Fuel Moisture & Fuel Types

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

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).

Dead fuel classes and fire behaviour

Dead fuels are classified by time-lag — the time required for a fuel to exchange approximately 63% of the moisture difference between itself and the surrounding air.

Time-lag class	Fuel description	Response time	Fire behaviour role
1-hour	Fine grass, dry pine needles, thin twigs (<0.6 cm diameter)	<1 hour	Primary vector of fire spread in surface fires. On a hot, dry afternoon, they can drop to critically low moisture in a matter of hours. The FFMC tracks fine fuel moisture directly.
10-hour	Small branches, thick grass stems (0.6–2.5 cm diameter)	~10 hours	Sustain fire spread and contribute to flame length. Determine whether a fire can persist through the night. Slower to dry out, but also slower to rehydrate after rain.
100-hour	Branches 2.5–7.5 cm diameter	~100 hours (~4 days)	Respond to multi-day drought. Feed a fire’s intensity once it is established. The DMC tracks this layer.
1000-hour	Large logs, stumps (>7.5 cm diameter)	Weeks	Reflect prolonged seasonal drought. The DC (Drought Code) tracks this layer. Relevant for deep-burning fires and smouldering persistence.

Note

The FWI system’s three moisture codes — FFMC, DMC, and DC — map directly onto these fuel layers. A high DC after a long dry summer means that even large woody fuels are available for combustion, supporting extreme fire intensity once a fire is established.

The combination of classes 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.

DC — Drought Code: deep fuels

The Drought Code (DC) represents the moisture content of deep organic layers — duff, large dead wood — equivalent to 1000-hour fuels. It evolves over a timescale of several weeks and only responds to significant and prolonged rainfall events. A single rain shower has no meaningful effect on the DC; recovery requires consecutive days of substantial precipitation.

The following table summarises how each fuel class maps to a FWI moisture index:

Class	Examples	FWI index
1-hour	Grasses, dead leaves	FFMC
10–100 hour	Fine and medium branches	DMC
1000-hour	Dead wood, deep duff	DC

Some indices (FFMC, DMC, DC) apply to dead fuels; others such as LFMC (Live Fuel Moisture Content) apply to live vegetation. These two categories follow fundamentally different dynamics — dead fuels respond directly to atmospheric conditions, while live fuel moisture depends on plant physiology and deep soil moisture reserves.

Caution

The DC cannot be interpreted without spatial and seasonal reference — see the Historical Percentiles section in Weather Models.

Key thresholds

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. 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.

Live fuel moisture

Live fuel moisture (LFM) is a separate variable from dead fuel moisture. Green vegetation does not ignite as readily as dead material, but it is not fireproof. Two mechanisms are relevant:

Drought stress. As soil moisture in deep layers is depleted over a prolonged dry period, plants reduce water uptake and live fuel moisture falls. When LFM drops below critical thresholds — typically around 80–100% for shrubs — live fuels begin to contribute actively to fire intensity and spread. The Soil Moisture page describes the soil layers relevant to this process.

Resinous species. Mediterranean shrubs — cistus, rosemary, heather, maquis — contain volatile resinous compounds that burn even at relatively high moisture content compared to non-resinous fuels. This makes Mediterranean shrubland structurally more flammable than northern European forest understory at equivalent moisture conditions.

Under prolonged summer drought, when deep soil moisture is depleted and live fuel moisture falls into the critical range, these shrubs can contribute to crown-level fire intensity that would not occur with the same wind and weather conditions earlier in the season.

Grass curing

Grass curing is the process by which annual grasses transition from green (living) to cured (dry, dead) as the dry season progresses. It is one of the most operationally important fuel variables in Mediterranean and grassland fire environments.

• Uncured green grass (0–30% cured) does not carry fire under most conditions
• Partially cured grass (30–70% cured) carries fire under moderate to severe weather
• Fully cured grass (>95% cured) is among the fastest-spreading fire fuel in the landscape — rate of spread can exceed that in forest fuels under equivalent wind conditions

Curing is driven by temperature, atmospheric humidity, absence of rainfall, and photoperiod. In Mediterranean landscapes, the typical seasonal progression:

• May–June — curing begins in coastal and low-elevation areas
• July — full curing common at most elevations in southern Europe
• August–September — persistent curing across the landscape
• After summer rains — partial recovery possible, but curing resumes rapidly when dry conditions return

Grass fires are a primary operational challenge in early season (spring, when forest fuels remain relatively moist) and in areas where land use change — agricultural abandonment, reduced grazing pressure — has produced large continuous grass carpets across formerly managed land.

Note

The FWI system was calibrated for Canadian boreal forest fuels. It does not explicitly capture grass curing dynamics. In grassland-dominant landscapes — common in Spain, Portugal, Italy, and Greece — operational tools specifically designed for grass fuels (such as the McArthur Grassland Fire Danger Rating System) provide more appropriate fire behaviour estimates during early and late season.

The live/dead fuel continuum

In practice, most fire environments contain a mixture of live and dead fuels at different stages of moisture response. The ratio between them shifts over the season and between weather events. After a multi-year drought sequence:

1. Deep soil moisture is depleted (Soil Moisture, DC)
2. Live fuel moisture in shrubs and trees falls toward critical thresholds
3. Dead fuel accumulation increases as drought-stressed plants shed material
4. The landscape becomes simultaneously more flammable and more available for high-intensity fire

This transition from a landscape where fire is controlled by weather to one where fire behaviour is controlled by fuel availability is a key concept for understanding why some fire seasons are structurally more dangerous than others, independent of any individual weather event.

Météo-France FWI variant

Météo-France uses a modified version of the Canadian FWI that adjusts the wind input used in the final index calculation. In the standard Canadian formulation, the FWI is computed using mean wind speed. The Météo-France variant incorporates 25% of the gust speed added to the mean wind, in order to better reflect the peak mechanical stress experienced by fine fuels during wind events.

In practice, this means that the Météo-France FWI will produce higher values than the standard Canadian FWI under identical conditions when significant gusts are present — a common scenario during Mistral, Tramontane, or Foehn episodes. When comparing FWI values across datasets or tools, it is important to verify which wind input convention was used.

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.

Caution

The indicators displayed in Wildflyer are derived from numerical models (GFS, ECMWF). They are estimates at the resolution of a kilometre-scale grid. Ground-truth observations — local weather stations, direct measurements, visual reports from crews — are the primary reference. Model outputs should always be interpreted in the context of local knowledge and real-time observations.

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.
• Viegas, D.X. & Viegas, M.T. (1994). A relationship between rainfall and burned area for Portugal. International Journal of Wildland Fire, 4(1): 11–16.
• European Commission Joint Research Centre (2023). Current wildfire situation in Europe. EFFIS Annual Reports.
• Noble, I.R., Bary, G.A.V. & Gill, A.M. (1980). McArthur’s fire-danger meters expressed as equations. Australian Journal of Ecology, 5(2): 201–203.
• Météo-France (2023). Indice Forêt Météo (IFM) — Note méthodologique. Direction de la Climatologie et des Services Climatiques.