The ice season 1998/1999 was average
As usual, the freezing process started in early November in the northern part of the Bay of Bothnia. Meanwhile, new ice was formed in the easternmost part of the Gulf of Finland, too, about three weeks earlier than the average. The ice cover expanded somewhat during November and in early December the ice situation was normal for the time of the year. In mid-December the weather became milder and the freezing process came to a halt. In late December new ice was formed in the northern Bay of Bothnia. In the eastern part of the Gulf of Finland the thin ice broke up and rafted at places.
In January the weather was varied. At first it was mild and windy and the ice in the northern Bay of Bothnia rafted and ridged. A cold spell ensued and ice was formed in all sea areas. The Quark was covered with new ice on January 10th. In the Gulf of Finland, eastern part, the ice reached as far as Gogland. In mid-January the mild and windy weather led to a break-up of the thin new ice and the ice floes in the northernmost Bay of Bothnia and in the Gulf of Finland drifted against the fast ice forming ridges. The weather changed cold again at the end of January and the Bay of Bothnia became entirely covered with ice on January 26th , i.e. over a week later than the average.
In early February a ridged ice belt formed off the fast ice edge in the northern part of the Bay of Bothnia. There was a fairly level ice cover off the coast, although thinner than usual. In the southern Sea of Bothnia and in the western Gulf of Finland there was less ice than usual. In early February the ice volume increased gradually in both sea areas. The ice cover reached its largest extent on February 11th, when the Bay of Bothnia was all ice-covered, the ice in the Sea of Bothnia reached 20-30 nautical miles off the coast, there was new ice in the northern part of the Sea of Åland and the ice cover in the Gulf of Finland extended to the line Utö-Pakri. At this point the ice covered as much as 157 000 km². In mid-February the ice cover in the Sea of Bothnia and in the western Gulf of Finland broke up and the floes drifted northeastward forming ridges along the Finnish coast but at the end of the month new ice was formed in these areas again.
In early March the ice in the Bay of Bothnia ridged along the Swedish coast whereas a lead covered with thin level ice was formed along the opposite Finnish coast. In the Gulf of Finland the ice remained along the Finnish coast for the whole month while the Estonian coast was ice-free to the area southeast of Gogland.
April was warmer than average and the ice started to rot and melt. The Sea of Archipelago was ice-free in mid-April, about a week earlier than the average. Soon afterwards, the Sea of Bothnia and the western part of the Gulf of Finland were ice-free, too, and at the end of the month, i.e. at about the average time, this was true of the Quark as well. The Gulf of Finland, eastern part, was ice-free by the end of April, about a week later than average. The month of May was colder than usual which meant that the break-up of ice was delayed; thus the last ice floes on the open sea in the Bay of Bothnia melted only in early June, about a week later than the average.
The thickness of the fast ice was 70–85 cm in the Bay of Bothnia, northern part, and 50–60 cm in the southern part, 40–50 cm in the Sea of Bothnia, 25 cm in the Sea of Archipelago and 40–60 cm in the Gulf of Finland. The ice thickness on the open sea was 40–60 cm in the Bay of Bothnia, northern part, and 20–40 cm in the southern part. In the Sea of Bothnia and in the Gulf of Finland, western part, it was 10–30 cm and in the Gulf of Finland, eastern part 25–50 cm.
The length of the ice season was average in the Bay of Bothnia, the Quark, the northern part of the Sea of Bothnia and in the Sea of Archipelago. In the southern Sea of Bothnia it was more than a week shorter than usual and in the Sea of Åland five weeks shorter. In the western part of the Gulf of Finland the ice season was about a week and in the eastern part about four weeks longer than usual.