It's 40 days since St Swithin's Day.
When it rained.
n.b. It may not have rained for you but it rained where I was and that's what counts. I had to hide in a hedge near Heathrow to avoid getting drenched, and I thought ah well, rain every day until August 24th.
n.b. Obviously the St Swithin's legend has been disproved as rubbish, obviously, because dead Saxonbishops don't affect our weather. But I always enjoy testing a hypothesis with real data.
Here then is my day by day record of the 40 days after St Swithin's Day 2025.
If a day was wet - even one drop - I turned the square blue.
If a day was dry - i.e. no rain - I turned the square yellow.
I call it a SWITHINOMETER.
15
WET
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
n.b. Yes I know technically we don't know the colour of today's final square.
But I've seen the weather forecast for here in Norfolk and it's going to be yellow.
I will of course come back and change the table if this turns out to be incorrect.
n.b. Ah, it was incorrect. A few spots of rain on the windscreen outside the wedding barn mean I've had to recolour the table.
And indeed rewrite the post because it flipped the overall results.
You can see how the 40 days started out being mostly wet - St Swithin was doing well.
There were only five dry days in the first three weeks.
But then the weather changed (from low-pressure dominated to generally anticyclonic).
And only seven days in the last three weeks have been wet.
Here are the overall results.
July 15th
wet days
dry days
2025
wet
21
19
Another success there for the dead Saxon bishop.
His saint's day was wet, and the majority of the last 40 days have been wet.
It's by the slimmest of margins, but wet has beaten dry.
n.b. Technically it was supposed to be 40 wet days. But UK weather doesn't do 40 consecutive days of exactly the same thing, so I always count a majority as a win.
I've been recording the post-Swithin weather in my diary since 1980 so I have over 40 years of data.
I blogged about this in some depth back in 2022, so won't trawl over my four decades of personal data again.
But in summary St Swithin's Day has been dry 23 times and wet 18 times.
And Swithin then only predicted the dominant weather 24 times.
Here are the best St Swithin's Day predictions since 1980.
July 15th
wet days
dry days
1989
dry
7
33
1990
dry
7
33
In both 1989 and 1990 Swithin predicted dry weather and 82% of the subsequent days were dry.
The most successful 'wet' prediction was in 1985.
July 15th
wet days
dry days
1985
wet
32
8
No other year comes close.
But some predictions have been appallingly incorrect. Here are the worst two.
July 15th
wet days
dry days
1995
wet
6
34
2016
dry
33
7
Since 1980 Swithin's prediction has only been correct 54% of the time.
That's barely better than a coin flip.
In that time there have also been five 50/50 ties.
I should say this is all very dodgy data.
It's based on where I was in the country, not a specific location.
It's based on my observations, so I may have missed light rain while I was asleep.
Also my definition of wet is at least one raindrop, whereas most weather stations require more than a 'trace'.
Indeed, several of the blue squares in this year's table represent a mere brief splash, not a proper shower.
If I check the data from my favourite weather station in Hampstead, I get very different results.
July 15th
wet days
dry days
2025
dry (not wet)
7 (not 21)
33 (not 19)
This is because we've had a lot of days this summer with only a tiny bit of rain, which hasn't officially registered.
Indeed August in London has been ridiculously dry overall.
But I can finish off with one genuinely good conclusion.
This is because I have over 1600 days of data since 1980.
And on every single one of those days I have asked myself "did it rain today?"
Did it rain today?
yes
no
1980-2025
44%
56%
A lot of those will have been brief showers that didn't wreck the day.
But if you've ever thought "it rains quite often during the British summer, doesn't it?"
...the answer is yes it does.
