Every town on this site carries its own thirty-year rainfall record, month by month — what a normal one brings, and what the driest and the wettest on record actually delivered. Read all of it at once and one thing comes out immediately.
The average is the least interesting number in the file.
London, August
A normal August in London brings 59.5mm of rain. Across the last thirty years, one August brought 0.9mm. Not a dry-ish month — a month with essentially no rain in it at all, in a city whose reputation is the opposite.
Norwich has had an April with 0.5mm against a normal 42mm. Bristol has had an August with 1.3mm against a normal 72mm. Across the whole list, 709 town-months have at some point delivered under five millimetres.
An average is a summary of thirty different Augusts. It describes none of them.
The swing is the story
The widest single month on the list is July in Mumbai, which has run from 247mm to 1,150mm across the same thirty years — a gap of nearly a metre of rain, in one month, in one place. January in Cairns is second at 68mm to 901mm.
Those are not rounding errors around a central figure. They are two entirely different months that happen to share a name, and no planning built on the average between them survives contact with either.
Some places barely have a calendar
Divide a town's wettest month by its driest and you get a number that says nothing about how wet it is, only about how much the time of year decides.
Leeds comes out at 1.3 — its wettest month brings about a third more than its driest, and a gardener there can largely ignore the calendar.
Mumbai comes out at 1,706. A normal July there brings 682mm; a normal January brings 0.4mm. That is the monsoon, and it is the most extreme thing on this list by a wide margin — Sacramento, the next one down, is 309.
Which is the argument for checking
Not for checking a forecast — for checking what actually landed. A record this wide is a record you cannot plan against, and the only honest way to know whether your garden has had enough is to add up what reached it.