The FWI index: what fire danger is and how it is measured
"Extreme fire danger" is a common warning in summer. Behind those colour-coded maps is usually the Fire Weather Index (FWI). This guide explains what it measures, what it does not, and how to use it.
1. What the FWI is
The Fire Weather Index is part of the Canadian fire danger rating system, the most widely used in the world. It estimates how far the weather favours a fire starting and spreading. It does not predict where fires will happen: it tells you how a fire would behave if one did start.
2. What it is calculated from
The system combines daily observations or forecasts of:
- Air temperature.
- Relative humidity.
- Wind speed.
- Accumulated rainfall.
From these it works out the moisture of different fuel layers, from fine litter to deep organic matter, and from that the ease of spread and the energy available. Because it has "memory", several hot, dry days in a row push the index up even if one particular day is not especially hot.
3. The danger classes
Copernicus EFFIS, the European Forest Fire Information System, calculates the FWI daily from forecasts by the European Centre for Medium-Range Weather Forecasts (ECMWF) and groups it into six classes: low, moderate, high, very high, extreme and very extreme. On our fire danger today page we show that class for every province in Spain.
4. What the FWI does not take into account
- The type and amount of vegetation: the same value does not mean the same thing in a dense pine forest as in irrigated farmland.
- The terrain, which speeds fire up on slopes.
- The causes of fires, which in Spain are mostly human (negligence and arson).
- Local phenomena such as dry thunderstorms or sudden wind shifts.
That is why the authorities combine it with other information when deciding on alerts and restrictions.
5. FWI, AEMET and regional indices
AEMET, the Spanish meteorological agency, publishes forecast maps of meteorological fire danger for several days ahead. Some autonomous communities also produce their own indices adapted to their territory, such as Galicia's daily fire risk index (IRDI). They may not match EFFIS exactly: when in doubt, the official warning from your region always prevails.
6. High-risk season and restrictions
Each autonomous community sets a legally defined high-risk season, normally in summer, during which burning, barbecues in the countryside or the use of certain machinery are banned or limited. On days of extreme danger, extra restrictions such as closing access to natural parks may apply.
7. How to use this information
- Before a hike or farm work, check the fire danger in your province.
- With high danger or above, avoid any activity that could produce flames or sparks.
- Check the restrictions in force on your regional government's website.
- If you see smoke or fire, call 112. Here is a guide on what to do if there is a wildfire nearby.
Frequently asked questions
Does today's fire danger apply to tomorrow?
Not really: the index is recalculated every day from the weather forecast. For the days ahead, check the fire danger forecasts from AEMET or Copernicus EFFIS.
Why is the danger extreme when it is not that hot?
Because the FWI accumulates the effect of previous days. After a dry spell with low humidity and wind, the vegetation can be so dry that the danger is extreme even if that day's temperature is nothing exceptional.
Does low danger mean fires cannot happen?
They can. Most fires in Spain are caused by human activity and can start at any danger level. With a low index they simply spread with more difficulty.
Updated: September 14, 2026 · incendiohoy.es