The ice formation in the innermost bays of the northern part of the Bay of Bothnia began with the end of November. At the beginning of December there was 1–5 cm thick new ice in the inner archipelago of the Bay of Bothnia. For the time of the year the situation was normal. The cold weather continued in the beginning of December, and the amount of ice increased. The first Finnish icebreaker began assistance on 13th of December. After this, the weather became milder. December and the first third of January ended up milder than normal.

Then weather became colder again and the rest of January was colder than normal, even exceptionally cold. In early February, the cold weather continued, and on the 7th of February, the culmination of the ice winter was reached – the extent of ice reaching 100 000 km².

After this, the weather became milder, and the southern winds pushed the ice fields together. The rest of February was unusually mild. In the beginning of March the ice extent was only 40 000 km². This small an ice cover has in the beginning of March previously been experienced in the years 1992 and 2008.

March was also warmer than usual. In the end of March the ice-covered area was unusually low, only about 20 000 km². April was also warmer than average and at the end of the month, ice appeared only in the Bay of Bothnia. Finally, the last ice melted in the Bay of Bothnia on 14th of May. The Baltic Sea became ice-free nearly two weeks earlier than usual.

The maximum thickness of the fast ice was 50–65 cm in the Bay of Bothnia, 25–40 cm in the Sea of Bothnia, 10–30 cm in the Archipelago Sea and 20–40 cm in the Gulf of Finland. The thickness of the pelagic ice was 30–60 cm in the Bay of Bothnia and 15–35 cm in the Gulf of Finland.

Jouni Vainio and Patrick Eriksson