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

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

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

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

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

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.

<< click for Newer posts

click for Older Posts >>

click to return to the main page

...or read more in my monthly archives
Jan24  Feb24  Mar24  Apr24  May24
Jan23  Feb23  Mar23  Apr23  May23  Jun23  Jul23  Aug23  Sep23  Oct23  Nov23  Dec23
Jan22  Feb22  Mar22  Apr22  May22  Jun22  Jul22  Aug22  Sep22  Oct22  Nov22  Dec22
Jan21  Feb21  Mar21  Apr21  May21  Jun21  Jul21  Aug21  Sep21  Oct21  Nov21  Dec21
Jan20  Feb20  Mar20  Apr20  May20  Jun20  Jul20  Aug20  Sep20  Oct20  Nov20  Dec20
Jan19  Feb19  Mar19  Apr19  May19  Jun19  Jul19  Aug19  Sep19  Oct19  Nov19  Dec19
Jan18  Feb18  Mar18  Apr18  May18  Jun18  Jul18  Aug18  Sep18  Oct18  Nov18  Dec18
Jan17  Feb17  Mar17  Apr17  May17  Jun17  Jul17  Aug17  Sep17  Oct17  Nov17  Dec17
Jan16  Feb16  Mar16  Apr16  May16  Jun16  Jul16  Aug16  Sep16  Oct16  Nov16  Dec16
Jan15  Feb15  Mar15  Apr15  May15  Jun15  Jul15  Aug15  Sep15  Oct15  Nov15  Dec15
Jan14  Feb14  Mar14  Apr14  May14  Jun14  Jul14  Aug14  Sep14  Oct14  Nov14  Dec14
Jan13  Feb13  Mar13  Apr13  May13  Jun13  Jul13  Aug13  Sep13  Oct13  Nov13  Dec13
Jan12  Feb12  Mar12  Apr12  May12  Jun12  Jul12  Aug12  Sep12  Oct12  Nov12  Dec12
Jan11  Feb11  Mar11  Apr11  May11  Jun11  Jul11  Aug11  Sep11  Oct11  Nov11  Dec11
Jan10  Feb10  Mar10  Apr10  May10  Jun10  Jul10  Aug10  Sep10  Oct10  Nov10  Dec10 
Jan09  Feb09  Mar09  Apr09  May09  Jun09  Jul09  Aug09  Sep09  Oct09  Nov09  Dec09
Jan08  Feb08  Mar08  Apr08  May08  Jun08  Jul08  Aug08  Sep08  Oct08  Nov08  Dec08
Jan07  Feb07  Mar07  Apr07  May07  Jun07  Jul07  Aug07  Sep07  Oct07  Nov07  Dec07
Jan06  Feb06  Mar06  Apr06  May06  Jun06  Jul06  Aug06  Sep06  Oct06  Nov06  Dec06
Jan05  Feb05  Mar05  Apr05  May05  Jun05  Jul05  Aug05  Sep05  Oct05  Nov05  Dec05
Jan04  Feb04  Mar04  Apr04  May04  Jun04  Jul04  Aug04  Sep04  Oct04  Nov04  Dec04
Jan03  Feb03  Mar03  Apr03  May03  Jun03  Jul03  Aug03  Sep03  Oct03  Nov03  Dec03
 Jan02  Feb02  Mar02  Apr02  May02  Jun02  Jul02 Aug02  Sep02  Oct02  Nov02  Dec02 

jack of diamonds
Life viewed from London E3

» email me
» follow me on twitter
» follow the blog on Twitter
» follow the blog on RSS

» my flickr photostream

twenty blogs
our bow
ian visits
broken tv
blue witch
on london
the great wen
edith's streets
spitalfields life
round the island
wanstead meteo
christopher fowler
the greenwich wire
bus and train user
ruth's coastal walk
round the rails we go
london reconnections
from the murky depths

quick reference features
Things to do in Outer London
Things to do outside London
London's waymarked walks
Inner London toilet map
20 years of blog series
The DG Tour of Britain
London's most...

