Finland's winter climate

The annual changes in temperature are of crucial importance for Finland's climate. It is natural to distinguish between seasons using thermal criteria, with seasons defined by the daily mean temperatures of 0°C and 10°C.

However, with this method the limits and lengths of seasons can vary greatly from year to year. Since Finland is located in the zone of prevailing westerlies where tropical and polar air masses meet, weather types can change quite rapidly, particularly in winter. In winter, the mean temperature remains below 0°C, but warm air flows can raise the daily high above 0°C at times.

Winter usually begins in mid-October in Lapland and during November in the rest of Finland, though not until December in the southwestern archipelago. It thus takes about two months for winter to proceed from Lapland to Åland. The sea and lakes retard the progress of winter. Winter is the longest season, lasting for about 100 days in southwestern Finland and 200 days in Lapland.

North of the Arctic Circle, part of the winter is the period known as the polar night, when the sun does not rise above the horizon at all. In the northernmost extremity of Finland, the polar night lasts for 51 days. In southern Finland, the shortest day is about 6 hours long

Permanent snow falls on open ground about two weeks after winter begins. The snow cover is deepest around mid-March, with an average of 60 to 90 cm of snow in eastern and northern Finland and 20 to 30 cm in southwestern Finland. The lakes freeze over in late November and early December. The ice is thickest in early April, at about 50 to 65 cm. In severe winters, the Baltic Sea may ice over nearly completely, but in mild winters it remains open except for the far ends of the Gulf of Bothnia and the Gulf of Finland.

The coldest day of winter comes well after perihelion, at the end of January everywhere except in the islands and coastal regions, where the slower cooling of the sea delays the coldest period until the beginning of February. The lowest temperatures in winter are from -45°C to -50°C in Lapland and eastern Finland; from -35°C to -45°C elsewhere; and -25°C to -35°C in the islands and coastal regions. The lowest temperature recorded in Helsinki is -34.3°C (1987). The lowest temperature recorded at any weather station in Finland during the past 100 years is -50.4°C (1985)

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