The Finnish Ice Service of the FIMR classifies the severity of the Baltic Sea ice seasons into five classes: extremely mild, mild, average, severe, and extremely severe. Classification is done according to maximum extent of ice cover in 1720-1996.
The maximum ice extent has been calculated for the day when the annual maximum has been reached. In calculations ice concentration, thickness or ice deformation degree have not been taken in account. The classification is based on the area of the total ice extent.
Charts below show three maximum ice conditions illustrating three kinds of severity classes. Season 1994 had similarities to season of 2003, and 1991 was a typical season in the 1990s. The charts illustrate, how even mild seasons effect winter navigation in Finland, Sweden, Russia, Estonia and Latvia. Only when ice season turns into severe, like in 1986, the ice has a major impact to the winter navigation of the other Baltic Sea countries.
1986 severe 337 000 km² | 1991 mild 122 000 km² | 1994 average 206 000 km² |
It should be noticed, that mild ice season could not be easy in perspective of ice navigation, and on the other hand, severe seasons might not be difficult. Average seasons, like 2003, are the hardest for ice navigation. The warm and windy periods between cold periods cause ice drift followed by ice pressure and ridging in the ice fields. Cold and calm periods increase amount of ice, which will with next windy period drift against the close drift ice edges, and brash ice barriers are formed, difficult to force.
The severity of ice seasons in 1720-2002. Explanations of the Finnish words in the chart:
hyvin leuto - exremely mild
leuto - mild
keskimääräinen - average
ankara - severe
hyvin ankara - extremely severe
