Surveys of Finland's climate are usually based on observations made by meteorological stations in what is known as a climatically normal period (30 years).
The climate of a region is ultimately determined by the radiation energy of the sun, and its distribution and temporal fluctuations. In Finland, the level of radiation changes considerably with the seasons and is also conspicuously different in the north and south. Due to cloud cover, the highest annual radiation levels are usually attained throughout Finland before aphelion, usually in early June. The daily maximum normally occurs before noon, also due to cloud cover. The annual amount of sunshine is highest (1900 hours) in the southwestern maritime and coastal regions and lowest (1300 hours) in eastern Lapland.
Cloud cover is especially abundant in Finland in autumn and winter, increasing from the northwest towards the southeast; about 65% to 85% of the sky is then covered in cloud. There are more overcast days (mean cloud cover over 80%) than clear days (mean cloud cover under 20%) or partly cloudy days. Clear days occur most frequently in May and June, and least frequently in November and December.
The mean annual temperature is about 5.5°C in southwestern Finland, decreasing towards the northeast. The 0°C mean limit runs slightly to the south of the Arctic Circle. Due to the warming influence of the Arctic Ocean, the coldest spot in Finland in terms of average annual temperature is not in the far north of Lapland, but in the northwest corner of Finland.
Temperature differences between regions are greatest in January, when the difference between southern and northern Finland is about 12°C; in June and July, this figure is only about 5°C. The continental nature of the climate is testified to by the difference in mean temperatures between the coldest and warmest months, that is, 20°C in the southwestern archipelago and 28°C in central Lapland. At the same latitudes elsewhere, this temperature difference varies even more greatly, from a few degrees in the North Atlantic to over 60°C in central Siberia.
Since Finland is located in the zone of westerly air disturbances, there are great variations in air pressure and winds, especially in winter. In the whole country, the wind blows most commonly from the southwest and least commonly from the northeast. The average wind speed is between 2,5 and 4 m/s inland, slightly higher on the coast and 5 to 7 m/s in maritime regions. Wind speeds are typically highest in winter and lowest in summer.
Moderate winds are typical of Finland; high winds are rare, particularly inland. Storms, with a wind speeds over 20 m/s, occur a few times every month on the open sea in autumn and winter. In spring and summer, storms are rare; inland storms are rare in all seasons, although occasionally devastating local tornadoes or downbursts may occur during summertime.
Humidity is dependent primarily on temperature. The humidity of the air is highest in July and August and lowest in February. Like temperature, humidity decreases towards the north. Relative humidity reaches its mean high in November and December (90%) and its mean low in May and June (65% to 70%). The figure does not vary very much within any region in any season.
Fog is most common in autumn, in southern and southwestern Finland, usually at night and early in the morning. In winter, though, fog can occur in daytime too. Early winter is often quite foggy in the 'fog corridor', about 40 to 80 km from the coast.
The Finnish climate is characterized by irregular rains caused by rapid changes in the weather. Only summer showers and thunderstorms show some sort of regularity, with rain occurring mostly in the afternoon and early evening. The annual precipitation in southern and central Finland is between 600 and 700 mm except near the coast, where precipitation is slightly less, particularly in Ostrobothnia. In northern Finland, where about half of the precipitation is snow, the annual precipitation is only about 600 mm.
The annual variation in precipitation is similar throughout the country. The driest month is March. The precipitation gradually increases until July and August, or until September and October on the coast, after which it decreases towards winter and spring. The lowest annual rainfall may be 200 to 300 mm and the highest annual rainfall 700 mm in northern Finland and 900 to 1100 mm elsewhere. The highest recorded daily precipitation is about 150 mm, excluding a few rare exceptions (such as 198.4 mm in Lahnus, Espoo, on July 21, 1944). Excluding the coastal regions, at least half of the days in a year have some form of precipitation (at least 0.1 mm). Daily precipitation exceeding 10 mm is recorded on 10 to 15 days every year.
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