Relative Humidity (RH) is a measure of how much water vapor is in the air compared to the maximum amount possible.
Why RH Matters for Wildfire
Relative humidity directly affects how quickly wildland fuels dry out and how readily they ignite. When RH drops, moisture evaporates from fine fuels (grasses, leaf litter, small-diameter dead wood), lowering their fuel moisture content and increasing flammability. RH is one of the primary weather drivers, alongside wind speed, temperature, and fuel moisture, that determines fire behavior.
Low RH accelerates the drying of dead fuels, particularly 1-hour (1H) and 10-hour (10H) fuels, which respond quickly to ambient conditions. When sustained low RH combines with high temperatures and wind, conditions become favorable for rapid fire spread, higher flame lengths, and difficult suppression.
Key Thresholds
RH values used in operational wildfire decision-making generally follow these ranges:
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Above 50%: Fuels retain moisture. Ignition probability is low and fire spread is limited under most fuel and terrain conditions.
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25% to 50%: Conditions become increasingly favorable for fire activity. Fine dead fuels begin drying significantly. Fire managers monitor trends closely in this range.
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15% to 25%: Conditions are conducive to sustained fire spread. In many US jurisdictions, RH at or below 15% (combined with sustained wind) triggers Red Flag Warnings from the National Weather Service (NWS).
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Below 15%: Extreme drying conditions. Fine fuels can reach critically low moisture levels within hours. These conditions are frequently associated with large, fast-moving fires.
In Canada, RH is also used in the concept of temperature/relative humidity crossover, where fire behavior is expected to increase when the RH value is less than or equal to the air temperature (for example, 30°C and 30% RH). This crossover condition is a widely used field indicator for elevated fire activity.
RH in Technosylva Products
In Wildfire Analyst (WFA) and Wildfire Operations, RH is delivered as a forecasted weather variable at two meters above ground level. RH forecasts are available at hourly intervals across the full forecast horizon.
RH is also a key input to derived metrics such as the Fire Potential Index Composite (FPIC). RH influences the fuel moisture components that feed into Technosylva's fire potential calculations.