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

Figure 1 – Courtesy of Twitter

The Met Office have claimed on Twitter (fig 1) that the bit of St Swithun day weather lore nonsense about 40 days of rain has never come true – I beg to differ, and I can prove it using data that is freely downloadable from the Met Office website itself. 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 am 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 (fig 2), the last time it occurred was as recently as 2015, in fact it seems to have occurred fairly regularly in the last twenty years for some reason.

Figure 2

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, but not to zero. Interestingly, the 15th of July 1931 did herald a spell of 40 days of rain, and here’s a detail graph to prove it (fig 3).

Figure 3

For any of my loyal followers, you may have read a similar blog from me a couple of years ago, but I lost that when I deleted the old WordPress blog in a moment of madness a couple of years ago and thought that I would rewrite it.