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Weather and Climate | Climate in Finland | The seasons
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.
In winter, the mean temperature remains below 0°C, but warm airflows 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 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. 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 this century is -51.5°C (1999).
In spring, the mean daily temperature rises from 0°C to 10°C. Spring begins in early April in Åland and the southwestern archipelago and later in April elsewhere, except for northernmost Lapland, where it does not begin until early May. Thus, spring begins a month earlier in the south than in the north. Its duration ranges from 45 to 65 days, being longest in the islands and coastal regions, because of the coolness of the sea. Once the mean daily temperature exceeds 5°C, the thermal growing season is considered to have begun. This take place about one month after the beginning of spring: at the end of April in southern Finland and at the end of May in northernmost Lapland.
For the real growing season to begin the snow must melt; this depends on the amount of snow, elevation and the position of the region relative to the sea. Open areas lose their snow cover within two to three weeks of the beginning of spring, whereas on average the snow in the forest smelts about two weeks later. The lakes usually become ice-free soon after the growing season begins in April in southwestern Finland, in May in the interior, and in June in Lapland.
In summer, the mean daily temperature is consistently above 10°C. Summer usually begins in late May in southern Finland and lasts until mid-September. Summer in Lapland starts about one month later and ends a month earlier than the south coast.
The regions north of the Arctic Circle are characterized by 'polar days', when the sun does not set at all. The northernmost parts of Finland have 73 such days every year. Even in southern Finland, the longest day (around Midsummer) is nearly 19 hours long. The warmest day of the year comes about one month after aphelion, i.e. around July 20, in the whole of Finland.
The highest summer temperatures in the Finnish interior are from 32°C to 35°C. Near the sea and in the islands, temperatures over 30°C are extremely rare; the highest temperature ever recorded in Helsinki is 31.6°C. The highest temperature ever recorded is from July 9, 1914, when 35°C was exceeded in several places (the maximum being 35.9°C in Turku). Heat waves, with a maximum daily temperature exceeding 25°C, occur on an average of 10 to 15 days per summer inland in southern and central Finland, and 5 to 10 days in northern Finland and on the coast. In the course of the summer, thunderstorms occur on 8 to 14 days in the interior and 4 to 8 days on the coast and in northern Lapland.
In autumn, the mean daily temperature remains below 10°C. Autumn begins around the last week of August in northern Finland and about one month later in southwestern Finland. The growing season ends in autumn when the mean daily temperature drops below 5°C. This occurs around the last week in September in northern Finland and in late October or early November in southwestern Finland.
Thus, the average length of the growing season is 180 days in the southwestern archipelago, 140 to 175 days elsewhere in southern and central Finland, and 100 to 140 days in Lapland. The first snow falls in northern Finland in September, and elsewhere in October.
Look at also: The onset of thermal seasons
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