Wildfire weather field guide
Fuel Moisture vs Relative Humidity: what is the difference?
Relative humidity describes moisture in the air. Fuel moisture describes water held in vegetation or dead fuels. The atmosphere influences fuels, but the two percentages are not interchangeable.
Core distinction
Air moisture vs fuel moisture.
NWCG defines relative humidity as the ratio of moisture in the air to the maximum amount the air could contain if saturated. Fuel moisture content is the amount of water in fuel expressed relative to its oven-dry weight.
An atmospheric variable that can change quickly with temperature, weather and time of day.
A property of the fuel itself. Dead and live fuels respond on different timescales.
Dead fuels
Fine dead fuels can respond quickly to the atmosphere.
NWCG defines fine fuel moisture as the probable moisture content of fast-drying fuels with a timelag of one hour or less, such as grass, leaves, needles and small twigs. These fuels can respond relatively quickly to changes in humidity and drying conditions.
Live fuels
Living vegetation behaves differently.
NWCG defines live fuel moisture as the ratio of water to dry plant material in living plants. Seasonal physiology, drought, species and site conditions all affect it; it does not simply track hourly atmospheric humidity.
- Dead fine fuels can dry and recover rapidly.
- Larger dead fuels respond more slowly.
- Live fuels have biological controls as well as weather controls.
- Nighttime RH recovery can increase moisture in fine dead fuels.
Fire weather
Critical conditions combine weather and fuels.
National Weather Service Red Flag criteria can combine low relative humidity, strong wind and dry-fuel indicators. Exact thresholds vary by region, which is why a humidity value alone is not a universal fire-danger rule.
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