The Finnish Meteorological Institute (FMI) has announced that it will introduce a new ‘normal period’ at the beginning of October 2021, based on weather observations recorded between 1991-2020.
Normal periods are calculated across 30 years and describe weather averages and variation ranges in the recent past. The statistics help place the present weather in historical context, for example by making comparisons to see if the past month was colder or warmer than usual. In addition, statistics can be used for anticipating what the weather is most likely to be like at a certain time of the year, or what kinds of conditions people should prepare for.
Pauli Jokinen, FMI meteorologist, said, “Weather conditions vary from one year to the next. The weather conditions of an individual year do not give an accurate image of the climate of a specific locality, for example. To get that, we need to examine longer-term averages.”
The 30-year period covered by the normal period is long enough for exceptional weather conditions in a single year not to significantly affect the average, but it is also short enough for the climate not to undergo significant changes in the period.
For the first time, the FMI has produced both weather statistics and marine statistics from the same period. The marine statistics now being calculated include sea level height and sea surface temperatures as well as the area covered by sea ice.
The mean temperature of Finland in the new normal period is approximately 2.9°C, which is about 0.6º warmer than the previous normal period, running from 1981-2010. Compared with the 1961-1990 period, the average temperature has already risen by about 1.3º. The greatest change has been in December, and the smallest has been in June and October.
The annual precipitation in Finland with the new normal period is approximately 609mm. The amount has increased by about 2% from the previous period, 1981-2010, and by about 9% from the 1961-1990 period. Growth has been greatest in the winter months, from December to February. In August, precipitation has decreased.
“The changes that have been observed between normal periods are especially underscored when we look at winters in the south. The length of the thermal winter has decreased in the southwest of the country by more than two weeks compared with the previous normal period 10 years ago,” said Jokinen.
The duration of the permanent snow cover has decreased by one to two weeks in southern and central parts of the country, and by even more in areas near the coasts. In connection with updating the normal period statistics, the definitions of some weather terms have been updated, such as the rare and exceptional weather phenomena.