Enlarging the severe-hail database in Finland by using a radar-based hail-detection algorithm and e-mail surveys

Collecting hail reports to build and grow a climatology is challenging in a sparsely populated country such as Finland. Previous work has built a hail climatology using mainly microfilmed newspapers and newspapers' Internet databases. To continue to grow the dataset, a new approach was tried during summers 2008, 2009 and 2010 involving verification of a radar-based hail-detection algorithm. The output of the hail algorithm was monitored for high probabilities of hail for each convective cell. If the output from the hail algorithm reached a specified level, an e-mail survey was sent to business, libraries, village associations, local emergency personnel, hotels or summer cottage renters in areas suspected of receiving hail. If no e-mail addresses from the affected areas were found, telephone calls were tried instead.

This approach resulted in a large increase in the number of severe-hail reports during the past three summers. All the received hail reports were documented (even non-severe; excluding the graupel), giving a much better idea of hail occurrence in Finland. During summer 2010, the hail experiment was expanded to include the most experienced storm spotters. The experiment group divided into four areas (south, west, east, and north), with each responsible individual overseeing their own area.

Our study shows yet another way of successful hail-report collection in a sparsely populated country. A lot of valuable data has been gathered about the skill of the probability of hail from the algorithm in Finland. Results showed a 70% increase in severe-hail cases when the three-year period (2008–2010) was compared to a climatological study period (1930–2006). Furthermore, the expansion of the hail survey in 2010 resulted in a decrease in the replies to our sent e-mails, but a vast increase in hail reports, severe-hail reports, and severe-hail days compared to any prior hail season.