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Table of Contents:

  1. Introduction to Field Guide
  2. Fire Behavior Fuel Model Key
  3. Relative Humidity Computations
  4. Using Dead Fuel Moisure Tables (1-Hour Timelag)
  5. Wind Measurements and Calculations
  6. Calculating Slope Percent
  7. Using the Worksheet and Fire Behavior Tables
  8. Determining Fire Size for Point Source Fires
  9. Fire Behavior Interpretations

Introduction to The Field Guide

This field guide was developed for use with the S-390 Fire Behavior Course. It is designed to give you a quick and easy reference for Unit XI, the final examination and for future field use.
The guide is divided into eight sections, and contains all the tables, formulas, keys, procedures and other aids as introduced in the S-390 units for performing a fire behavior analysis. This includes procedures for determining input values of fuel model, fuel moisture, midflame windspeed, and percent slope. It presents a system of worksheet, tables and formulas to perform calculations of fire behavior. This system allows you to process input data using the mathematical equations in the fire behavior prediction model as developed by the Northern Forest Fire Laboratory at Missoula, Montana.
Calculations using the tables should be the same as those made with the Texas Instrument Model 59 computer-calculator with the fire behavior program chip, or with the fire behavior nomograms. The tables are generally much easier to use than the nomograms, and give most of the output values available from the TI-59. Output values from the tables are generally close to those obtained from the TI--59.
A worksheet for predicting fire behavior is presented on page 4. It has 36 lines for entries. Two columns allow two sets of calculations on the same sheet. The first portion of the worksheet requires site and other input data needed for calculations. The second portion takes you through the fire behavior calculations and gives you the output values. References or instructions are made in many of the lines as to field guide sections, tables, calculations, or repeat entries. The sequence of.entries was designed to permit forward progression through the field guide when completing the worksheet.
One column of the worksheet on page 4 is completed to provide an example of entries to be made. See the steps to completing the worksheet in sections 6 and 7. There are blank worksheets in the back of the guide suitable for reproduction.


Fire Behavior Fuel Model Key

  1. Primary carrier of fire is grass. Expected rate of spread is moderate to high, with low to moderate intensity.
    1. Grass has a relatively fine structure, is generally below knee level and is easy to walk through. Model 1
    2. Grass has thick, coarse stems, is above knee level, and is difficult to walk through. Model 3
    3. Mixture of grass and litter beneath open timber or brush overstory that does not burn. Model 2

    Fuel Model Descriptions - Grass Group (PDF)

  2. Primary carrier of fire is brush. Expected rate of spread and intensity are both moderate.
    1. Vegetation type is southern rough or low pocosin. Model 7
    2. Live fuels absent or sparse with no capability to reduce fire spread rate. Model 6
    3. Live fuel moisture can have a significant damping effect on the fire behavior.
      1. Brush is about knee deep with a light loading of 1-H TL fuels. Model 5
      2. Brush is close to head high with a heavy loading of 1-H TL fuels. Model 4

    Fuel Model Descriptions - Shrub Group (PDF)

  3. Primary carrier of fire is debris beneath a timber stand.
    1. Live fuels are present in sufficient quantity to influence fire behavior. The load of 100-H TL fuels is heavy. Model 10
    2. Surface fuels are mostly foliage litter, with little or no live fuel.
      1. 1-H TL load strongly predominates; and 100-H TL fuels are sparse. Foliage litter is long needle pine or hardwood leaves, loosely compacted. Model 9
      2. 1-H and 10-H TL fuel load combined is about equal to 100-H TL load. Foliage litter is short needle coniferous or small hardwood leaves, tightly compacted. Model 5

    Fuel Model Descriptions - Timber Group (PDF)

  4. Primary carrier of fire is slash.
    1. Slash is not continuous. Other ground fuels must be present to help carry the fire. Average slash depth is about 1 foot. Model 11
    2. Slash is continuous or nearly so. Other surface fuels need not be present to carry the fire. Average slash depth is about 3 feet. Model 13
    3. Slash generally covers the ground, though there may be bare spots or areas of light coverage. Average slash depth is about 2 feet. Model 12

