Seasonal change is dealt with in meteorology through heat accumulation. The so-called thermal autumn ends when average temperatures fall below zero so that the accumulation of heat begins to decrease. At the same time, the thermal winter begins.
In principle, a thermal winter can be defined even if there is only one day during the winter, when the average temperature remains below zero. In this case, winter would be that one day.
However, in mild winters with average temperatures on both sides of the zero, it is difficult to determine the time of the thermal winter between the winter months. We cannot predict with certainty whether the cold season will come or whether the weather will continue to be mild.
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Sometimes, the exact start and end dates of a thermal winter can only be determined in late spring. This will be the case this winter, at least in the south.
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However, this is not a unique situation, as in the winter of 2007-2008 the determination of the thermal winter in southern Finland was equally difficult. The daily mean temperature did not settle for longer than below zero until mid-March.
Also, the thermal spring in mild February and March can only be defined afterwards.
So let us now look at how this year is progressing.
Ville Siiskonen, acting as Agent,
meteorologist, Finnish Meteorological Institute.
Published in Science Journal 4/2020