A climatology of large hail in Finland

Large hail can be life-threatening to public and very damaging to property. Thunderstorms with pea size hail in Finland are not very uncommon but severe hail is a rare event. Hail events have damaged cars and crops, broken windows in several cases and caused even slight injuries. In fact, very little is known about hail climatology in higher latitudes around the world. Finland is situated roughly between 60 and 70 degrees North with both oceanic and continental influences. This study will give some insight into high latitude severe hail climatology in Northern Europe.

This climatological study examines the temporal and spatial distribution of large hail (2 cm or larger) in Finland. Cases have been collected from old newspapers, from eyewitness observations and in recent years from the storm spotter reports. The place, date, time and possible damage were documented as accurately as possible, but few cases were left out due the unclear size or time information. This study includes large hail cases in Finland from the 1930xs to today.

The large hail season in Finland extends from late May through mid September. Some small hail has also occurred during winter. The peak month of large hail in Finland is July. More than half of the cases take place in July. June is slightly more favourable for large hail than August. May and September have received about 5 % of all the cases together. The largest hailstones (> 4 cm) tend to occur from late June to early August. There are large inter-annual variations in hail frequency. In recent years even more marginally severe hail cases have been reported. On the other hand, during the war years in the late 1930xs and early 1940xs, there were hardly any reported severe weather events. Some warm summers are related to higher than average hail reports. In cold summers, only a few cases, if any, is expected. The diurnal hail distribution shows that roughly 90 % of the cases occur between 1200 and 2200 local time. The diurnal peak for large hail is 1400 – 1800 local time.

Western and central Finland seems to have more severe hail cases than the most southern, eastern and northern parts of Finland. Lots of reports have been received from near the largest cities, which is natural in country that is sparsely populated. The largest hailstones (> 4 cm) have occurred in the middle and eastern parts of the country. The largest, 8 cm, hailstone ever reported and photographed in Finland occurred in August 1968 in the southeastern part of the country. The distribution of maximum hail size shows that almost half of the large hail cases are 2 – 3 cm in diameter. The two highest categories (6 – 7 cm and 7 – 8 cm) represent about 10 % of all cases.