St Swithun’s forecast proved correct in 20 of the last 86 summers

Courtesy of the UKMO

The Met Office claim that the forty days of rainfall promised to follow as wet St Swithun has never happened, in fact they say that there has never been a spell of rain that has lasted for forty days, well that would be pushing it a bit, and I don’t think that was what the old saint meant anyway. He more realistically meant that each of the next forty days would be wet, and in that regard I think I can prove that he was indeed correct quite a number of times by using data that is freely downloadable from the Met Office website. According to the UKP daily rainfall figures for England and Wales, St Swithun has been proved correct in at least 20 of the last 86 summers. Of course, I’m stretching a point to its absolute limit with this assertion, because the UKP series is derived from gridded rainfall data, and I’m counting days with at least 0.01 mm of rainfall, but as you can see from the table below, the last time it occurred was as recently as 2015 and in fact it seems to have occurred quite regularly since 1997 for some reason. In other regions of the UK St Swithun is proved right just, if not more often than it is in England and Wales.

0.01 mm or more of rainfall

For the purists amongst you who like to define a ‘rain’ day as being a day with at least 0.2 mm of rainfall or more, then the number of wet St Swithun forecasts is drastically reduced of course, but not to zero.

0.2 mm or more of rainfall

Interestingly the 15th of July 1931 in the first year of the UKP series did herald a spell of forty days with 0.2 mm or more of rainfall, and here’s the graph to prove it.

England & Wales Rainfall 1931

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