    Fuel Model Descriptions - Slash Group (PDF)



Relative Humidity Computations

  1. Determining relative humidity and dew point temperature from a psychrometric table:
    1. Select the proper table for your elevation.
    2. Obtain dry and wet bulb temperature readings from a psychrometer.
    3. Find the wet bulb temperature at top of table, then move down that column.
    4. Find the dry bulb temperature at left side of table, then move horizontally to right.
    5. Find the intersection of wet bulb column and dry bulb line, which is a block with two numbers.
    6. The lower number in block is relative humidity, the upper is dew point.
  2. Using dew point temperature to determine relative humidity at various temperatures within a fixed or stationary air mass.
    1. Locate wet and dry bulb temperature readings on psychrometric table.
    2. At intersection of these entries, determine the relative humidity and dew point.
    3. Although relative humidity will change with changes in temperature, the dew point will remain the same.
    4. Select a new dry bulb temperature and locate its line in the table.
    5. Move horizontally to the right until you find the block with the same dew point as 2 above.
    6. The relative humidity number in the block with that dew point is your new relative humidity.
  3. Determining temperature and relative humidity for a projection site above or below the observation site:
    1. If projection site is above the observation site, subtract the lapse rate* per 1000 feet of rise.
    2. If projection site is below the observation site, add the lapse rate* per 1000 feet of drop.
    3. Determine relative humidity for projection site using the dew point procedure as in B. on previous page.

    *Use 3.5°/1000 feet for a stationary air mass (usual), and 5.5° / 1000 feet for moving air mass (i.e., foehn)

Table 2A: Pressure 30 Inches of Mercury (PDF) (Elevations 0 - 500 feet above sea level)
Table 2B: Pressure 29 Inches of Mercury (PDF) (Elevations 501 - 1900 feet above sea level)
Table 2C: Pressure 27 Inches of Mercury (PDF) (Elevations 1901 - 3900 feet above sea level)
Table 2D: Pressure 25 Inches of Mercury (PDF) (Elevations 3901 - 6100 feet above sea level)
Table 2E: Pressure 23 Inches of Mercury (PDF) (Elevations 6101 - 8500 feet above sea level)


Using Dead Fuel Moisture Tables (1-Hour Timelag)

Step 1 - Obtain the following site data for determining dead fuel moisture:

  • Time of day
  • Month
  • Dry bulb temperature
  • Relative Humidity
  • Aspect (N, S, E, W, Level)
  • Exposure of fine dead fuels to sun or not
  • Go to Step 2

Step 2 - Consider location/elevations for predictions:

  1. Predictions for same site as data gathered. Go to Step 3
  2. Predictions for site above or below the data site (daytime only with no inversions). Go to Step 7

Step 3 - Consider day or night conditions:

  1. Daylight hours (0800 - 1959) Go to Step 4
  2. Nighttime hours (2000 - 0759) Go to Step 9

Step 4 - Determine reference fuel moisture from dry bulb temperature and relative humidity using table 3a.

Go to Step 5

Step 5 - Consider exposed or shaded fuel conditions:

  1. At least 50% of fine dead fuels are exposed to sun. Use "Clear and/or no canopy" portions on tables 3B - 3D.
  2. At least 50% of fine dead fuels are shaded from clouds or canopy.


Step 6 - Determine fuel moisture correction value considering month, time of day, and aspect:

  1. May, June, July--use table 3B.
  2. February, March, April/August, September, October-use table 3C.
  3. November, December, January--use table 3D.

Go to Step 8

Step 7 - Determine dead fuel moisture content for prediction sites above or below data site on same slope (daytime only with no inversions). Follow these substeps:

  1. Determine elevation difference between data site and prediction site.
  2. Calculate new temperature for prediction site using usual lapse rate of 3.5° Fahrenheit per 1000 feet. See unit IV, page 8.
  3. Determine relative humidity for prediction site by following procedures (using dew point) as given in unit IV, page 18.
  4. Calculate dead fuel moisture content by using new dry bulb temperature and relative humidity for prediction site.

Go to Step 4

Step 8 - Determine dead fuel moisture content (daytime) by adding correction value (step 6) to reference fuel moisture (step 4).


