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Fuel Moisture I

posted Jul 6, 2014, 4:27 PM by Joe Christy
Fuel Moisture in the context of wildland fire has been intensely studied and consequently has grown technical.

First, the theory
At its most basic level, Fuel Moisture is defined to be the the percentage of the weight of the fuel that is water. In other words, [(Wet Weight – Dry Weight)/Dry Weight] x 100. Wet weight is easy. You just weigh a stick. Dry weight is more daunting. You take the same stick and dry it in an oven until the weight stops changing, taking care to keep the oven temperature below the ignition temperature of the stick - fire changes everything.
The first distinction to be drawn is between live fuel and dead fuel. What the Ben Lomond RAWS(1), which collected the data that you link to, measures is dead fuel moisture. Live fuel moisture, despite being the more important factor in wildland fire, is impossible to measure reliably and accurately because once you cut the stick to get the wet weight, it's dying and it's totally dead by the time you get the dry weight.
The second distinction is among 1-hour, 10-hour, 100-hour, and 1,000 hour fuels. The time is what is known as the lag time; the time necessary for the fuel to respond to changes in atmospheric moisture. This is more of a rule of thumb. It's supposed to be the length of time that your stick has to dry in the oven before it reaches a constant weight. Roughly, 1-hour fuels are grasses and twigs up to 1/4" in diameter; 10-hour fuels are between 1/4" and 1" in diameter, and so on. Traditionally, 10-hour dry fuel moisture is what gets measured, and in the old days it got measured once a day, because it took the better part of a day to get the dry weight. Nowadays, automated weather stations use a standard set of "10-Hr Fuel Sticks"(2), which are dried and instrumented when manufactured then plugged into the weather station.

On to the practice
There have been unusually extensive periods since the first of the year when the (10-hour, dead fuel moisture) has been in the single digits. That's what accounts for the spotting(3) we've seen around our prescribed burns that I've mentioned several times in the Battle Mountain News this year.
Obviously, what determines whether an unextinguished spot fire turns into a wildland fire is whether it ignites the surrounding live shrubs and trees. This is where the live fuel moisture of 1,000 hour fuels is crucial. Remember that 1,000-hour dead fuel moisture would take over 40 days to measure, and that live fuel moisture is impossible to measure.
The common rule of thumb for determining when live fuel moisture is perilously low is when it's crunchy. Take a live leaf or a small branch from a manzanita or a scrub oak and bend it with your hand. In the winter it will bend before breaking, if it breaks at all. In wildfire season, leaves crackle in your hand and sticks snap.
The most sure-fire way to determine hazardous live fuel moistures is to observe what happens around a fire burning under the canopy. That's what I was referring to yesterday. Some of the piles threw 10'- 15' flames when we first lit them and the flashy fuels within caught fire. One of those flames carried the fire into a smallish branch that we ordinarily would have cut down with the pole saw, which was broken, so we hit it with back-pumps and called the engine. What was most sobering is that we saw a scar in a live, ~6" moss covered branch at least 50' up a tree catch fire, which is when we called the engine and decided to stop igniting new piles and only feed the existing piles *very* slowly.

An example: Fuel Moisture in MBUAPCD regulations
Monterey Unified Air Pollution Control District [MBUAPCD] is concerned with, duh, air pollution. The smoke from a fire is composed of gases and particles from the fuel that haven't burned. White smoke is mostly steam. Darker, colored smoke has the unhealthy stuff in it. How does it get there? Primarily in the steam. In the fire, water in the fuel boils off. As it heats, the water distills a soup of chemicals in the wood, including dumplings of solid material, and tiny particles of the soup with even tinier dumplings are carried off with the steam in the smoke column.
Tying into the lag-time discussion, the MBUAPCD mandatory drying times for burn materials - up to 2 inches in diameter: dry 30 days; 2—6 inches in diameter:  dry 60 day; and over 6 inches in diameter: dry 180 days - correspond to 720-hour, 1,440-hour, and 4,320-hour fuels.

"In theory there is no difference between theory and practice. In practice there is." - Yogi Berra
3) I.e. small "spot" fires, caused by embers from the fire, hoisted from the fire by convection, landing on the duff outside the containment rings of the fires.