The Fire Weather Index (FWI) is an international standard for assessing daily fire risk in wildland areas. It indicates how conducive the weather conditions are to the ignition and spread of fire, as well as how intense a potential blaze could be. Simply put, the FWI answers the question: could a fire break out today, how fast might it grow, and how difficult would it be to control?
The origins of the FWI date back to the 1960s, when Canada developed a fire danger assessment system as part of the Canadian Forest Fire Danger Rating System (CFFDRS). The system was built on decades of research into the effects of weather and fuel moisture on fire behavior. Although the index was originally designed with Canadian forests in mind, numerous trials and studies have shown that it performs well across various biomes and climatic regions worldwide. Since 2007, the FWI has also been an official tool for assessing fire risk within the European Union.
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Three Basic Indicators – Fuel Moisture Codes
FFMC – Fine Fuel Moisture Code: Indicates the degree of dryness of vegetative fine fuels, such as leaves, needles, or grasses. It is the most dynamic component, responding even to weather changes over just a few hours, such as light rainfall or a temperature shift. A high FFMC value means that the vegetation is dry and ready to ignite, even a small spark can cause a wildfire. Conversely, low values should be interpreted as indicating high litter moisture, and therefore low susceptibility to fire. This code is used, among other things, to represent potential ignition, that is the readiness of vegetation to catch fire from an appropriate source of energy.
DMC – Duff Moisture Code: Reflects the moisture content of the duff layer located a few centimeters below the surface, within loosely compacted, decomposed organic soil layers (humus layer). This layer responds more slowly to weather changes than fine fuels (those referred to by the FFMC value), which is why a brief rain shower is often insufficient to moisten it. A high DMC indicates that a fire will be more difficult to extinguish, and mop-up operations may be more challenging due to greater depth of burning. An important application of the DMC is in assessing the risk of ignition caused by lightning strikes.
DC – Drought Code: This is the slowest-changing index over time. It refers to the deeper, densely compacted organic layers of the soil (fermentation layer) and accumulates the effects of prolonged drought. Very high DC values indicate that the entire organic layer and the soil are dried out, which can lead to (under)ground fires that are particularly difficult to detect and extinguish. Its main application is to indicate the seasonal effects of drought on forest fuels and the potential smoldering of deep organic layers and large surface logs.
The three basic components above (FFMC, DMC, and DC) describe the fuel moisture in three different layers of the forest ecosystem, each with a different rate of response to weather changes. For each of these three fuel layers, there is one moisture code, and their values increase as moisture decreases. The FFMC, which refers to fine surface material, responds the fastest. Just a few hours without precipitation, combined with sunny and windy weather, can cause a significant increase, signaling high flammability of the surface litter. The DMC, which reflects the moisture of the duff layer located a few centimeters (2–7 cm) below the surface, changes more slowly, requiring a longer period of drought or rainfall to noticeably shift its value. The DC, in turn, represents the deeply embedded organic layers and reacts very slowly, accumulating the effects of long-term weather trends. Based on these three indices, two intermediate indices (ISI and BUI) are calculated, which directly indicate the potential fire’s behavior and its ability to spread rapidly.
Two Intermediate Indices
ISI – Initial Spread Index: Derived from combining the calculated FFMC with the measured wind speed. It indicates the potential rate of fire spread. A high ISI means that the fuel is very dry and the wind is strong, resulting in a highly dynamic and fast-moving fire. Even if the amount of fuel is limited, the fire will spread rapidly and may cover a significant area in a short time. This is particularly important for operational units in the first minutes after fire detection.
BUI – Buildup Index: Calculated from the DMC and DC, it reflects the longer-term dryness of the forest environment. This index indicates how much fuel is available in the deeper layers and how long it can burn. A high BUI value means the fire will have plenty of fuel and can persist in the area for a long time. Even with a moderate ISI (not a very fast rate of spread), a high BUI means that once a fire starts, it can burn for a long time and with high intensity. For firefighting command units, this signals the potential for a prolonged suppression operation.
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Final Index
FWI – Fire Weather Index: The final index is produced by combining the ISI and BUI into a single numerical assessment of fire danger. The index value is dimensionless, meaning it has no unit of measurement. The FWI is a nonlinear indicator, which means that its increase and final value are not directly proportional to the input data. As the name suggests, the index is intended to indicate how today’s weather affects fire conditions. High readings of this index indicate that a fire may be both fast-moving and difficult to extinguish. Higher values signify greater intensity and larger flames.
Depending on the geographical region, the interpretative scale of the FWI may vary; however, its general purpose is to assign index values to the corresponding fire danger classes, ranging from low to extreme. For example, in Europe, data presented within the European Forest Fire Information System (EFFIS) are classified, interpreted, and displayed on maps using a six-level fire danger scale.
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The highest category, “Very Extreme,” was only introduced in June 2021 to better reflect the level of risk in areas that, during summer heatwaves, especially around the Mediterranean, regularly exceeded the threshold for “Extreme” danger. This category covers areas where the FWI value surpasses 70 points. In the EFFIS system, this scale is applied consistently across all countries under analysis, allowing for coherent spatial comparisons of fire danger levels in Europe, North Africa, and the Middle East.
The FWI is not just an analytical tool but a practical operational indicator. It can be used as a single value, but typically, its individual components are also considered, providing a comprehensive picture of the situation. The primary function of the FWI is to deliver a reliable, comparative assessment of potential fire intensity, enabling emergency services to allocate personnel and resources more effectively and to better plan and adjust firefighting operations according to the anticipated development of the fire. At the same time, the FWI serves as a universal indicator of fire danger in forested areas, used both for preventive measures and for public communication or managing access to forests. It is not merely a theoretical index, it is a tangible tool that helps protect people, the environment, and property in everyday practice.
The Fire Weather Index is a precise early warning signal. When properly interpreted, it can determine the success of an entire rescue operation. Modern wildfires are faster, more intense, and harder to control. The FWI offers a chance to stay one step ahead, but only if we know how to read and use it correctly.
