine a reference fuel moisture for day or night by entering dry bulb temperature and relative humidity into a table. Next, you will determine a fuel moisture correction value from the tables by considering the month, time of day, aspect, and fuel exposure to sun or shade. The correction value is then added to the reference fuel moisture to get the adjusted dead fuel moisture. Now turn to pages 18 and 19. There are a series of nine steps that will direct you to the proper tables. You may not need to use all the steps. For example, in step 2, there are two different conditions of which you will select one. You then go on to the step under that selection. You will go only to those steps to which you are directed.
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  • Spring '04
  • MIchealJenkins
  • fuel moisture

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