Fuel Moisture

One of the key factors determining how fast a wildfire moves and spreads in the chaparral is fuel moisture – the amount of water that has been absorbed by the fuel. Here we're talking about “dead moisture” – the moisture that is in dead wood, brush, and grass. “Live moisture” is the moisture present in living plants, and this is largely independent of atmospheric conditions. The notion that high fuel moisture retards fire makes instant sense to everybody – if the fuel is wet or damp, it isn't going to catch fire as quickly as perfectly dry fuel, nor will it burn as intensely. But dig down into the notion of fuel moisture a little more, and there are some complexities that are well worth knowing…

Fuel moisture is determined in the end by atmospheric humidity and precipitation. However, there is a lag time between a change in atmospheric conditions and a change in the moisture of any particular fuel. Light, small fuels (such as dead grass) will very quickly respond to changes in the atmospheric conditions; heavy fuel (such as large downed logs) will change very slowly. This lag time is critical for firefighters, as it is a major determinant of whether fire will spread quickly or slowly through a particular area at a particular time. Here's how they classify fuel moisture lag time:

Fuels are classified into four categories by which they respond to changes in moisture. This response time is referred to as time lag. The four categories are:

• 1-hour fuels: up to 1/4 inch in diameter.
• 10-hour fuels: 1/4 inch to 1 inch in diameter.
• 100-hour fuels: 1 inch to 3 inches in diameter.
• 1000-hour fuels: 3 inches to 8 inches in diameter.

Examples of one-hour fuels are grass, leaves, mulch and litter. Fuel moisture in these fuels can change within one hour according to factors such as temperature, rain, humidity and shade. Conversely, larger diameter fuels such as deadfalls, brush piles, etc., take up to 1,000 hours to respond to changes in environmental factors.

Fuel moisture can be determined by clipping and immediately weighing the sample before oven drying it to a constant weight. Then the following formula can be used to determine percent fuel moisture: [(Wet Weight – Dry Weight)/Dry Weight] x 100.

When I first saw fuels classified as “1 hour” or “10 hour” fuels, I thought that meant how long the fuel would burn. But that's not it at all – the time is instead a reference to the lag time between a change in relative humidity and the consequent change in fuel moisture.

In a desert climate such as ours, there is a daily cycle of relative humidity: we are typically very dry during daylight hours, and very variable during nighttime (but averaging about 80% RH). That means that fuels with lag times below about 15 hours will dry out every day, and at some point during the day they will become “flashy”, meaning prone to quick ignition, quick fire growth, and will burn intensely. Since we rarely have more than a week of very dry nights, fuels with lag times of more than about a week (roughly 200 hours) will never completely dry out, and those fuels are much less flashy (and therefore much less dangerous).

Firefighters constantly use fuel moisture information when assessing the risks of any particular area during an incident. For example, the morning after a humid night, a field full of tall, dead grass (about 1 hour fuel) may be perfectly safe – but a couple of hours after sunrise, when the air has been dry for an hour, that same place may be very dangerous.

Homeowners can use fuel moisture information as well, when deciding what has to be cleared from near their homes (or other structures). Plants with plenty of live moisture are best of all; plants (or plant materials, such as leaves, needles, and wood chips) with short lag times are the worst.