The wettest season begins: November is London's rain record-holder. All the numbers — and three jobs before the first storms

Every British conversation starts with the weather — so you may as well have the best lines in the room: exact numbers. We took the full observation record of the Heathrow weather station — the Met Office publishes it monthly back to 1948 — and worked out when it actually rains in London.

When does it rain in London — by the numbers?

The annual rainfall chart (1991-2020 normals) runs like this: quietest in spring — March 39mm, April 42mm; summer is wetter than assumed — August collects 54mm; but the true peak is autumn: September 50mm, October 65mm and November 67mm, the outright record-holder. By season: autumn 181mm, winter 161mm, summer 147mm, spring 127mm. The pattern is simple: the shorter the day, the wetter it gets — the Atlantic storm season winds up from October and holds the city into January. Planning something for 'the dry month'? That is March, not July.

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Is it true that it always rains in London?

The great myth of British weather collapses against one figure: 616mm of rain a year. That is less than Moscow, Rome or New York. London rain wins not by volume but by character: fine, frequent and arriving with wind — hence the cult of forecasts and the famous small talk. The rainy days are many; the litres are few.

What has changed over the record?

The Heathrow series lets us compare eras. The average daily maximum temperature over 1951-1980 was 14.3°C; over the last thirty years, 15.9°C — plus 1.6 degrees. Rainfall is up slightly, but the real change is how it arrives: in bursts. Four of the station's five wettest years on record came after 2000 (the record: 2014, with 864mm). Mild winters and rare but heavy deluges are the new normal — and what that does to bills and insurance, this series covers on Thursday.

Three jobs before the first storms

First: sign up for the Environment Agency's free flood warnings on GOV.UK — by postcode, phone call, text or email; the same service checks whether your address sits in a risk zone. Second: look at your gutters and downpipes before November, not after — a blocked gutter turns 'the wet month' into a wet bedroom. Third: open your home insurance policy and check it covers storm and flood — how buildings differs from contents and what matters for renters is in our home insurance guide. Tomorrow in this series: what Britons wear in the rain, and why nobody here wears a down jacket in winter.