Relative Humidity

Unit: Percentage (%)

Relative humidity (RH) is the percentage of water vapour in the air relative to the maximum it can hold at that temperature. It changes throughout the day — rising at night as temperatures fall, dropping during the afternoon as temperatures rise.

Why it matters for fire weather

RH directly controls the moisture content of fine fuels — grasses, dead needles, and small twigs. These fuels reach equilibrium with the surrounding air in less than an hour. When RH drops, fine fuels become immediately flammable; when it rises (overnight), they absorb moisture and become harder to ignite.

RH is one of four inputs to the FWI system and strongly influences the Fine Fuel Moisture Code (FFMC), which tracks the moisture of surface litter and fine fuels (Van Wagner, 1987).

However, RH has an important limitation: it doesn’t account for temperature. At the same RH, hotter air dries fuels much faster than cooler air. VPD captures this combined effect, which is why it is increasingly used alongside RH in fire weather assessment.

How it works

RH is measured with hygrometers at 2 m height inside a shielded weather enclosure (following WMO standards). Because RH depends on temperature, it follows a predictable daily cycle:

• Early morning (06:00–09:00): RH is typically 70–90% — fuels absorb moisture overnight
• Afternoon (13:00–17:00): RH typically drops to its daily minimum — the most relevant window for fire weather

This daily cycle is why the FWI system uses noon observations — capturing conditions close to the afternoon minimum.

Key thresholds

Like temperature, RH thresholds are most meaningful in combination with other variables. The values below are drawn from French and European operational guidance.

RH	Physical significance
> 60%	Fine fuels are absorbing moisture from the air. Ignition is difficult without an intense heat source.
40–60%	Fine fuels begin to lose moisture. Fire indices start to rise, especially with elevated temperature.
30–40%	The French fire weather system marks 40% RH as a first meteorological alert threshold (Valabre/ECASC). Fine fuels dry rapidly.
20–30%	Significant drying. French guidance escalates at 30% RH. Fine fuels are highly receptive to ignition.
< 20%	Extreme atmospheric dryness. At the third French alert level (< 20%), virtually all fine fuels are at their lowest possible moisture content. Combined with heat and wind, these conditions have accompanied major Mediterranean fire events.

Note

Low RH alone does not mean high fire danger. A cool, humid morning can transition to very low afternoon RH — this is normal daily behaviour. What matters is the combination: when low RH coincides with high temperature (producing high VPD), wind, and antecedent drought, fire indices reach levels associated with significant fire behaviour.

How to read it in Wildflyer

Relative humidity appears on the weather timeline and in the expert view. Watch for the afternoon minimum — this is the most fire-weather-relevant RH of the day. For a more complete picture of atmospheric drying power that accounts for temperature, also check VPD.

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.