Precipitation

Unit: Millimetres (mm)

Precipitation is measured as accumulated rainfall over a given period — typically 24 hours for fire weather index calculations, and hourly for real-time monitoring.

Why it matters for fire weather

Rain is the primary natural mechanism for wetting fuels and reducing fire danger. But the effect of rainfall on fire weather is not straightforward — it depends on how much rain falls, how long it lasts, and how dry the fuels were before the rain arrived.

• Fine fuels (surface litter, grass, twigs) respond quickly — even a few millimetres of rain can temporarily wet them. But they also dry quickly, sometimes within hours of the rain stopping.
• Deeper fuel layers (duff, organic soil) require sustained, heavier rainfall to recover. The FWI system models this through three progressively deeper moisture codes, each with its own rainfall threshold (Van Wagner, 1987).
• Background drought accumulates over weeks and months. Short rainfall events reduce surface flammability but do little to address the deep drought that drives the most severe fire seasons.

How it works

The FWI system uses 24-hour accumulated precipitation (measured at noon) as one of its four daily inputs. Rainfall affects the three fuel moisture codes at different thresholds, reflecting how much rain is needed to penetrate each fuel layer:

Fuel moisture code	Rainfall threshold	Fuel layer	Response time
FFMC (Fine Fuel Moisture Code)	0.5 mm	Surface litter, fine twigs (≤ 2 cm)	~16 hours
DMC (Duff Moisture Code)	1.5 mm	Loosely compacted organic material (5–10 cm)	~12 days
DC (Drought Code)	2.8 mm	Deep, compact organic layers (10+ cm)	~52 days

Rainfall below these thresholds is ignored for that code — it doesn’t reach the corresponding fuel layer. The increasing thresholds reflect canopy interception, reduced infiltration, and the greater water-holding capacity of deeper layers (Van Wagner, 1987).

The French fire weather system uses a similar concept through the Réserve (water reserve) calculation, which tracks a daily water balance: Réserve(J) = Réserve(J-1) + Rainfall − Evapotranspiration (Valabre/ECASC). This directly models how precipitation replenishes soil moisture and how evaporation depletes it.

Key thresholds

Rainfall thresholds describe how different amounts of rain affect different fuel layers. These values are drawn from the FWI system’s mathematical formulations (Van Wagner, 1987).

24h rainfall	Physical significance
< 0.5 mm	No effect on any fuel moisture code. Fine fuels may not even get wet. Trace amounts are common in dew and very light drizzle.
0.5–1.5 mm	Wets surface fuels only (FFMC responds). Fine fuels get a brief reprieve, but dry again within hours if conditions are warm and windy. Duff and deep layers are unaffected.
1.5–2.8 mm	Surface and duff layers affected (FFMC and DMC respond). One to two days of reduced surface flammability. Deep drought remains unchanged.
2.8–10 mm	All three fuel layers affected. Surface fuels wet, duff layer begins to recover. Deep drought (DC) starts to respond, though slowly given its 52-day time lag.
10–25 mm	Meaningful recovery across all fuel layers. Multiple days of reduced fire indices. The FFMC may return to low-danger levels for several days.
> 25 mm	Substantial recovery event. Duff and deep organic layers receive significant moisture. In Mediterranean conditions, these events are rare during summer and can mark the effective end or interruption of the fire season.

Note

The absence of rain is often more telling than any individual rainfall event. Prolonged dry spells — weeks or months without significant precipitation — are the foundation of severe fire seasons. The Drought Code (DC) tracks this cumulative deficit, and in Mediterranean summers it routinely reaches values of 400–600, reflecting 3–5 months of persistent drought (Dimitrakopoulos & Bemmerzouk, 2011).

Timing and duration

Not all rain events are equal, even at the same total amount:

• Brief, intense showers — much of the water runs off rather than infiltrating into fuel layers. Surface fuels get wet but deeper layers see little benefit.
• Prolonged, steady rain — allows water to percolate through the fuel profile. The same total amount over several hours is more effective at recovering fuel moisture than a single heavy burst.
• Morning rain followed by hot afternoon — gives a false sense of security. Fine fuels (FFMC response time ~16 hours) can dry back to dangerous levels by mid-afternoon if temperature is high and relative humidity drops.
• Consecutive rain days — the most effective at reducing deep drought. The DC’s 52-day response time means only sustained wet periods truly recover deep fuel moisture.

How to read it in Wildflyer

Precipitation appears on the weather timeline as both recent accumulation and forecast amounts. Use it to assess:

• Recent rain — has there been enough to affect fuel moisture codes? Check the thresholds above against the 24h total.
• Dry spell duration — consecutive days without significant rain are a key indicator. The fire indices (especially DC and DMC) reflect this accumulating drought.
• Forecast rain — upcoming precipitation can signal when current fire weather conditions may ease. But verify the amount — light scattered showers (< 2 mm) have minimal effect.

Sources

• Van Wagner, C.E. (1987). Development and structure of the Canadian Forest Fire Weather Index System. Forestry Technical Report 35, Canadian Forest Service.
• Valabre/ECASC. Tableaux des Indices — Fire Weather Indices Reference Tables. Training materials, Service Départemental d’Incendie et de Secours.
• Dimitrakopoulos, A.P. & Bemmerzouk, A.M. (2011). Evaluation of the Canadian fire weather index system in an eastern Mediterranean environment. Meteorological Applications, 18(1): 83–93.