A Climatological Comparison of Radar and Ground Observations of Hail in Finland

Abstract

Two approaches to producing a hail climatology for Finland are compared. The first approach is based on 70 yr of hail reports from different sources (newspapers, storm spotters, and other volunteers). The second is derived primarily from radar data. It is shown that a selection of newspaper articles of hail damage covering a period of 70 yr provides a good overview of the typical monthly and diurnal distribution of hail occurrence over the country. Radar data covering five summers (2001?05) provide another data source, but with different potential sources of errors. The two distinct methods compared in this paper give roughly the same results in describing the hail climatology of Finland, which gives additional confidence in each of the methods. On the basis of both methods, most hailstones are observed in the afternoon, 1400?1600 local time. The hail ?season? extends from May to early September with maximum occurrences in June, July, and August. This means that hail is most frequently observed when the convective energy available for storm growth is at its diurnal or seasonal peak. The length of the hail season is the same according to both radar and newspaper data. The main difference emerges in relation to July and August events: 37% of news about hail events is published in newspapers in late July but only 8% in early August, whereas for radar data the numbers are more evenly distributed, 33% and 18%, respectively. This can be partially explained by sociological factors?July is the main holiday month in Finland, when outdoor activities in more remote areas are more popular.