read the archive
Apr24  Mar24  Feb24  Jan24
Dec23  Nov23  Oct23  Sep23
Aug23  Jul23  Jun23  May23
Apr23  Mar23  Feb23  Jan23
Dec22  Nov22  Oct22  Sep22
Aug22  Jul22  Jun22  May22
Apr22  Mar22  Feb22  Jan22
Dec21  Nov21  Oct21  Sep21
Aug21  Jul21  Jun21  May21
Apr21  Mar21  Feb21  Jan21
Dec20  Nov20  Oct20  Sep20
Aug20  Jul20  Jun20  May20
Apr20  Mar20  Feb20  Jan20
Dec19  Nov19  Oct19  Sep19
Aug19  Jul19  Jun19  May19
Apr19  Mar19  Feb19  Jan19
Dec18  Nov18  Oct18  Sep18
Aug18  Jul18  Jun18  May18
Apr18  Mar18  Feb18  Jan18
Dec17  Nov17  Oct17  Sep17
Aug17  Jul17  Jun17  May17
Apr17  Mar17  Feb17  Jan17
Dec16  Nov16  Oct16  Sep16
Aug16  Jul16  Jun16  May16
Apr16  Mar16  Feb16  Jan16
Dec15  Nov15  Oct15  Sep15
Aug15  Jul15  Jun15  May15
Apr15  Mar15  Feb15  Jan15
Dec14  Nov14  Oct14  Sep14
Aug14  Jul14  Jun14  May14
Apr14  Mar14  Feb14  Jan14
Dec13  Nov13  Oct13  Sep13
Aug13  Jul13  Jun13  May13
Apr13  Mar13  Feb13  Jan13
Dec12  Nov12  Oct12  Sep12
Aug12  Jul12  Jun12  May12
Apr12  Mar12  Feb12  Jan12
Dec11  Nov11  Oct11  Sep11
Aug11  Jul11  Jun11  May11
Apr11  Mar11  Feb11  Jan11
Dec10  Nov10  Oct10  Sep10
Aug10  Jul10  Jun10  May10
Apr10  Mar10  Feb10  Jan10
Dec09  Nov09  Oct09  Sep09
Aug09  Jul09  Jun09  May09
Apr09  Mar09  Feb09  Jan09
Dec08  Nov08  Oct08  Sep08
Aug08  Jul08  Jun08  May08
Apr08  Mar08  Feb08  Jan08
Dec07  Nov07  Oct07  Sep07
Aug07  Jul07  Jun07  May07
Apr07  Mar07  Feb07  Jan07
Dec06  Nov06  Oct06  Sep06
Aug06  Jul06  Jun06  May06
Apr06  Mar06  Feb06  Jan06
Dec05  Nov05  Oct05  Sep05
Aug05  Jul05  Jun05  May05
Apr05  Mar05  Feb05  Jan05
Dec04  Nov04  Oct04  Sep04
Aug04  Jul04  Jun04  May04
Apr04  Mar04  Feb04  Jan04
Dec03  Nov03  Oct03  Sep03
Aug03  Jul03  Jun03  May03
Apr03  Mar03  Feb03  Jan03
Dec02  Nov02  Oct02  Sep02
back to main page

the diamond geezer index
2023 2022
2021 2020 2019 2018 2017
2016 2015 2014 2013 2012
2011 2010 2009 2008 2007
2006 2005 2004 2003 2002

my special London features
a-z of london museums
E3 - local history month
greenwich meridian (N)
greenwich meridian (S)
the real eastenders
london's lost rivers
olympic park 2007
great british roads
oranges & lemons
random boroughs
bow road station
high street 2012
river westbourne
trafalgar square
capital numbers
east london line
lea valley walk
olympics 2005
regent's canal
square routes
silver jubilee
unlost rivers
cube routes
Herbert Dip
capital ring
river fleet

ten of my favourite posts
the seven ages of blog
my new Z470xi mobile
five equations of blog
the dome of doom
chemical attraction
quality & risk
london 2102
single life
april fool

ten sets of lovely photos
my "most interesting" photos
london 2012 olympic zone
harris and the hebrides
betjeman's metro-land
marking the meridian
tracing the river fleet
london's lost rivers
inside the gherkin
seven sisters

just surfed in?
here's where to find...
diamond geezers
flash mob #1  #2  #3  #4
ben schott's miscellany
london underground
watch with mother
cigarette warnings
digital time delay
wheelie suitcases
war of the worlds
transit of venus
top of the pops
old buckenham
ladybird books
acorn antiques
digital watches
outer hebrides
olympics 2012
school dinners
pet shop boys
west wycombe
bletchley park
george orwell
big breakfast
clapton pond
san francisco
children's tv
east enders
trunk roads
little britain
credit cards
jury service
big brother
jubilee line
number 1s
titan arum
doctor who
blue peter
peter pan
feng shui
leap year
bbc three
vision on
ID cards