Since 1998 IM has been using the weather index of fire risk of the Canadian system FWI (Fire Weather Index)

The FWI was developed by the Canadian Forest Service and is used by various countries in the world, particularly in Europe. Through the use of this index, it is possible to estimate fire risk from the state of the various fuels present on forest soil, which is indirectly determined through observation of meteorological elements.

To estimate the fire risk of the Canadian system FWI, the following values, observed at 12 UTC, are taken into consideration: air temperature, relative humidity, wind speed and amount of the precipitation occurred over the past 24 hours (12-12 UTC). The FWI is a cumulative index, which means that the index value for one day reflects both the conditions observed on that day and their evolution along the time elapsed since the beginning of the index estimation.


The FWI is made up of 6 sub-indices estimated on the basis of the meteorological elements through which different possible states of the soil are assessed. The final FWI is then distributed, according to a scale of fire risk at district level, by five risk classes: Low, Moderate, High, Very High and Maximum, which correspond to the scale used during forest fire summertime, between 15 May and 14 October.


Since 2002 the FWI has been daily estimated by Instituto de Meteorologia without interruption along the year, with operational use in actions of forest fire prevention and combat, including in wintertime, when a new scale, also at district level, is used. This scale has only three levels: Low, Medium and High.


FWI Components

 

• FFMC (Fine Fuel Moisture Code) – This index classifies the fine dead fuels, with rapid drying rate, according to their moisture content. So, it corresponds to the degree of flammability of these fuels, which are on the soil surface. The moisture content of these fuels at 12 UTC on a given day depends on the moisture content at the same hour on the previous day, the precipitation (mm) occurred in 24 hours (12-12 UTC) and the air temperature (ºC) and relative humidity (%) at 12 UTC on the day considered. Wind speed only influences the drying rate of these materials.
• ISI (Initial Spread Index) - This index of initial spread of the fire depends on the FFMC sub-index and the wind speed (km/h) at 12 UTC.
• BUI ((Build up Index) - This index is a factor of evaluation of the vegetables that can build up a fire ("heavy" fuels on the soil) and is estimated from two of the sub-indices: DMC e DC.
• DMC (Duff Moisture Code)- This index expresses the moisture content of humus and woody materials of medium size up to nearly 8 cm under the soil surface. The humus index is estimated from the precipitation occurred in 24 hours (12-12 UTC), the air temperature and relative humidity at 12 UTC and the humus index of the previous days.
• DC (Drought Code) - his index is a good indicator of the effects of seasonal drought on forest fuels (humus and woody materials of larger size), which are under the soil surface, 8 to 20 cm deep. This drought index is obtained from the precipitation occurred in 24 hours, the temperature at 12 UTC and the previous day DC.
• FWI (Fire Weather Index) - This is the final index of the Canadian system, and is estimated as a function of its sub-indices ISI and BUI.