Relative humidity (RH), the atmospheric moisture content, is a primary environmental factor influencing how wildfires ignite, spread, and intensify. RH expresses the amount of water vapor in the air as a percentage of the maximum amount the air can hold at that specific temperature. Low RH means the air is dry and readily pulls moisture from its surroundings, creating conditions highly favorable for fire. Conversely, high RH saturates the air, slowing the drying process of potential fuels and dampening fire potential.
How Atmospheric Moisture Transfers to Fuel
The mechanism linking atmospheric moisture to fire behavior is the constant exchange of water vapor between the air and dead vegetative matter, known as fuel. This process directly determines the Fuel Moisture Content (FMC), which is the weight of water contained in a fuel source relative to its oven-dry weight. Dead fuels, such as leaves, pine needles, and small twigs, constantly absorb or release moisture until a balance is reached.
This point of balance is called the Equilibrium Moisture Content (EMC). EMC is the moisture level a fuel attains when exposed to constant atmospheric conditions. High RH results in a high EMC for the fuel, while low RH leads to a low EMC. Fine fuels, like grass and litter, react quickly to changes in RH, sometimes changing moisture content hourly, while larger fuels, such as branches and logs, respond much more slowly, requiring days or weeks to reflect a shift in humidity.
Impact on Fire Ignition and Spread
The practical effect of humidity is most apparent in how it alters the energy required to start and sustain a fire. When RH is high, the resulting high FMC requires a significant portion of heat energy to first boil off the water within the fuel. This vaporization process consumes latent heat, preventing the fuel from reaching its ignition temperature. Consequently, high humidity makes ignition difficult and slows the rate of spread, as the fire must pre-heat and dry out wet fuel ahead of the flame front.
The opposite occurs when RH is low, drying fine fuels to a low FMC. With less water to vaporize, the fuel requires less energy to ignite, making it receptive to sparks or firebrands. Once ignited, the fire spreads rapidly because heat energy focuses on combustion, leading to higher flame temperatures and increased heat transfer. In these dry conditions, an established fire exhibits more intense behavior, including longer flame lengths and a faster rate of spread.
Using Humidity to Predict Fire Danger
Fire management agencies routinely monitor RH because it is a direct and dynamic input into fire weather forecasting and risk assessment. Fire weather observations, often taken at 2 p.m., include RH to estimate the moisture content of one-hour timelag fuels (the fastest-drying fuels). This data is integrated into complex models, such as the U.S. National Fire Danger Rating System (NFDRS), to calculate fire danger components.
The Ignition Component (IC) within NFDRS uses RH and other factors to estimate the probability that a firebrand will cause a sustained fire. A volatile condition known as the “humidity crossover” or “30-30-30 rule” is a strong indicator of extreme fire behavior. This occurs when the temperature is 30°C or higher, the RH is 30% or lower, and wind speeds are 30 km/h or greater. The combination of low RH and high temperature creates a tipping point where fuels are extremely dry and fires are difficult to control.