The forest fire index

The forest fire warning system is designed to inform the general public and the fire authorities about forest fire risk caused by dry terrain. The Finnish Meteorological Institute meteorologist on duty has the final decision on the validity of the forest and grass fire warning.

In addition to weather observations (past weather) and forecasts (upcoming weather), the meteorologist assesses terrain's fire sensitivity using an index estimating terrain drought. The index is based on the assessment of dryness of the terrain using weather observations and a specifically designed calculation model.

The calculation model is used to evaluate the half-open treeless terrain (e.g. clear cutting) humus surface moisture. Open areas will dry faster than stand-intensive and thus more likely represent a flammable type of terrain. The model has been developed at the Finnish Meteorological Institute for Finnish conditions.

The model calculates the humus content in the surface moisture for layers with thickness of 3 and 6 centimetre. The thinner layer (3 cm) option is used for grass fire index and the thicker layer (6 cm) for forest fire index calculation.

Forest fire with brown smoke.
Kuva: Juho Aalto.

Grass fire warnings based on the related index may be issued from March to May for those snow-free areas where the grass is not completely green. The forest fire index, for its part, is the basis for evaluating risk of forest fires.

How we calculate forest fire index

For the calculation of the forest fire index, the so-called grid-based method applies, whereby estimating the value of weather quantities in a regular set of points, i.e. grid, where each point describes an area of a certain magnitude around it i.e. lattice screen. The grid in use is a uniform coordinate projection grid covering the entire country in a 10 km x 10 km lattice.

The model input consists of air humidity, wind speed, air temperature, solar radiation and precipitation. Solar radiation is assessed using a weather prediction model. Temperature, air humidity and winds speed are measured at observation stations. Precipitation is estimated using a weather radar network covering nearly the entire country and data from automatic weather stations.