Unit 10

Upon completion of this unit, students will be able to:

1. Define critical live fuel moisture and the thresholds for various fuel types.

2. Identify three methods for obtaining live fuel moisture.

3. Describe the relationships between relative humidity, wind, and moisture content of fine and large fuels.

4. Explain how the amount and duration of precipitation and soil moisture affect moisture content of fine and large fuels.

5. Define the fuel moisture timelag concept and its value to firefighters and fire managers.

6. Describe how fuel moisture is determined for dead fuels in each of the four timelag categories.

7. Define moisture of extinction, how it varies in natural fuel complexes, and how it affects wildland fire ignition and spread.

8. Determine fuel moisture content for fine dead 1-hour timelag fuels from fuel moisture tables during daylight conditions.

FUEL MOISTURE CONTENT FOR FINE DEAD 1-HOUR TIMELAG FUELS FROM FUEL MOISTURE TABLES DURING DAYLIGHT CONDITIONS

A. Eight inputs are needed to calculate the fine dead fuel moisture:

• Dry bulb temperature • Relative humidity

• Month • Shaded or unshaded

• Time of day • Slope

• Aspect • Site location

• All inputs can be entered on the Fine Dead Fuel Moisture worksheet.

• The temperature and relative humidity are necessary to determine the reference fuel moisture.

• The other inputs are necessary to determine the fuel moisture correction.

1. Reference Fuel Moisture Table (Table 2)

• Determine a reference fuel moisture (RFM) by selecting the appropriate dry bulb temperature and relative humidity from Table 2.

• Table 2 is for RFM during daytime hours. You must know both dry bulb temperature and relative humidity from your site.

• Notice the ranges of temperatures on the left, and ranges of humidities across the top.

• With the appropriate temperature range, move horizontally until you intersect with the column for the appropriate humidity ranges at the top.

• At this intersection you have a RFM content percent.

2. Fine Dead Fuel Moisture Content Corrections For Day

• Determine a fuel moisture correction (FMC) value from the tables by considering the:

• Month

• Cloud and/or canopy cover shading

• Time of day

• Site location elevation difference

• Aspect

• Slope percent

• The correction value is then added to the RFM to get the adjusted fine dead fuel moisture (FDFM).

• Table 3 gives you correction values for the months of May, June, and July; this table is only valid for those three months.

  • Notice that there are two sections to the table.

• The top section is for unshaded surface fuels, while the bottom part is for shaded surface fuels.

• After making the proper selections of aspect and slope on the left, and time of day from the top, you will find FMC value at the point of intersection.

• Fine Dead Fuel Moisture Correction Tables 4 and 5 are used for the months of February, March, April/August, September, October, and November, December, January, respectively.

• Make sure you are using the correct monthly table.

3. Site Location

There is one other input that warrants additional explanation – site location or elevation change on your worksheet.

• This is based roughly on the lapse rate, which is approximately 3.5° F per 1,000 under average dry conditions.

• If the atmosphere is very unstable, we learned that the lapse rate, or change in temperature over elevation, could be as much as 5.5° F per 1,000, or even more.

This comes into play as we consider the location of where we are taking our weather readings relative to where the fire is.

• If the fire is either 1,000 feet above or below the location where the weather is taken, we do not need to make any adjustments; it is presumed that the temperature within that 2,000 feet will be within an acceptable variation.

• If the fire is greater than 1,000 feet and less than 2,000 feet either above or below us, we can and do need to make an adjustment.

Elevations greater than 2,000 feet above or below the predicted site will require a new temperature and relative humidity reading.

• If the fire is either 1,000 feet above or below the location where the weather is taken, then we use the L column from the table. 

• If the fire is greater than 1,000 feet and less than 2,000 Above us, we use the values in the A column from the table. 

• If the fire is greater than 1,000 feet and less than 2,000 Below us, we use the values in the B column from the table.

Use the fine dead fuel moisture calculation tables and worksheet to answer the questions below.

1. What is the reference fuel moisture (RFM) for the following daytime situations?

a. Temperature 85 °F, relative humidity 22 percent

RFM _____

b. Temperature 60 °F, relative humidity 62 percent

RFM _____

2. What are the fuel moisture correction (FMC) values for the following situations?

a. August 20, at 1200, mid-slope location, east aspect, and cloudy sky (shaded fuels)

FMC _____

b. May 10, at 1400, south aspect, 20 percent slope, clear with unshaded fuels.

FMC _____

3. What is the fine dead (1-hour) fuel moisture (FDFM) content at the following fire locations? Record your solution on the fine dead fuel moisture worksheet.

Read the next two situations carefully!

a. It is November 19, at 1500. Fuels are exposed to the sun on a west aspect. Readings from a belt weather kit taken 1,500 feet below the fire give a dry bulb temperature of 92 °F, and a relative humidity of 16 percent. The slope is 40 percent.

RFM _____ + FMC _____ = FDFM ______

b. It is October 12, at 1700. Fuels are shaded on a north aspect, but under clear skies. Readings from a belt weather kit taken 1,400 feet above the fire give a dry bulb temperature of 75 °F, and a relative humidity of 28 percent. The slope is 20 percent.

RFM _____ + FMC _____ = FDFM _____