diamond geezer

 Thursday, August 25, 2022

One of our most famous pieces of weather lore, right up there with "red sky at night...", is this rhyme related to St Swithin's Day.
St Swithin's day if thou dost rain
For forty days it will remain
St Swithin's day if thou be fair
For forty days 'twill rain nae mare
It's superstitious nonsense, obviously, because the atmosphere doesn't realise the date is July 15th and because an English summer is always somewhat changeable. A dead bishop interred in Winchester cannot affect our weather.

But there can be a nugget of truth in the saying. Some summers the jet stream settles to our north displacing all the wet weather and some years it settles over us bringing in an endless sweep of rain-bearing depressions.

I mention this because August 24th marks exactly 40 days after St Swithin's Day, which means today is the ideal day to look back and tot up this summer's weather to test St Swithin's hypothesis.
July 15th: dry
St Swithin's Day 2022, it may not surprise you to hear, was dry.
July 16th - August 24th: dry, dry, wet, wet, wet, dry, wet, dry, dry, dry, dry, dry, dry, dry, dry, wet, dry, dry, dry, dry, dry, dry, dry, dry, dry, dry, dry, dry, dry, dry, wet, wet, wet, dry, dry, dry, dry, dry, wet, dry
And it's only rained on nine of the subsequent 40 days, the other 31 being bone dry, which is exactly what you might expect in a drought-ridden summer.

n.b. my count is a personal count based on where I am. If it rains where I am, even slightly, then this counts as a wet day. If it doesn't rain where I am, not even a drop, then this counts as a dry day. It's an appallingly self-centred way of measuring rainfall, so meteorologically very suspect, but it does have the advantage of being consistent over time.

 July 15thwet daysdry days
2022dry931

Well done St Swithin... not spot on, but pretty close.
"'Twill rain nae mare" is a pretty much perfect summary of the last six weeks.

But one year tells us nothing. What we need is a lengthier series of data over several years and then we might be able to see some patterns. Fortunately I've been recording St Swithin's Day weather in my diary since 1980, because of course I have, and also counting how many of the subsequent 40 days were wet or dry.

 July 15thwet daysdry days
1980wet1327
1981dry1129
1982wet1525
1983dry931
1984wet1228
1985wet328
1986dry1624
1987wet2911
1987dry1921
1989dry733

In 1980 St Swithin was very wrong - July 15th was wet but most of the subsequent days were dry. 1982 and 1984 were similarly off-key, with a slew of dry days following a wet July 15th. But there are three stonking successes here, namely 1983 and 1989 which were forecast to be dry and it hardly rained afterwards, and 1985 which was forecast to be wet and it carried on chucking it down. So maybe there is something in all this after all.

Let's continue.

 July 15thwet daysdry days
1990dry733
1991wet1327
1992dry2119
1993wet2218
1994wet1426
1995wet634
1996dry1624
1997dry1228
1998dry1822

1990 was a proper success, a full-on sunny summer following a prediction of fair weather. But 1995 was a disaster, almost exactly the same split as 1990 but predicted to be wet throughout. Most of the other years were a bit more balanced. If you're counting, St Swithin has only swung the wrong way here seven years out of nineteen.

Sadly I stopped recording the summer's weather in my diary in 1999 because I was too focused on other events. It took until 2004 for me to pick up the baton again, and I've recorded it diligently ever since.

 July 15thwet daysdry days
2004wet2416
2005dry2020
2006dry2020
2007wet2119
2008dry2416
2009wet2020
2010wet1624
2011wet2119
2012dry1822

These are much closer totals, with the number of wet and dry days never dropping below 16 or rising above 24. We also have the first three dead heats - 2005, 2006 and 2009 - making St Swithin's original prediction irrelevant. If you're counting, July 15th has been a dry day just as often as it's been wet.

 July 15thwet daysdry days
2013dry1921
2014dry1624
2015wet1921
2016dry337
2017dry2317
2018dry1426
2019dry1822
2020wet1426
2021dry2020
2022dry931

Again most of these years have fairly equal totals for wet and dry days, but there are two glaring outliers. 2016 was ridiculously wet, but followed a dry July 15th so St Swithin was wrong. And this year has been ridiculously sunny, and followed a dry July 15th so St Swithin was right. It's interesting that over the last decade July 15th has been dry a lot more often than it's been wet, whereas beforehand my data was running fifty-fifty.

Like I said a single year tells us nothing, but maybe we can draw some overall conclusions from the period 1980-2022.

• St Swithin's Day was dry 58% of the time and wet 42% of the time.
• The subsequent 40 days were mostly dry 63% of the time and mostly wet 29% of the time.
• St Swithin correctly predicted the general gist of the subsequent 40 days 55% of the time.

That's not much better than flipping a coin. It'd be more accurate if the rhyme just said...
Whatever St Swithin sees in the sky
For forty days 'twill likely be dry
• Only 5 summers out of 38 have been more than 75% dry (including this year).
• Only 2 summers out of 38 have been more than 75% wet (1985 and 2016).
• St Swithin only predicted five of these correctly.

More usefully...

• On average, the 40 days after St Swithin's Day include 18 days when rain falls and 22 dry days.
• i.e. an English summer is mostly dry.

And most importantly...

• The St Swithin's Day rhyme has never been true.


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