Fuel Moisture

Dead fuels are those wildland fuels whose moisture contents are controlled exclusively by changing weather conditions. Examples include dead herba- ceous fuels, dead roundwood, fallen dead leaves and needles, and the litter of the forest floor. For purposes of fire behavior modeling, dead fuels are divided into four “timelag” categories: 1-hour, 10-hour, 100-hour, and 1000-hour fuels. The shorter the timelag, the more responsive the fuel is to changing weather conditions. For Example 1-hour fuels only take on the order of one hour to respond to changing weather conditions, which explains why fire danger can be very high even right after a heavy rain if the subsequent weather conditions allow the 1-hour fuels to dry out. Samples are taken from standing dead trees, shrubs, or grasses. Dead fuel moisture can also be calculated from observed or forecast weather data. Model calculations of 1-hour, 10-hour, 100-hour, and 1000-hour fuel moisture are routinely made at all Oklahoma Mesonet weather tower sites and can be found in the “FIRE” section of OK-FIRE (http://okfire.mesonet.org) under “Current/Recent Fire Danger” and “Fire Danger Forecasts.” Fuel moisture readings can be found in the “FIRE” section of OK-FIRE under “Current/Recent Fire Danger” and “Fire Danger Forecasts.” at http://okfire.me- sonet.org.

Time Lag Fuel Classes

Time Lag Fuels

One-hour fuels are less than 0.25 inch in diameter (also dormant grasses).

Time Lag Fuels

Ten-hour fuels are 0.25 inch to 1 inch in diameter (also leaf litter and pine needles).

Time Lag Fuels

One hundred-hour fuels are 1 inch to 3 inches in diameter.

 

Time Lag Fuels

One thousand-hour fuels are 3 inches to 8 inches in diameter.

Recommended Fuel Moisture Conditions for Conducting Prescribed Fires

You should burn when 1-hour fuels are between 7 to 20 percent.

1-hour fuels <5 percent: spotfires certain

1-hour fuels >11 percent: spotfires rare

1-hour fuels >20 percent: fire may not spread

10-hour fuels burn best when between 6 to 15 percent moisture

10-hour fuels more than 15 percent: fire may not burn in certain fuel types.