The Finnish Meteorological Institute (FMI) operates fourteen tide gauges, mareographs, on the Finnish coast. Measurement started at Hanko in 1887 and at Helsinki in 1904. The other stations were established in the 1920s, the latest at Rauma in 1933.
The difference between the highest and the lowest measured water level is over two meters at all the stations except at Föglö in the Archipelago Sea, where it is a bit lower. The water level fluctuations are strongest in the ends of the Bay of Bothnia and the Gulf of Finland: at Kemi, Oulu and Hamina the total variation is over three meters. The extreme sea levels are measured during autumn or winter.
Water level variations in the Baltic Sea are influenced by both the slow changes in the total water amount in the Baltic Sea and the short-term local variations. The local variations are mainly controlled by wind and air pressure as well as the ice conditions in winter. The tide has an effect of only a few centimeters at the Finnish coast.
The standing waves, seiches, in the sea are mainly caused by the wind and the air pressure. The seiche is a typical phenomenon for the combined basin of the Baltic Proper and the Gulf of Finland, and its effect on the sea level is emphasized in the ends of the bays. On a small scale, a similar phenomenon can be seen in a bathtub.
If a low pressure is dominating on the area, and a strong wind is blowing from the right direction simultaneously with a seiche piling water up towards the end of a bay, the water level may rise very high in the bay. If the total water amount in the Baltic Sea has accumulated several tens of centimeters over the average in advance, a possibility for an exceptionally high water level exists. The water level rose extremely high in the Gulf of Finland and on the east coast of the Baltic Proper in this kind of circumstances on 9th Jan 2005.
The sea level variations have seasonal differences because of the annual periodicities in the behaviour of the wind and the air pressure. The average sea level is at its highest in December and at its lowest in April-May. The short-term variability of the sea level is also strongest in winter, November-January, and smallest during summer, in May-July. Individual years, however, may differ greatly from each other, and this kind of average annual periodicity is not evident every year.