Step 9 - Determine dead fuel moisture content (nighttime) from dry bulb temperature and relative humidity using table 3e.


Tables 3A, 3B, 3C, 3D, 3E, 3F, 3G (PDF)


Wind Measurements and Calculations

  1. Standard wind measurements are taken on fires at:
    1. The 20-foot level using an anemometer (taken in a clearing or 20 feet above all vegetation.
    2. Eye level using a wind meter or a hand held anemometer.
  2. Fire behavior calculations require midflame windspeed values.
    1. The 20-foot wind values from forecasts or measurements can be adjusted to midflame windspeeds.
    2. Eye level wind measurements can generally be used for midflame windspeeds.
  3. The wind (correction) factor is used as an input for calculations of:
    1. Adjusted rate of spread (see section 6).
    2. Adjusted fireline intensity (see section 6).
    3. Effective windspeed (see sections 6 and 7).

Midflame windspeed is affected by fuels overstory and terrain (PDF)
Table 4A: Wind Adjustment Factors (PDF)
Table 4B: Correction Factors fo Wind (PDF)


Calculating Slope Percent

Aids for determining slope percent from topographic maps:

Slope Indicator , which matches contour lines on a USGS 7-1/2 minute quadrangle map.
Contour Tables , which are for USGS 15 minute and 7-1/2 minute quadrangle.

Formula 1: Use on any contour map or in the field.
  Slope percent =
rise in elevation
X 100
horizontal distance
Formula 2: Use on any contour map.
  Slope percent =
contour interval X no. of contours
X 100
horizontal distance

Steps to determining slope percents using formula 1:

  1. What are the contour intervals on the map (feet)?
  2. What is the map scale (inches per mile)?
  3. What are the elevations at points on a slope (use contour lines)?
  4. What is the difference in elevation between those points?
  5. What is the distance between points in feet (use distance scales)?
  6. Calculate slope percent using the formula.

Table 5A: Slope Conversion from Topographic Maps and Table 5B: Correction Factors for Slope (PDF)
Table 5C: Contour Overlay (uses 7.5 minute quad.) (PDF)
Table 5D: Contour Overlay (uses 15 minute quad.) (PDF)
Slope Indicator Overlay (PDF)
Table 5E: Slope Degrees/Percent Conversion (PDF)


Using the Worksheet and Fire Behavior Tables

Step 1: Determine primary input values.

See workbook units I - X and/or field guide sections 1 - 5. (enter values on worksheet lines 1 - 14, 16)

Step 2: Determine effective windspeed factor.

  1. Enter fuel model and midflame windspeed into table 4B; read wind factor at intersection. (Worksheet lines 15 and 18)
  2. Enter fuel model and slope percent into table 5B; read slope factor at intersection. (Worksheet line 17)
  3. Calculate the effective windspeed factor. (Worksheet line 19)
    Effective windspeed factor = wind factor + slope factor

Step 3: Determine fire behavior values for no wind or slope.

  1. Select the proper fire behavior tables for fuel model (6A series and 6B series)
  2. Enter dead fuel moisture and live fuel moisture values into table 6A-?; read rate of spread in chains/hour at intersection. (Worksheet line 22)
  3. Enter dead fuel moisture and live fuel moisture into table 6B--?; read fireline intensity in BTUs/Ft/S at intersection. (Worksheet line 25)

Step 4: Calculate adjusted fire behavior values.

  1. Adjusted rate of spread = ROS X (WF + SF + 1)
  2. Adjusted fireline intensity = FI X (WF + SF + 1)

When ROS = rate of spread for no wind or slope
FI = fireline intensity for no wind or slope
WF = wind factor
SF = slope factor

Step 5: Determine other fire behavior values.

  1. Enter fireline intensity into table 6C; read flame length to closest foot. (Worksheet line 28)
  2. Enter percent shading, temperature and dead fuel moisture into table 6D; read percent probability of ignition at intersection. (Worksheet line 29)

Notice of over-predictions of fire behavior

Rate of spread and fireline intensity may be overpredicted by tables 6A and 6B under certain conditions when fuel moistures are relatively high and midflame windspeeds are high. The weak flames produced at the high moisture contents will be overpowered by strong winds and the fire will be quenched rather than accellerated by the wind. Chances of this occurring are greatest in fuel models 1, 6, and 8. Table 6E is presented as a guide when working with these fuel models.

