Fuel Moisture (1h/10h/100h)
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Unit: Percentage (%) — weight of water relative to dry weight of the fuel
Fuel moisture is classified by time-lag — the time it takes for a fuel particle to reach two-thirds of its equilibrium moisture content with the surrounding air. The smaller the fuel, the faster it responds to weather changes.
Why it matters for fire weather
Section titled “Why it matters for fire weather”Different fuel sizes play different roles in fire behaviour. Understanding which fuels are dry tells you what kind of fire to expect:
- 1-hour fuels determine whether a fire can ignite and initially spread
- 10-hour fuels sustain fire spread and contribute to flame length
- 100-hour fuels determine whether a fire will be persistent and deep-burning
The FWI system models similar timescales through its moisture codes: the FFMC corresponds roughly to 1-hour fuels, the DMC to 10-hour/100-hour fuels, and the DC to 1000-hour fuels (Van Wagner, 1987).
How it works
Section titled “How it works”The three classes
Section titled “The three classes”| Class | Fuel type | Response time | Examples |
|---|---|---|---|
| 1-hour | Fine fuels | < 1 hour | Dead grass, needles, small twigs (< 6 mm diameter) |
| 10-hour | Small fuels | ~10 hours | Twigs and small branches (6–25 mm diameter) |
| 100-hour | Medium fuels | ~100 hours (~4 days) | Larger branches (25–75 mm diameter) |
Fine fuels respond directly to current temperature and relative humidity — they can dry to flammable levels within an afternoon and recover moisture overnight. Larger fuels integrate weather over longer periods, smoothing out daily fluctuations.
What each class tells you
Section titled “What each class tells you”1-hour fuels track hourly weather. High afternoon temperatures and low RH dry them immediately. They recover overnight when humidity rises. When 1-hour fuel moisture drops below approximately 10%, ignition becomes easy and fire spreads readily through fine surface fuels.
10-hour fuels smooth out daily cycles. They carry moisture information from the last half-day or so, giving a picture of whether conditions have been consistently dry rather than just dry this afternoon.
100-hour fuels reflect multi-day trends. When these are dry (below approximately 20% moisture), it indicates sustained dry weather over several days. Fires burning in dry 100-hour fuels consume more material, produce more heat, and are more difficult to fully extinguish.
The combination tells the story: dry 1-hour fuels mean a fire can start; dry 10-hour and 100-hour fuels mean it will be intense and persistent.
Key thresholds
Section titled “Key thresholds”These approximate moisture thresholds are drawn from fire behaviour research and represent the points where fire characteristics change significantly. Actual thresholds vary by vegetation type and region.
| Class | Moisture level | Physical significance |
|---|---|---|
| 1-hour | < 10% | Fine fuels are highly receptive to ignition. Fire spreads readily through surface litter. |
| 1-hour | < 5% | Extreme dryness in fine fuels. Rapid fire spread and high ember production. |
| 10-hour | < 15% | Active fire spread with moderate intensity. The bulk of surface fire fuel is dry enough to sustain combustion. |
| 10-hour | < 8% | High-intensity surface fire. Significant flame lengths. |
| 100-hour | < 20% | Larger branches participate in combustion. Fires become persistent. |
| 100-hour | < 12% | Full involvement of medium fuels. Fires burn deeply and are slow to fully extinguish. |
How to read it in Wildflyer
Section titled “How to read it in Wildflyer”Fuel moisture values appear in the expert view. Because 1-hour fuels respond so quickly, they track closely with the afternoon VPD and relative humidity values. The 10-hour and 100-hour values provide context on whether the broader fuel profile is drying or recovering.
Sources
Section titled “Sources”- Van Wagner, C.E. (1987). Development and structure of the Canadian Forest Fire Weather Index System. Forestry Technical Report 35, Canadian Forest Service.
- Anderson, H.E. (1982). Aids to determining fuel models for estimating fire behavior. USDA Forest Service General Technical Report INT-122.
- Rothermel, R.C. (1972). A mathematical model for predicting fire spread in wildland fuels. USDA Forest Service Research Paper INT-115.