Unit 11

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

1. Describe the four common denominators of fire behavior on tragedy wildland fires.

2. Describe extreme fire behavior characteristics and recognize fire environment influences that contribute to extreme fire behavior.

3. Describe the three stages of crown fire development and identify the key factors and indicators leading to crown fire development.

4. Identify the three factors that contribute to the spotting problem and describe the conditions associated with each factor.

5. Define the probability of ignition, describe its use, and determine it using tables.

6. Define firewhirls (vortices), the conditions under which they are likely to develop, and their implications to wildland fire behavior.

7. Explain the difference between wind-driven and plume-dominated fires.

A. Probability of ignition is a rating of the probability that a glowing firebrand will cause a fire, providing it lands on receptive fuels.

1. It is not related to the likelihood of a firebrand being produced, or:

• Being the right size and shape to be transported still glowing by convection and/or winds.

• Having available fuels where it lands.

2. It states the chance (probability) that the firebrand will cause an ignition when the right kind of firebrand lands on the right kind of fuel.

• When the probability of ignition is 70 percent, if 10 glowing firebrands land on receptive fuels (grass, needles, punky wood) there will be seven ignitions.

• Whether a wildland fire actually results from the ignition depends on the fire environment.

B. Probability of Ignition Table 

1. The probability of ignition is determined from:

• Fuel shading

• Fine dead fuel moisture

• Dry bulb temperature

2. Choose the table half with the correct fuel shading, either less than, or more than, 50 percent cloud and/or canopy cover.

3. Read the probability of ignition at the intersection of the proper fine dead fuel moisture column and dry bulb temperature row.

Using the chart above, determine probability of ignition for the following:

#1 – Clear day, no canopy cover

• Fine dead fuel moisture = 5%

• Dry bulb temperature = 86 °F

• Probability of ignition =

#2 – Clear day, dense canopy

• Fine dead fuel moisture = 6%

• Dry bulb temperature = 75 °F

• Probability of ignition = 

Remember: Probability of ignition relates only to the chance of initial ignition.

Fine Dead Fuel Moisture and Probability of Ignition

1.     It’s August 15th at 1600 hours local time. Fuels are shaded on a south aspect with 31% slope. Weather observation taken 1200 feet upslope from the fireline are: Dry bulb 81° F  and relative humidity 15%. 

What is fine dead fuel moisture? 

What is the POI?

2.     It’s July 23rd at 1300 hours local time. Fuels are model 6 on a west aspect with 31% slope. Weather observation at the location are: Dry bulb 75° F  and relative humidity 14%.  20% cloud cover.

What is fine dead fuel moisture 1500 feet above this location?

What is the POI?

3.     Determine the probability of ignition based on the following: (1 pt.)

Dry bulb temperature: 94°

Fine dead fuel moisture: 3%

Sky weather: 10% cumulus clouds

Fuels: grass

Probability of ignition =       

4.     Determine the probability of ignition based on the following: (1 pt.)

Dry bulb temperature: 72°

Fine dead fuel moisture: 8%

Sky weather: Sunny and clear

Fuels: dense timber

Probability of ignition =