Calculating One Hour Fuel Moisture And Probability of Ignition (PI)

Step 1: Determine Reference Fuel Moisture (RFM)

Day Time Hours 0800 – 1959 (Table A)
Night Time Hours 2000 – 0759 (Table B)

  1. Dry bulb Temperature is determined for the time period in question either by measurement or forecast.
  2. Using the intersection of dry bulb temperature (F) and Relative Humidity (%) determine reference Fuel Moisture (RFM) in percent.

Step 2: Using the Dead Fuel Moisture Content Corrections, match the table having Spring/Fall months (Table C), summer months (Table B) or Winter months (Table D) with the appropriate month.

  1. Determine if your Fine Dead Fuels are shaded (> 50% shading of surface fuels) or exposed (<50 shading of surface fuels). Shading can be due to cloud cover and/or canopy cover.
  2. Determine the aspect of the point (area) for which you are calculating Probability of Ignition (PI).
  3. Determine the slope: (0-30%) or (>30%).
  4. Calculate the Dead Fuel Moisture Content Correction for each aspect, slope, appropriate time of day, and the elevation differences between the location of the projection point and the RH/Temp site location
  1. A – 1000 feet – 2000 feet above site
  2. L – 1000” of site location
  3. B – 1000 feet to 2000 feet below site

Step 3: Determine Fine Dead Fuel Moisture by adding the Reference Fuel Moisture (step 1) with the Fuel Moisture Correction (step 2).

Step 4: To determine Probability of Ignition:

  1. Use the Fine Dead Fuel Moisture (Step 4), shading present (Step 2a), and the dry bulb temperature (step 1b)
  2. Using the Probability of Ignition Table to determine your PI.

Step 5: Nighttime Calculations

  1. Use Table E for Reference Fuel Moisture
  2. Use Table F for Dead Fuel Moisture Content Corrections
  3. Use Probability of Ignition Table as in Step 4.