“That we should experience such a high average temperature in Stockholm in May – 16.1 degrees – is an occurrence which, statistically speaking, happens just three times in a million years. So it was unusually warm; exceptionally, even, compared with what we tend to experience in May,” says Gustav Strandberg, climate researcher at SMHI's Rossby Centre. He has made a statistical analysis of one of the longest series of temperature measurements in Sweden – the “Stockholm temperature series” – with temperatures recorded from 1756.
The next highest average monthly temperate for Stockholm is from 1993, when the May average was 13.9 degrees, 2.2 degrees below this year’s average. This is also rare, statistically speaking only occurring once in over six hundred years. The average monthly temperature in Stockholm for May is 10.5 degrees, based on the meteorological normal period 1961-1990, which is used in today’s weather comparisons.