How Long Do Wildfires Last? Factors that Affect Duration
Nobody wants to be caught in the middle of a wildfire, but unfortunately, sometimes it is inevitable. When you find yourself in that situation, the most common question is: how long will this wildfire last? The answer varies significantly. Some fires are extinguished in a matter of hours, while others burn for weeks or even months. Understanding what determines wildfire duration helps explain why some fires become long-duration events and what communities can expect during extended fire incidents.
Wildfire duration depends on several factors, including fire size, fuel availability, weather conditions, terrain, and the effectiveness of suppression efforts. While individual fire lengths vary widely, the average duration has increased dramatically over the last several decades. According to the USDA, individual wildfire length has risen from an average of 6 days in the 70s to 52 days in the 2000s, highlighting how important it is to be informed and prepared.
What Factors Determine How Long a Wildfire Lasts?
Understanding the factors that determine wildfire duration helps explain why some fires are quickly extinguished while others become long-duration incidents.
Factors That Affect Both Spread and Duration
The same factors that contribute to wildfire spread also contribute to how long a wildfire lasts, and the two are often interconnected. Available fuel, weather conditions, and topography all play roles in both how fast a fire moves and how long it continues to burn.
Available Fuel
The amount and type of fuel available directly affect how long a fire can last. Moisture level, chemical makeup, and density are all factors to consider when determining fuel flammability.
Moisture level significantly impacts burn time. Live vegetation contains high levels of moisture and takes longer to burn because the fire must first eliminate the moisture, while dead or dry vegetation burns much faster. Some plants, shrubs, and trees contain oils or resins that promote combustion, causing them to burn more easily, quickly, or intensely than those without.
Fuel availability and composition play a significant role in determining how long a fire lasts. If no fuel is available, a fire's spread is quickly stunted and can then be controlled. If fuel is abundant, however, a fire's spread and continuation become much harder to control.
Weather Conditions
Weather significantly affects fire duration. Heavy winds, high temperatures, and dry conditions all contribute to fire behavior and duration. Heavy winds exacerbate fires by carrying a fresh supply of oxygen and pushing fires toward new fuel sources. High temperatures increase the ignition susceptibility of fuels, while low humidity means drier fuels that are more likely to catch fire. These conditions often facilitate the start of a fire and will continue to exacerbate it until the conditions change.
Topography
The topographical features of an area can help or hinder a fire. For example, a fire that ignites at the bottom of a steep slope will spread much faster up the hill than vice versa. This is because rising hot air will preheat fuels, making them more susceptible to ignition. On the other hand, a rocky slope can prevent the spread of a fire due to its lack of fuel and the wide gap of open space.
Unique Factors for Wildfire Duration
Beyond the factors that affect both spread and duration, there are unique considerations specific to wildfire duration. How terrain affects accessibility and how suppression resources and management are deployed are particularly important for understanding how long a fire will last.
How Terrain Affects Accessibility and Firefighting Efforts
When a wildfire is first identified, the initial response is always a direct attack, where firefighters get as close to the fire as possible and keep it as small as possible until it is extinguished. Engine crews pump water from tanker trucks while firefighters on the ground dig containment lines and create safe zones to work from. If successful, an engine crew can extinguish a fire before it grows larger.
If the initial response doesn't work, strategies switch to an extended attack or prolonged action plan, which involves more resources and takes longer to resolve. Terrain, or topography, is one of the main reasons a prolonged response may be needed. Steep hills, mountain paths, or rocky ground often call for an indirect attack, which includes strategies that consider a fire might be fought over a longer time than a direct attack. Difficult terrain not only extends firefighting timelines but also creates significant safety challenges, as firefighters may face increased entrapment risks when working in remote or inaccessible areas.
Suppression Resources and Management
A vigorous initial attack on a wildfire can greatly reduce the likelihood that the fire will grow and cause substantial damage, making a successful initial attack the best chance of extinguishing a fire quickly. However, a successful initial attack depends on deploying the right number and type of firefighting resources in a timely manner. However, the right kind or number of resources might not always be available.
According to the USFS, "While most fire managers have a clearly defined goal of minimizing the number of escaped fires, they face substantial uncertainty about the number, location, and intensity of fires and they have limited funds to acquire suppression resources or construct operating bases."
Therefore, if the appropriate resources are not deployed in an initial attack, a wildfire is much more likely to persist for an extended period.
How Do We Define When a Wildfire is Over?
Understanding when a wildfire is truly over requires knowledge of the phases of wildfire suppression. According to the National Wildfire Coordinating Group, wildfire suppression follows several distinct phases from discovery to final extinguishment.
- Discovery - Marks when a fire is first detected and reported, triggering the initial response and resource deployment.
- Initial Attack - The first suppression action taken to control a fire. Firefighters work to contain the fire quickly before it can grow larger, and if successful, the fire may be extinguished during this phase.
- Extended Attack - If the initial attack is not successful, the fire moves into this phase, which involves more resources and longer-term suppression strategies requiring more complex planning and coordination.
- Contained - When firefighters establish control lines around the fire's perimeter, the fire is considered contained. A fire can be 100% contained but still actively burning within those control lines.
- Controlled - The fire reaches controlled status when it is no longer spreading and poses minimal threat. All hot spots that pose an immediate threat to control lines have been extinguished, and the fireline has been strengthened to prevent flare-ups.
- Mop up - During this phase, firefighters work to extinguish all remaining hot spots, embers, and smoldering materials within the burn area. This phase ensures the fire cannot reignite and is critical for truly putting the fire out.
- Fire Out - The final phase marks when all burning has stopped and the fire is completely extinguished. This is when a wildfire is truly over, though monitoring may continue to ensure no new fires emerge from the burn area.
While a fire may be considered "out" when it reaches 100% containment and control, full extinguishment through mop up and reaching a fire out status can take days, weeks, or even months for large fires, particularly in areas with deep organic soils that can smolder for extended periods.
Why Understanding Wildfire Duration Matters
There is no single answer to how long wildfires last. Some fires are extinguished in hours, while others burn for weeks or months. Understanding the factors that influence duration, including fuel availability, weather conditions, topography, terrain accessibility, and suppression resources, helps explain why fires last different lengths and what to expect during an active fire incident.
Recognizing that average wildfire duration has increased dramatically, from 6 days in the 1970s to 52 days in the 2000s, helps set realistic expectations about how long fires may burn. Understanding the phases of wildfire suppression helps clarify when a fire is truly over versus when it is simply contained or controlled.
During an active wildfire, staying informed through official sources and following evacuation orders remain essential. Understanding that fires can persist for extended periods, and knowing the factors that contribute to longer duration, helps communities prepare appropriately and make informed decisions about safety and evacuation timing.