In autumn the weather was variable, as a warm September was followed by a colder than normal October, which in turn was followed by a mild November. The coastal waters cooled down by the end of October, which was cold considering the time of year; the first ice was observed at the end of October/beginning of November in the inner bays of the Hailuoto and Kokkola regions. At the beginning of November, sea surface temperatures in the pelagic areas were about one degree above seasonal average.

The first half of November was slightly colder than normal, whereas the second half was considerably milder than normal, which was reflected in the very slow cooling of the marine waters and in the melting of the first ice. When November changed to December, only the innermost bay ends were covered with ice in the Finnish sea areas. Compared to average temperatures, at the end of November the seawater temperatures were one degree above the long-term values.

Early December was still quite mild, but soon the weather got colder and ice began to form in the sea areas. In mid-December Finland was hit by an "Arctic cold" and the formation of ice in the sea areas quickly began. In mid-December there was ice in the northernmost part of the Gulf of Bothnia, the Vaasa archipelago and the ends of the Gulf of Finland. Seawater temperatures in the sea areas surrounding Finland were below six degrees. Compared to the long-term averages, the temperatures were still a half degree to one and a half degrees higher than normal. The colder than average weather lasted until the end of the month and, all in all, the month was colder than average in most parts of the country.

At the beginning of 2010, the weather continued to be cold. At that time, the Bay of Bothnia was ice-covered, with the exception of the most central pelagic areas. There was new ice in the Quark and also in areas off the fast ice edge along the Sea of Bothnia coast. There was thin ice in the archipelago of the Gulf of Finland, and in the Russian territory the pelagic ice was about ten centimetres thick. Ice was forming even in the Gulf of Riga. Seawater temperatures in the sea areas surrounding Finland were below three degrees. Compared to the long-term averages, no great differences existed in the temperatures any longer - the values were either about half a degree below or above normal.

The ice formation in the Gulf of Bothnia started about one to four weeks later than average, whereas the ice formation in the Gulf of Finland started even three weeks earlier than average.

The cold weather continued and more ice formed. On January 8, the entire Bay of Bothnia was covered with ice; a week later the amount of ice also increased in the Gulf of Finland and Gulf of Riga. After mid-January, southern winds made the ice field in the Bay of Bothnia drift towards northeast and an open water zone was created in the pelagic area on the Swedish side. January 27 was the windiest day of the month; the wind increased to storm-like conditions in the western sea areas. In the Bay of Bothnia, the ice drifted rapidly to the north-northeast, causing severe pressure in the ice field. During the following days, the ice drift also caused intense ice pressure in the Gulf of Finland. The low January temperatures, which in some places were abnormally low, ended with freezing temperatures and, on the last day of the month, the ice-covered area amounted to 175,000 km².

Approaching mid-February, new ice had formed quickly in the Sea of Bothnia and the Sea of Åland, by which time the ice had also spread to the northern Baltic Sea. In the Bay of Bothnia and Gulf of Finland the ice had grown even thicker.

The very cold weather continued in the second half of February, when arctic air flowed from the east into the Finnish sea areas. On February 17, the area covered in ice expanded to 244 000km², marking the maximum area for the winter. This took place almost two weeks earlier than normal.

At that time, the Gulf of Finland and the Gulf of Bothnia were covered with ice, with the exception of an approximately 6,500 km²-wide area in the eastern Sea of Bothnia off Rauma and Pori. The Gulf of Riga was also thoroughly covered with ice. In the northern Baltic Sea the ice edge ran from near Almagrundet to Glotovi and further on to the south-southwest. The central Baltic Sea off the coastline had a thin ice cover north of Öland and Klaipeda. The southern Baltic Sea off the coast of Germany was covered with thin fast ice in the insular areas. There was also thin ice and new ice in the Kattegat and Skagerrak.

On the third weekend of February (19 to 21/02/2010), the wind increased to storm readings and dangerously strong ice pressure occurred in the ice field of the Gulf of Finland. These stormy winds reduced the frozen area considerably. As the wintry weather continued, the frozen area expanded again, reaching nearly 244,000 km² on February 25. In winter 2009/2010, the temperature remained below zero for an unusually long period of time without a break - a total of 60 days. The frosty winter ended on February 26 and, with the winds blowing from the south, the frozen area began to decrease.

March was the fourth consecutive month which was colder than normal in Finland. Frost and strong winds alternated; with the frost, the frozen area expanded from time to time, only to decrease again because of the strong winds. In the second half of the month, mild weather and rainfall melted the ice in the northern Baltic Sea and the Gulf of Finland. The fast ice in the coastal regions of the Gulf of Finland and the southern Sea of Bothnia began to rot, while open spots appeared in the inner archipelago. In turn, there were large areas of open water close to the Estonian coast.

In April, the sea ice continued to rot and melt. The ice edge in the Gulf of Finland quickly shifted to the line Tiiskeri - Gogland - Bolshoy Tyters. In the Sea of Bothnia, the ice field was compressed, and areas of open water appeared off the coastal regions. Melting continued and the Gulf of Finland and the Sea of Bothnia were free of ice on April 19. In the Sea of Bothnia, the remaining ice disappeared at the end of April, while in the Gulf of Finland and the Bay of Vyborg it melted on May 2. At the time, in the Bay of Bothnia, winter lingered on: the coasts remained in 30-80 cm thick fast ice, and east of the line Rödkallen - the Quark there was 30-60 cm thick, compact and ridged drift ice.

In the following one and a half weeks, the coastal ice and the scarce pelagic drift ice in the southern part of the Sea of Bothnia melted away. In mid-May, there was an unusually long and widespread heat wave, considering the time of year. It also caused the ice field in the Bay of Bothnia to rot and melt away at a high speed. On May 17, the coastal ice in the Bay of Bothnia was gone, while large areas of even thick ice in places still remained in the pelagic areas.

The impact of the heat continued and, consequently, made the water warm up and the ice to melt. One week later, on May 24, only a few thicker floes of ice remained in the Quark, while there were some slightly larger areas of drift ice in the central part of the northern Bay of Bothnia. The last ice in the Bay of Bothnia melted at the end of May and the Bay of Bothnia was ice-free on May 31. 

The time of the final melting, compared to normal, varied greatly. In the coastal waters of the northern Bay of Bothnia the final disappearance of ice took place two to four weeks earlier than average and in the Gulf of Finland about one week to more than a month later than average.

The duration of the ice winter in the northern Bay of Bothnia was over a month shorter than average. In the southern Bay of Bothnia and in the Vaasa archipelago the ice winter was one and a half to two weeks shorter than average, whereas it in the Quark was two weeks longer than average. In the other sea areas surrounding Finland the length of the ice winter was a week (the metropolitan area) to more than a month (western Gulf of Finland) longer than normal.

In the Finnish part of the Bay of Bothnia, the maximum thickness of the fast ice was between 40 and 85 cm, in the Sea of Bothnia between 30 and 60 cm, in the Archipelago Sea between 15 and 50 cm and in the Gulf of Finland between 20 and 60 cm. The thickness of the ice in the pelagic Bay of Bothnia was 30–60 cm, in the Sea of Bothnia 10–40 cm, in the Gulf of Finland 30–45 cm, in the Sea of Åland 10–35 cm and in the northern Baltic Sea 10–35 cm.

Jouni Vainio and Patrick Eriksson, Ice Service