"The fire rage every way [...] the wind mighty high and driving it into the City; and every thing, after so long a drought, proving combustible, even the very stones of churches." That was Samuel Pepys, writing in his diary on September 2, 1666, the first day of the Great Fire of London. His account underscores the timeless influence of weather on fire outbreaks and spread. Even today, an ill-fated combination of temperature, humidity, rain, and wind can transform a stray spark into a city-wide inferno.
As climate change alters weather patterns, fire risks are changing too. Areas previously considered low-risk may emerge as new hotspots, while in regions already prone to wildfires the fire season is getting longer and more intense. We are already witnessing such shifts in the Wennington wildfire during the August 2022 UK heatwave and the Palisades Fire during the winter of 2024-2025.
To stay ahead of these evolving fire threats, anticipation is crucial. This article explores one of the primary tools used to predict and prepare for wildfire risks: the Fire Weather Index (FWI).
Case in point: by combining the FWI and land cover data, Callendar identified over 300 hazardous industrial sites in France at risk of wildfires, with risks projected to increase by around 50% by 2050 on average. This study prompted France to enact a new law, tightening undergrowth clearing regulations near industrial sites.
Fire Weather Index and climate in brief
The Fire Weather Index originates from Canada. It was developed by the Canadian Forest Service from the 1960s, its current version dates from 1984. It is now widely regarded as the reference indicator for anticipating wildfires, especially since the European Union started using it as a shared benchmark for assessing the risk of forest fires in 2007. Other noteworthy uses include the French "Météo des forêts" (a forest weather report broadcast daily in summer since 2023) or an adaptation of the FWI to anticipate fire risk in Malaysia...
The purpose of the Fire Weather Index is to assess the probability of a fire starting and spreading based on weather conditions: temperature, precipitation, wind, and relative humidity. Indirectly, it also takes into account sunshine.
The FWI is presented as a number, usually between 0 and 50. The higher it is, the greater the risk of fire. Thresholds can vary, but the risk is often considered high around 30 and very high from 45.
The FWI is calculated from two components:
The Initial Spread Index (ISI), which represents the likelihood of an ignition turning into a fire and the speed at which it will expand in its early phase.
The Buildup Index (BUI), which represents the quantity of fuel available for combustion that the fire will find to feed itself.
To simplify, we can say that when the ISI is low, there is little chance that a fire will break out. And when the BUI is low, there is little chance that it will be serious.
Both components are based on complex mathematical models, but both depend on temperature, relative humidity, and precipitation. The ISI also takes wind speed into account. It is therefore possible to assess the risk of fire in the next few days using weather forecasts. It is also possible to anticipate long-term risks based on climate projections (provided a multivariate correction method is used, but that's another subject).
The Initial Spread Index: Will a spark die out or ignite a blaze?
Let's take a closer look at what drives the various FWI components. The Initial Spread Index is itself calculated from the wind speed and the Fine Fuel Moisture Code (FFMC).
The FFMC quantifies the humidity of the forest litter: are the dead leaves, pine needles, twigs, etc., soaked or dried out? Or to put it another way: if a cigarette butt falls on the ground, is it more likely to go out or spark a fire?
Since the Fine Fuel Moisture Code depends on the moisture of the surface layer of the soil, it drops rapidly when it rains. On the contrary, it increases with temperature (hence evaporation).
The Buildup index: How much fuel is available?
The Buildup Index is a numeric rating of the amount of fuel available for combustion. It is also calculated from two other indices:
the Duff Moisture Code (DMC),
the Drought Code (DC).
The Duff Moisture Code is quite comparable to the Fine Fuel Moisture Code. Both represent the quantity of light fuel available to burn, but the DMC takes into account a larger layer of soil, up to about 5 to 10 cm deep. It is calculated from temperature, relative humidity and precipitation.
The Drought Code is an indicator of medium- to long-term dryness. For example, it will be higher in arid climates or after a seasonal drought. Its goal is to represent the quantity of water in the soil, logs, large wood debris, small watercourses, wetlands, etc. It only takes into account temperature and precipitation and is only an aggravating factor: if the Duff Moisture Code is already low, the Drought Code has no effect on the available fuel.
In addition to weather variables, both calculations take into account the duration of daylight, so the result varies with latitude and time of year.
Summary of the FWI computation process
If we summarize the calculation of the forest weather index, it gives this:
Although the intermediate indicators are relatively complicated, it is quite simple to understand their role: the Fine Fuel Moisture Code, Duff Moisture Code, and the Drought Code are all moisture indicators, but with different inertia. If a few dry days follow a rainy period, the FFMC will quickly reach high values because the surface layer of the soil dries rapidly. The DMC will gradually increase, and it will take much longer for the DC to change.
Use cases for the Fire Weather Index under the threat of climate change
With rising temperatures, shifting precipitation patterns, and more frequent extreme weather events, climate change is already having a discernible impact on fire risks. The likelyhood of the 2024 wildfire in Brazil, 2023 months-long blaze in eastern Canada and 2019-2020 bush fire in Australia were all increased due to climate change, resulting in devastating economic, environmental and human losses.
Climate change is creating new conditions for wildfires, including longer fire seasons, more extreme fires due to drought, forest degradation and increased fuel load. In this era of heightened wildfire risk, using climate projections to compute future Fire Weather Index values is an invaluable tool. Use cases include, for example:
Early Warning System: The FWI helps fire managers and policymakers anticipate and prepare for high-risk conditions, allowing for proactive measures like controlled burns and public alerts.
Resource Allocation: By pinpointing when and where the risk is highest, the FWI aids in allocating firefighting resources more effectively, saving both lives and property.
Adaptation Planning: Long-term projections using the FWI can guide land use planning and inform strategies to build resilience in the face of increasing wildfire threats.
Real Estate and Insurance: Factoring wildfire risk into housing prices and insurance premiums can help homeowners and insurers make more informed decisions.
The road ahead is challenging, but with tools like the FWI, we are not driving blind. By understanding and anticipating wildfire risks, we can take action to protect our communities, forests, and wildlife.
Take Action with Callendar. At Callendar, we are a leading startup specializing in climate risk assessments. Our offer includes worldwide FWI projections that are compliant with the best available science and cross-referenced with satellite data to detect the presence of forests or other combustible land cover.
Need a wildfire risk assessment accounting for current and future climate? Contact us now!