Tables 6A-1 - 6A-6: Rate of Spread for no wind or slope, various fuel models (PDF)
Tables 6B-1 - 6B-6: Fireline Intensity for No Wind or Slope, various fuel models (PDF)
Table 6C: Flame Length and Fireline Intensity Relationship to Each Other, and Table 6D: Probability of Ignition (Percent) (PDF)
Table 6E: Limitations to Fire Behavior Calculations (PDF)


Determining Fire Size for Point Source Fires

Step 1: Calculate the total forward spread distance from origin.
Spread distance = adjusted rate of spread X projection time

Step 2: Enter fuel model and effective windspeed factor into table 7A; read effective windspeed at the intersection. Interpolate as necessary. (Worksheet line 34)

Step 3: Enter spread distance and effective windspeed into table 7B; read area in acres at intersection. Interpolate as necessary. (Worksheet line 35)

Step 4: Enter spread distance and effective windspeed into table 7C; read perimeter in chains at intersection. Interpolate as necessary. (Worksheet line 36)

Step 5: Use the fire shape for given effective windspeed and the total forward spread distance to plot the fire on a map.

Table 7A: Effective Windspeeds (PDF)

Table 7B: Area Estimations for Point Source Fires (PDF)

Table 7C: Perimeter Estimations for Point Source Fires (PDF)

Fire Shapes Associated with Effective Windspeeds (PDF)


Fire Behavior Interpretations

Ten Standard Firefighting Orders:

  1. Keep informed on fire weather conditions and forecasts.
  2. Know what your fire is doing at all times.
  3. Base all actions on current and expected behavior of fire.
  4. Have escape routes for everyone and make them known.
  5. Post lookouts when there is possible danger.
  6. Be alert, keep clam, think clearly, act decisively.
  7. Maintain prompt communications with your people, your boss and adjoining forces
  8. Give clear instructions and be sure they are understood.
  9. Maintain control of your men at all times.
  10. Fight fire aggressively, but provide for safety first.

Thirteen Situation That Shout "Watch Out"

  1. You are building a line downhill toward a fire.
  2. You are fighting a fire on a hillside where rolling material can ignite a fire below you.
  3. The wind begins to blow, increase, or change direction.
  4. The weather turns hotter or drier.
  5. You are on a line in heavy fuel with unburned fuel between you and the fire.
  6. You are in an area where the topography and/or cover makes travel difficult and slow.
  7. You are in unfamiliar country.
  8. You are in an area where firefighters are not familiar with local factors influencing fire behavior.
  9. You are attempting a frontal assault on afire with pumpers.
  10. Frequent spot fires are crossing the line.
  11. You cannot see the main fire, and you are out of communication with anyone who can see it.
  12. You do not clearly understand your assignment or instructions.
  13. You are drowsy and feel like taking a nap near the fire line.

Advice and Cautions to Firefighters -- Be Alert and Be Informed:

In light fuels where high rates of spread are possible, i.e., grass and/or brush.
In steep terrain where fire can make an uphill run, i.e., narrow canyons and box canyons.
To wind shifts and other weather changes---get latest forecasts, take observations, and watch the sky for indicators.
To traveling in unfamiliar country, hazardous terrain, and/or heavy fuels.
To what the fire is doing --post lookouts and keep in communications with others on the fire.
To means of escape , i.e., escape routes, safety islands, and fire shelters.
To prop wash and turbulence from low flying aircraft, i.e., helicopters.and air tankers.
To supervisors' orders and instructions, and be sure you understand them.

Fire Severity Related to Fuel Moisture Chart (PDF)

Fire Suppression Interpretations of Fireline Intensity and Flame Length Chart (PDF)

Example of Fire Activity Planning Chart (PDF)

Worksheet for Predicting Fire Behavior (PDF)


Copyright 2008, Michael Jenkins. Cite/attribute Resource . admin. (2005, September 28). Field Guide. 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