Synoptic Classification of Thunderstorms in Finland

Thunderstorm systems in Finland in 1998-2002 are classified into different types according to the synoptic-scale weather analysed on surface weather maps. A thunderstorm system is defined to include all individual thunderstorms within the country associated with the same surface synoptic factor, e.g. a front proceeding across the country. The factors are the cold front, warm front, occluded front and stationary front (with thunderstorm development nearly along the frontal line); and warm air mass, cold or polar air mass, and the pre-cold-frontal squall line. In addition, a distinction is made between air masses from the western sector, directly influenced by the Atlantic Ocean, and those from the eastern sector, having a more continental nature. Heavy- or moderate-thunderstorm systems occur mostly in the warm air mass and the cold front, and are also relatively frequent in the stationary front and the pre-cold-frontal squall line. Air-mass thunderstorms are more frequent than frontal storms, especially in the latter half of the warmest season, July-August. Thunderstorms, both frontal and air-mass, are most often associated with the western, Atlantic air mass, but a higher percentage of flashes is associated with the eastern, continental air mass. The thunderstorm-climatic variability between years can be characterized not only by the thunder days and flash density, but also by the nature of the air mass and the frontal activity.