The ice season 1996/1997 was mild but longer than usual, owing to a cold spring
The sea began to freeze in the northern Bothnian Bay in mid-November, which is about the average time. Freezing was negligible until mid-December, when the ice cover off the Bothnian coast began to expand and new ice began to form in the eastern Gulf of Finland. This is about a week later than the average. Simultaneously new ice began to form in the coastal areas of the Bothnian Sea. This is about the average time for that region. For the rest of December the freezing process continued in the normal manner.
In early January the weather turned colder, speeding up the freezing process to such an extent that the Bothnian Bay was ice-covered almost in its entirety on January 10th. Soon enough the weather turned mild and windy and in the Gulf of Bothnia the ice began to ridge off the Finnish coast. In late January there was a new cold spell, as a result of which the entire Bothnian Bay was covered with ice approximately two weeks later than the average. Again a milder and windier time followed and in the Bothnian Bay the ice began to ridge heavily off the Finnish coast.
Early February saw the beginning of a colder period during which the largest ice extent, 129 000 km², was reached on February 18th, when the Bothnian Bay, the Quark and the Archipelago Sea were ice-covered throughout; on the coast of the Bothnian Sea there was a 20 mile wide area of ice and in the Gulf of Finland the ice cover extended from the east as far as the longitude of Helsinki. In the northern Baltic there was a narrow band of new ice along the coast.
In late February and early March the ice in the Bothnian Bay drifted to the NE, ridging on the Finnish side. Meanwhile a large ice-free area opened up in the southern part of the bay and on the Swedish side. By mid-March the weather turned cold again and on March 24th the ice cover was almost as extensive as on February 18th. Then the ice cover slowly started to recede, the weather remaining chilly almost for the whole spring.
The Archipelago Sea, the archipelago of the southern Bothnian Sea and the western Gulf of Finland were ice-free by late April - early May, about a week later than the average. The northern part of the Bothnian Sea became ice-free in early May and the eastern Gulf of Finland in mid-May, about two weeks later than the average. The heavily ridged ice in the Bothnian Bay melted slowly, with the last patch of ice off Ulkokalla melting as late as mid-June, approximately four weeks later than the average.
The thickness of the fast ice was 40–85 cm in the Bothnian Bay, 10–40 cm in the Bothnian Sea, the Archipelago Sea and the western Gulf of Finland and 30–45 cm in the eastern Gulf of Finland. Ice thickness off the coast was 40–60 cm in the northern and 10–40 cm in the southern Bothnian Bay. In the eastern Gulf of Finland, ice thickness was 15–50 cm on the open sea.
The ice winter was slightly longer than usual in the Gulf of Bothnia, apart from the Åland Islands, where it was about seven weeks shorter than the average. In the Gulf of Finland it was average, except for the open sea area to the west of Gogland, where it was about six weeks shorter than the average.