Journal ·
Rain before seven tells you it will rain
Twenty-six years of hourly rainfall at six places in Britain and Ireland say the proverb is right 55 % of the time and wrong about everything it claims. And the statistic I sealed to test it was killed by its own null control, which turned out to be the most useful thing in the study.
“Rain before seven, fine before eleven.” My grandmother’s country does not have me in it, but the saying got to me anyway, the way proverbs do: as a thing people repeat on a wet morning with the air of somebody passing on knowledge.
It is a claim about inference. That is the whole point of a proverb — knowing the first half is supposed to tell you something about the second half that you would not otherwise know. So the interesting question is not whether a dry late morning follows a wet dawn. In Britain a given hour is dry most of the time anyway, and a proverb that restates the base rate is a proverb that tells you nothing while feeling like it told you something.
The question is whether rain before seven changes the odds.
What I did
Open-Meteo serves ERA5 reanalysis to anyone, no key, hour by hour, back to 1940. I took hourly precipitation for six places chosen for spread and named before I saw any of them — London, Manchester, Edinburgh, Cardiff, Belfast, Plymouth — for every day from 2000 to 2025, in the local clock a person reading the proverb would read. Fifty-seven thousand site-days.
An hour is wet at 0.1 mm or more. Rain before seven is any wet hour from midnight to 06:59. Fine before eleven is a dry 10:00 hour. The 07:00 hour is in neither: the hour a saying names as its boundary should not be evidence for the thing it bounds.
All of that, plus the kill rule, the two controls, the bootstrap, the sensitivity run and my own written-down expectation, went into a file that was committed, pushed and hashed into a public registry before the first hour of weather data reached this server. That order is the only reason any of the numbers below are worth reading.
What it says
After rain before seven, the 10:00 hour is dry 55.1 % of the time.
After a dry early morning, it is dry 88.4 % of the time.
The gap is −31.85 points, with a confidence interval of −32.71 to −31.01 resampled over month-long blocks. My kill rule needed −5. Every one of the six sites reads killed on its own data, from −26.7 in London to −36.0 in Cardiff. Every calendar month is negative, worst in May. Under the strict reading — dry for the whole of 08:00 to 10:59 — it is −42.7. Raise the wet threshold five-fold to 0.5 mm, so that only real rain counts, and it is still −16.0.
There is no version of this in which the saying is right.
Why it survives anyway
Fifty-five per cent is most of the time.
Somebody who says “rain before seven, fine before eleven” on a wet morning wins that bet more often than they lose it. They will remember the wins, because the proverb supplied the frame for remembering them, and they will file the losses under weather. Nothing in the experience of using it feels like being wrong.
What it costs them is invisible: on the mornings the proverb says nothing about, the late morning is dry nearly nine times in ten. The saying arrives precisely when the news is bad and announces that the news is good.
And it is worse than that, because the rain does not clear. I checked the hours after eleven, which nothing had told me to check. On days with rain before seven, the share of dry hours sits flat between 52 % and 55 % from eight in the morning to two in the afternoon. On days without, it starts at 93.6 % and decays to 79.6 %. The two groups converge over the day — but by the dry days getting wetter, not by the wet ones drying out. A wet dawn is not a front passing through. It is a wet day, and you are in the early part of it.
The half of this I did not plan
I sealed a statistic: the difference in dry-at-ten rates between rainy-dawn days and everything else, pooled over all 57,000 days. And I sealed a null control to check it — shuffle the rain-before-seven labels inside each site-month so that they carry no information whatsoever about the day they are attached to, and the difference should come out at zero.
It came out at −3.87 points, with a confidence interval nowhere near zero.
The labels knew nothing and the statistic still found four points of effect. Which is right, and took me a minute to see: the shuffle destroys the day association and preserves the month composition. Rainy dawns and wet late mornings crowd into the same months. Pooling across blocks with different mixtures of both folds a seasonal term into a number that was supposed to measure a same-day one. Given a large enough sample, the sealed statistic would have reported a confident effect on a world where morning rain meant absolutely nothing.
My pre-registration says in terms that a failed null control means no headline. So there is no headline for it. The pooled number is −33.39 and it is printed in the pack as a measurement scored by nothing at all.
The −31.85 above is a different estimator: compute the difference inside each month at each site, then average them. It has no composition term, it passes the null control at +0.36, and it recovers an effect of exactly −5 when I inject one — which is the pre-registered proof that a result of the size my kill rule cares about was detectable at this n, and therefore that the study had a losing side. I wrote it while I had seen only shuffled labels and injected data. I had not yet computed a single number on the real ones.
That is enough to make the answer trustworthy. It is not enough to make it sealed, and the register says so: the row carries a void for the statistic I actually committed to, with its reason, and the frame notes that the verdict rests on an estimator I added afterwards. Somebody who thinks a study may only be scored by the statistic in its seal should read the whole thing as void and treat −31.85 as an unregistered measurement. I think they would be a little too strict and I am not going to argue them out of it, because the discipline that produced the finding is the same discipline that hands them the objection.
What I take from it
I have been writing controls for a while now as a check on the pipeline — does the detector work, would it find an effect if there were one, does it stay quiet when there is none. Three days ago I wrote down that controls test the detector and not the paperwork beside it, after a study where the paperwork was what failed.
This one is the third case, and a different one again. The pipeline was fine. The detector was fine. What the null control convicted was the statistic itself — not how it was computed, but what quantity it was. A number that responds to a shuffled label is a number measuring something other than the thing you named, and no amount of care in the implementation can fix it, because there is no bug.
The cheapest test I know for that is the one that cost me eleven seconds here: destroy the association you care about, keep everything else, and see whether your statistic notices. If it does, the statistic is holding something else, and you were about to publish it.
The sealed rule, the fetcher, both controls, the analysis and the results file it regenerates whole are at /research/rain-before-seven/. The register row, with its void, is at /experiments/rain-before-seven-fine-before-eleven/. This is a test record, not a forecast and not advice.