Wind & Gusts

Unit: km/h (also displayed in m/s, mph, or knots depending on your organisation settings)

Wind speed is measured at 10 metres above ground level following WMO standards. Gusts are short-duration peaks (typically 3-second averages) within the sustained wind.

Why it matters for fire weather

Wind is the dominant driver of fire spread rate. In the FWI system, the relationship between wind and spread rate is exponential — doubling wind speed more than doubles the expected fire spread rate (Van Wagner, 1987).

What wind does:

• Drives fire spread — wind pushes the fire front forward and determines the direction of spread. The ISI (Initial Spread Index) models this as an exponential function of wind speed.
• Pre-heats fuel — wind tilts flames toward unburnt vegetation ahead of the fire, drying and heating it before the fire arrives.
• Transports embers — wind carries burning material (embers, bark flakes) ahead of the fire front, creating spot fires. In Mediterranean conditions, spotting distances of 5–10 km are common during strong wind events, and distances exceeding 20 km have been recorded (European Commission JRC, 2023).
• Supplies oxygen — increased airflow to the combustion zone raises fire intensity.

Wind is a direct input to the ISI component of the FWI system and to the HDWI (Hot-Dry-Windy Index).

How it works

The FWI system uses noon wind speed at 10 m height. The ISI wind function is exponential:

f(W) = exp(0.05039 × W)

At 0 km/h, the wind effect is 1. At 20 km/h, it is approximately 2.7. At 40 km/h, it reaches approximately 7.4 (Van Wagner, 1987). This exponential scaling means that high wind speeds produce disproportionately large increases in expected fire spread.

Wind at the surface (where fire actually burns) is typically weaker than the 10 m measurement due to friction from vegetation and terrain. However, turbulence, terrain channelling, and convective effects can create local wind speeds that exceed the measured value.

Key thresholds

Wind thresholds gain fire weather significance in combination with fuel dryness (reflected in FFMC, VPD) and drought conditions. The values below are drawn from European fire weather guidance.

Wind speed	Physical significance
< 10 km/h	Light winds. Fire spread is slow and primarily driven by slope and fuel conditions rather than wind.
10–20 km/h	Moderate winds. Fire spread rate increases noticeably. Wind begins to dominate fire direction.
20–30 km/h	Strong winds. Fire spread accelerates significantly. Ember transport becomes an important factor. The ISI wind function produces values 2.7–4.5× the baseline.
30–50 km/h	Very strong winds. French fire weather guidance uses 30 km/h sustained as a meteorological alert threshold, with 40 km/h gusts as a further escalation (Valabre/ECASC). Fire behaviour becomes wind-driven — spread is dominated by wind rather than terrain or fuel heterogeneity.
> 50 km/h	Extreme winds. Associated with the most destructive Mediterranean fire events. During the 2018 Mati fire (Greece), sustained winds of 50–60 km/h with gusts to 77 km/h drove the fire 5–10 km in 2–3 hours.

Note

Wind is a powerful driver of fire spread, but it does not act alone. A strong wind over saturated fuels after heavy rain will not produce significant fire behaviour. What makes wind dangerous is its combination with dry fuels — which is exactly what the ISI captures by combining wind speed with the Fine Fuel Moisture Code.

Gusts and wind direction

Gusts are short bursts of wind that exceed the sustained speed. They are particularly significant for fire weather because they:

• Throw embers far beyond the fire perimeter, creating spot fires
• Cause sudden accelerations in fire spread
• Can temporarily shift fire direction

Wind direction changes can be as consequential as wind speed. A shift in wind direction turns the fire’s flank into a new head fire — instantly widening the fire front. The 2017 Pedrógão Grande fire (Portugal) became catastrophic after a near-90° wind shift created a new, wide head fire from what had been a flank (European Commission JRC, 2023).

Mediterranean wind regimes

In the Mediterranean basin, regionally characteristic winds fundamentally alter fire behaviour:

• Mistral (Southern France) — cold, dry northwesterly; sustained 50 km/h, gusts exceeding 100 km/h
• Tramontana (Catalonia, Balearics) — dry northerly; similar intensity to Mistral
• Foehn (Italian/Swiss Alps) — adiabatic warming can raise temperatures 10–14°C in hours while dropping humidity
• Sirocco (Southern Mediterranean) — hot, dry Saharan air; temperatures 40°C+ with RH 10–20%

These wind events are the primary trigger for wind-driven fires — a fire behaviour category where wind overwhelms terrain and fuel effects, producing uniform spread with extreme spotting distances (Castellnou et al., 2009).

How to read it in Wildflyer

Wind speed and gusts appear on the weather timeline and in the expert view. Wind direction is shown as a compass bearing. Pay attention to:

• Afternoon winds — often the strongest, coinciding with peak drying conditions
• Gust-to-sustained ratio — large ratios indicate turbulent conditions that increase ember transport
• Direction changes — forecast wind shifts are critical context for understanding how fire indices may evolve through the day

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.
• Castellnou, M., Miralles, M., & Molina, D. (2009). Wildfire management in Mediterranean-type regions: paradigm change in Southern Europe. Wildfire, 18(3): 18–23.
• European Commission Joint Research Centre (2023). Current wildfire situation in Europe. EFFIS Annual Reports.