Temperature

Unit: Degrees Celsius (°C)

Air temperature measured at 2 metres above ground level, following the World Meteorological Organization (WMO) standard.

Why it matters for fire weather

Temperature is a key driver of fuel drying, but it does not indicate fire danger on its own. A hot day with high humidity and no wind may pose less fire risk than a mild day with bone-dry air and strong gusts.

What temperature does:

• Accelerates evaporation — warmer air increases the rate at which vegetation loses moisture. This effect is captured more precisely by VPD, which combines temperature and humidity into a single drying metric.
• Feeds fire weather indices — temperature is one of four inputs to the FWI system (alongside relative humidity, wind, and precipitation). It directly affects the Fine Fuel Moisture Code (FFMC) and Duff Moisture Code (DMC) calculations (Van Wagner, 1987).
• Shapes the atmosphere — high temperatures can destabilise the lower atmosphere, promoting convective activity that influences fire behaviour. The HDWI captures this by combining VPD with wind speed.

Temperature matters most in combination with other variables. The French fire weather system (Météo-France) tracks temperature alongside humidity and wind as separate thresholds that together determine danger level — not any single variable in isolation (Valabre/ECASC training materials).

How it works

Weather stations measure air temperature using shielded thermometers at 2 m height. For fire weather purposes, the most relevant reading is the afternoon maximum — typically between 13:00 and 17:00 local time — which coincides with minimum humidity and peak drying conditions.

The FWI system uses noon (12:00 local standard time) temperature as input. This single daily observation drives the moisture code calculations that track how fuels dry over time (Van Wagner, 1987).

Key thresholds

Temperature thresholds are only meaningful in combination with humidity, wind, and drought conditions. The values below indicate approximate levels used in European fire weather assessment, drawn from French operational guidance (Valabre/ECASC).

Temperature	Physical significance
< 25°C	Moderate drying rate. Fire danger depends heavily on humidity, wind, and antecedent drought.
25–30°C	Drying accelerates through the day. Combined with low RH (< 40%), fuels lose moisture noticeably.
30–35°C	Rapid daytime drying. French fire weather guidance uses 30°C as one of the meteorological alert thresholds (Valabre/ECASC).
35–40°C	Intense drying. At these temperatures, even vegetation that retained moisture overnight can become dry by early afternoon. French guidance escalates at 35°C.
> 40°C	Exceptional heat. In Mediterranean fire events (Pedrógão Grande 2017, Mati 2018, Greece 2023), temperatures above 40°C have coincided with catastrophic fire behaviour — but always in combination with low humidity, drought, and wind.

Note

Temperature alone cannot be read as a risk indicator. A cool day with low humidity and strong wind can be more dangerous than a hot, humid, calm day. Fire weather indices like the FWI and HDWI exist precisely to combine these factors.

How to read it in Wildflyer

Temperature appears on the weather timeline and in the expert view alongside other variables. Use it to understand the drying potential of the day, but always interpret it alongside relative humidity, VPD, and the fire indices that combine all factors.

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.
• European Commission Joint Research Centre (2023). Current wildfire situation in Europe. EFFIS Annual Reports.