The exceptionally mild winter 2014/2015 was followed by a summer that was colder than usual. The deviation from the long-term average was small, however, remaining below 1 °C across the whole country. In the sea areas, June and July were about 1.5 °C cooler than usual, whereas August was about 2 °C warmer than average.

Because the temperatures in June and July were relatively cool, sea water warmed up slowly in the Finnish coastal areas. At the end of June, the sea surface temperature rose above 15 °C in coastal areas. Compared to the previous two years, the highest seawater surface temperatures remained 3–5 °C lower. Due to a warm August, surface waters began to cool down between the end of August and beginning of September, which is considerably later than usual.

The average air temperature in September was higher than normal throughout the whole country. September was almost 3 °C warmer than usual in the Finnish sea areas and the seawater temperature in the Baltic Sea was higher than average. The average temperature in October, on the other hand, did not deviate largely from the statistical mean: the average air temperature was less than 1 °C lower than usual in the southern part of the country, while the northern part of the country was mainly less than 1 °C warmer than average. The sea areas were 0.5–2 °C warmer than usual, which at the beginning of November was reflected in sea water temperatures that were 1–3 °C warmer than usual in the Baltic Sea.

November was exceptionally warm in many areas. The month was even record mild in several areas in the southern and western parts of the country. Compared to the usual deviation, the temperature deviation in November was between just under 3 °C and about 5 °C. In sea areas, November was 3.3 … 5.1 °C warmer than average. As a whole, the average temperature in the autumn, i.e. between September and November, was exceptionally high in the whole country, as the temperature deviation in most of the country was over 2 °C, and in some areas in the north over 3 °C.

At the beginning of the winter, sea water in the Finnish territorial waters was still between just over 1 °C and just over 3 °C warmer, and the first ice did not form in the far reaches of the Bay of Bothnia until the last days of November. December was unusually mild in the southern and central parts of the country, in some areas even exceptionally mild. A December as mild as this repeats on average once in 10–30 years in the current climate. December was also considerably warmer in the Finnish sea areas, with average temperatures 4–5.5 °C higher than usual. At the end of December, sea water was still between more than 1 °C and about 3 °C warmer than average. Around the turn of the year, the weather cooled considerably and ice started to form. The icebreaker Kontio began assistance in the Bay of Bothnia on New Year's Eve – at that point the area covered by ice was almost 10 000 km².

January began with cold temperatures and cold weather continued for about three weeks. According to statistics, January was colder than average throughout Finland, with southern and central parts of the country even experiencing exceptionally low temperatures. The temperature deviation ranged primarily from four to seven degrees. The weather in the sea areas was 2–5 °C colder than average. In terms of temperature, the month was divided into two distinctly different periods: the first three weeks of the month were mostly very cold, but the end of the month saw a considerable increase in temperature, and south-westerly winds. The maximum ice extent of the winter 2015/2016 was reached at the end of the cold period, on 22 January. Ice then appeared along the whole Finnish coast and the thickness of fast ice was 10–50 cm. Most of the Bay of Bothnia was covered by 10–25 cm thick very close ice. The Quark had 5–25 cm of very close ice and new ice. In the coastal areas of the Sea of Bothnia, fast ice covered the archipelago and very close ice the area about ten nautical miles further out to the sea. The Archipelago Sea was covered by thin level ice. In the Gulf of Finland, the archipelago was covered by fast ice and the pelagic area east of the line between Hanko peninsula and Tyters mainly by 5–15 cm of very close ice. Väinämeri was covered by 10–20 cm of fast ice and the eastern and northern parts of the Gulf of Riga by very close ice. Over the rest of the Baltic Sea, some parts of the coastal areas were covered by new ice. The extent of the area covered by ice was 110 000 km², which means that, statistically, the winter was a mild one and most of the cold weather occurred at the beginning of the winter. By the end of January, the ice-covered area had shrunk to only 66 000 km².

Like in the previous year, February was unusually mild in many areas. The average air temperature in February was about 5 °C higher than usual in land areas, in the east even 6–7 °C higher than usual. The temperatures in the sea areas surrounding Finland were 3.5–5.5 °C higher than usual. The winter as a whole (December–February) was on average milder than normal and precipitation levels were unusually high in many areas; even record-breaking levels were observed in central parts of the country. During the mild February, the ice extent did not increase or decrease significantly and was 62 000 km² on the last day of the month.

March was 2–4 °C warmer than the long-term average. March was also warmer than average in the sea areas, with deviations between +2.3 … +3.2 °C. The extent of the ice cover did not vary significantly during the first three weeks in March, but started to decrease during the last week. At the end of March, an area of some 31 000 km² was covered by ice.

All in all, the ice extent during February and March remained relatively unchanged, while varying between 70 000 km² and 55 000 km². On frosty days, new ice formed in open areas, after which it again was compressed together by southerly winds. The ice was thickest in the northeastern corner of the Bay of Bothnia, where the thickness of the ice was mainly 50–75 cm. Most of the pelagic ice field was ridged ice and the thickness of level ice was 40–60 cm. In the Gulf of Finland, some ice remained only in sheltered bays and off Vyborg and St. Petersburg. Coastal ice gradually began to rot.

Like the previous months, April was warmer than usual – generally more than 2 °C warmer than average. In the sea areas, April was +1.7 … +2.2 °C warmer than average and the remaining ice cover was only 16 000 km² at the end of April. The Gulf of Finland became free of ice on 16 April, when the last pieces of ice melted in the Bay of Vyborg.

The warm weather at the beginning of May melted the remaining ice fairly fast and the last ice chart was published on 12 May, when rotten ice covered the remaining area of only about 800 km² in the central parts of the northern Bay of Bothnia. The Baltic Sea was completely free of ice on 14 May.

The first ice in the Bay of Bothnia, the Quark and the northern parts of the Sea of Bothnia started to form 3–4 weeks later than average, in the southern parts of the Sea of Bothnia and in the Archipelago Sea around the normal time, and in the Gulf of Finland slightly earlier than normal. The ice finally disappeared from the Gulf of Bothnia 2–4 weeks earlier than usual. The ice winter in the Gulf of Finland ended between just over one month and two months earlier than usual.

The maximum thickness of the fast ice was 50–75 cm in the Bay of Bothnia, 30–45 cm in the Sea of Bothnia, 5–20 cm in the Archipelago Sea and 25–40 cm in the Gulf of Finland. The thickness of the pelagic ice was 40–75 cm in the Bay of Bothnia and 5–25 cm in the Gulf of Finland.

The distance in which vessels had to navigate through ice was on 22 January 2016 from St. Petersburg to the ice edge 122 nautical miles and from Kemi altogether 144 nautical miles (divided into two legs of 64 and 80 nautical miles).

Jouni Vainio, Iceservice