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 of60 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.