Fire thrives in low humidity, as reduced atmospheric moisture significantly increases the intensity and speed of fire growth. In fire science, humidity is defined by the relative humidity (RH). This measures the amount of water vapor present in the air compared to the maximum amount the air can hold at that temperature. When RH drops, the environment becomes highly conducive to fire ignition and propagation, confirming a direct relationship between dry air and increased fire danger.
The Science of Combustion and Moisture
For any fire to start and sustain itself, three components must be present: heat, fuel, and an oxidizing agent (oxygen), a concept known as the fire triangle. Moisture acts as a natural inhibitor by disrupting the heat component. Water within a fuel source, such as wood or vegetation, prevents the material from reaching its ignition temperature quickly.
Water has a high specific heat capacity, requiring significant energy to raise its temperature. When heat is applied to moist fuel, that energy is first consumed by the water, converting it into steam. This process of vaporization acts as a powerful heat sink, drawing energy away from the combustion reaction. Consequently, wet fuel requires substantially more external heat and longer exposure to ignite compared to dry material.
How Low Humidity Dries Fuel Sources
The transition to high fire risk begins when atmospheric relative humidity (RH) falls, influencing the moisture content of dead vegetation. RH is distinct from fuel moisture content (FMC), which is the actual percentage of water in the combustible material. Low RH creates a steep moisture gradient between the air and the fuel, driving the process of desorption, or drying.
This drying occurs because the surrounding air aggressively pulls water vapor from the fuel until they reach an equilibrium moisture content. The speed at which a fuel dries is categorized by its “time lag,” which relates to the fuel’s size. Fine fuels, such as dead grasses, pine needles, and small twigs, are classified as 1-hour time lag fuels because their moisture content responds to RH changes within about an hour.
The rapid drying of fine fuels is concerning because they are the initial material that carries a fire from a spark to a sustained flame. Larger fuels, like heavy logs, have 100-hour or 1,000-hour time lags and dry out much more slowly. When fine fuels dry out, they become easily ignitable, allowing a fire to start with minimal energy and quickly spread to the larger, more resistant fuels.
Fire Behavior and Spread in Dry Conditions
Once fuel sources are dried by low humidity, the resulting fire burns with greater intensity and spreads more rapidly. Since less energy is wasted on vaporization, heat is channeled directly into the combustion process and transferred efficiently to adjacent materials. This efficient heat transfer allows the fire to pre-heat surrounding fuels faster, reducing the ignition delay for unburned areas.
A fire burning under low relative humidity often exhibits erratic behavior and a high rate of spread. This is frequently compounded by windy conditions, which supply more oxygen to the flames and push the fire front forward. Strong winds can also carry burning embers, a phenomenon known as spotting, far ahead of the main fire line. This combination of dry fuel and efficient heat transfer creates conditions difficult for fire suppression efforts to control.