September warmer than normal in Finland for the tenth time in succession

8.10.2018 14:28

The number of days this past summer where temperature climbed over 25 degrees Celsius fell only one day short of Finland's record. The growing period's thermal time sum is already at a record high.

Photo: Bengt Wikström

Photo: Bengt Wikström

According to statistics maintained by the Finnish Meteorological Institute, with the exception of Lapland, September was unusually and, in some places, exceptionally warm. The average temperature in September varied between approximately 14 degrees Celsius in southern areas to just under 8 degrees in the north of Finnish Lapland. The average temperature for the month was 1 to 3 degrees higher than usual. September has similar temperatures on average every 10 to 30 years, a bit more often in Lapland. 

In terms of temperature, the month was divided into two. The beginning and middle of the month were very clearly warmer than usual, whereas the end of the month was colder. The month's highest temperature, 26.1°C, was recorded in Pyynpää Rauma on 8 September. This was also the last day when temperatures of over 25 degrees were recorded in Finland this past summer. In total, there were 64 days when this happened. This fell only one day short of the record for days with temperatures exceeding 25 degree, which was 65 days in 2002. Although no temperatures exceeding 25 degree were recorded after 8 September, temperatures did climb to over 20 degrees in some places even after the middle of the month.

On 26 and 27 September, a storm dubbed Kuisma travelled over the country, which caused quite a bit of damage and power cuts especially in the southern and middle sections of the country. The strongest average wind speed measured in sea areas was 26.q metres per second in Kylmäpihlaja, Rauma. The strongest recorded overland gusts were around 23 metres per seconds.  After Kuisma passed, cold air spread over our country from the north and temperatures fell below zero during the last nights of the month in nearly the entire country. The month's lowest temperature, -9.5 °C , was recorded in Muonio village on 29 September.

A larger than typical amount of rain after a dry summer

September was unusually rainy in most of the country. Especially in the area stretching from South Karelia to Northern Lapland, the month's overall amount of precipitation was exceptionally high in some places meaning that this happens once every 10 to 30 years. The September level of precipitation varied from around 50 millimetres in Ostrobothnia and Western Lapland to over 100 millimetres in Eastern Finland. The month's highest overall amount of precipitation, 148.6 mm, was recorded in Koivuniemi, Virolahti. The Pelmaa station in Seinäjoki received the least precipitation with just 34.6 millimetres.  Laukansaari island in Punkaharju, Savonlinna had the most precipitation in the span of one day on 11 September with 56.7 millimetres.

The first snow recorded in statistics fell on 25 September in Kilpisjärvi village. The snow-depth that morning was 2 centimetres.

Thermal time sum for the growth period already at a record high

In the south of Finland, this year's thermal growing period began during the latter half of April, while in the north it began in early May. By the end of September, the accumulated thermal time sum was at a record high with the exception of the most northern portion of Lapland. For example, a total of 1,829 day degrees has accumulated at Helsinki Vantaa airport, which exceeds the airport's previous thermal time sum record from 2011 (1,829 day degrees). Typically, the thermal time sum continues to grow in the south of Finland until the end of October, which means that we will only know the final sum later on. However, in the north, the thermal growing period is nearly at its end.

The thermal time sum accumulates during days were the average temperature for the day is more than +5 degrees.  The sum includes the portion of daily average temperatures during the growth period that exceed +5 degrees. If the day's average temperature is less than +5 degrees during the growing period, the sum does not grow but it does not decrease either.

Further information:

Weather statistics for September (in Finnish)

Weather statistics from the climate service, tel.  0600 1 0601 (EUR 4,01/min + local network charge)
Weather forecasts from the meteorologist on duty 24h/day tel. 0600 1 0600 (EUR 3,85/min + local network charge)