Climatology of Severe Hail in Finland: 1930–2006

Abstract

A climatology of severe hail (2 cm in diameter or larger) in Finland was constructed by collecting newspaper, storm-spotter, and eyewitness reports. The climatology covered the warm season (1 May–14 September) during the 77-yr period of 1930–2006. Altogether, 240 severe-hail cases were found. The maximum reported severe-hail size was mainly 4 cm in diameter or less (65% of the cases), with the number of cases decreasing as hail size increased. In a few extreme cases, 7–8-cm (baseball sized) hailstones have been reported in Finland. Most of the severe-hail cases (84%) occurred from late June through early August, with July being the peak month (almost 66% of the cases). Most severe hail fell during the afternoon and early evening hours 1400–2000 local time (LT). Larger hailstones (4 cm or larger) tended to occur a little later (1600–2000 LT) than smaller (2–3.9 cm) hailstones (1400–1800 LT). Most severe-hail cases occurred in southern and western Finland, generally decreasing to the north, with the majority of the cases near population centers. The proportion of severe hail less than 4 cm in diameter is greatest over the agricultural area in southwestern Finland where crop damage caused by severe hail is more likely to be reported. The underreporting of hail is a particular problem across much of Finland because of the vast forest and lake areas, low population density, and relatively small hail swaths. Since the 1990s, a greater interest in severe weather among the general public and media, a storm-spotter network, improved communications technology, and an official Web site for reporting hail have increased the number of reported hail cases. According to the most recent 10 yr (1997–2006), Finland experiences an annual average of 10 severe-hail cases during 5 severe-hail days.