Source: The Library Of Congress Country Studies
Latitude is the principal influence on Finland's climate. Because of Finland's northern location, winter is the longest season. On the average, winter lasts 105 to 120 days in the archipelago and 180 to 220 days in Lapland. This means that southern portions of the country are snow-covered about three months of the year and the northern, about seven months. The long winter causes about half of the annual 500 to 600 millimeters of precipitation in the north to fall as snow. Precipitation in the south amounts to about 600 to 700 millimeters annually. Like that of the north, it occurs all through the year, though not so much of it is snow.
The Atlantic Ocean to the west and the Eurasian continent to the east interact to modify the climate of the country. The warm waters of the Gulf Stream and the North Atlantic Drift Current, which warm Norway and Sweden, also warm Finland. Westerly winds bring the warm air currents into the Baltic areas and to the country's shores, moderating winter temperatures, especially in the south. These winds, because of clouds associated with weather systems accompanying the westerlies, also decrease the amount of sunshine received during the summer. By contrast, the continental high pressure system situated over the Eurasian continent counteracts the maritime influences, causing severe winters and occasionally high temperatures in the summer.
Data as of 1988