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.
When assessing terrain flammability, the meteorologist has access not only to weather observations and weather forecasts, but also to a terrain drought evaluation index. 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 calculates the humus content in the surface moisture for layers with thickness of 3 cm and 6 cm. The thinner layer (3 cm) option is used for grass fire index and the thicker layer (6 cm) for forest fire index calculation.
Grass fire warnings based on the related index may be issued from April to May for those snow-free areas where the grass is not completely green. In addition, grass fire index may be used, if necessary, to assess ignition hazard on milled peat production areas. The forest fire index, for its part, is the basis for evaluating forest fire hazard.
The calculation of the surface moisture for Finland is based on data from all weather stations reporting at least once a day to the Finnish Meteorological Institute. Using these weather stations one gets a rough estimate of the regional variations of forest fire index, but estimates far from the stations are inaccurate.
Substantial improvement in local accuracy can be achieved using a lattice-based method, where the necessary weather variables are first interpolated into a regular grid lattice. 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 directly measured at about 20 automatic weather stations (AWS), and it can also be calculated from cloud observations obtained from 50 SYNOP observation stations. A total of 150 stations provide temperature and humidity measurements. The weather radar network covers almost the whole country. In areas where weather radar information is not available, rainfall measurements from automatic weather stations are used.