A climatology of tornadoes in Finland

Tornadoes are more common in Finland than previously believed. This study summarizes some features of tornado statistics in Finland from 1796 to 2003. This study includes two datasets. The historical dataset is from the period 1796-1996, covering the time period when no systematic tornado documentation was maintained at the Finnish Meteorological Institute. The new dataset, from 1997 to 2003, covers the period when the Finnish Meteorological Institute has been actively collecting information on tornadoes in Finland. The methods of collecting, evaluation and classification of tornado reports has been different for these two datasets. In this study, the definition of a tornado, methods to collect reports, and the credibility evaluation process are briefly discussed.

Tornadoes occur in Finland from May through October. More than two thirds of the events occur during the statistically warmest months, July and August. The distribution of waterspouts is shifted towards late summer compared to all tornadoes. The maximum month is in August, when half of the tornadoes start over water. Also, the maximum for significant tornadoes is in August when more than a quarter of all observed tornadoes are significant. In the period from 1997 to 2003 there has been an average of 10 tornado cases each year. Most of the tornadoes overland occur in the late afternoon and evening, peaking at 17-19 local time. The diurnal distribution of waterspouts is more scattered throughout the day than tornadoes on land.

Geographically, the tornado density is highest in eastern Finland, in south-central parts of the country, and over the Gulf of Finland. The concentration of cases in the eastern half of the country is more evident with the significant (F2 or stronger) tornadoes. The density is lowest in Lapland (northern Finland) and in some inland areas of western Finland. The density of waterspouts is high over the Gulf of Finland, but also in the lake district of eastern Finland.

The strongest tornado recorded in Finland was of F4 intensity. A total of 35 significant tornadoes have been observed. Most of the observed tornadoes were of F1 intensity or less. Tornadoes and waterspouts occurring at coast were of F1 intensity or weaker. Almost all tornadoes that started over water were weak. In central eastern Finland the portion of significant tornadoes of all kinds is higher than elsewhere in Finland. In Lapland and in large parts in western Finland, significant tornadoes have not been observed.