Long-term changes in the frequency of thunder days in the Baltic countries

Long-term changes in the thunderstorm climate of the Baltic countries and their relations with circulation weather types (CWT) during the period 1950-2004 were analyzed. The study area was located in northeastern Europe and encompassed Estonia, Latvia and Lithuania. Thunderstorm day (TD) data series collected by of observers at 40 meteorological stations were carefully checked for artificial inhomogeneities and used as the data source. During the study period, the average annual number of TDs decreased by about 24% in the Baltic countries with a rate of 0.9 TDs per decade. The maximum in thunderstorm frequency was observed during the period 1953-1963, and the minimum during 1990-1994. A long-term decrease in the TD frequency was accompanied by an increased frequency of northerly CWTs that are unfavorable for thunderstorm development. Meanwhile, the frequency of southerly and easterly CWTs that are favorable to thunderstorm formation decreased.