How much rain is a lot, in millimetres?

A forecast rarely says "wet" or "dry". It says a number in millimetres, and most people have no feel for what that number means until they are standing in it. Is 2mm an umbrella day or a non-event? Is 10mm the kind of morning that ruins a picnic? We went to our own weather data to find out, rather than guess.

Where these numbers come from

Rainy Rewards checks the Met Office forecast every morning for each of our partner venues, and logs the forecast total for that day against the trigger the venue has set. That gives us a real run of forecast checks to look at rather than a textbook definition. The figures below are drawn from our six Manchester Rainy Rewards venues, covering the mornings between 3 June and 14 September 2026, 293 checks in total.

Most mornings are under a millimetre

The single biggest bucket, by far, is the one nobody notices: 237 of the 293 checks, 81%, came in under 1mm for the day. That is the everyday texture of a British forecast, a trace of drizzle or a dry morning that never quite delivers what the clouds threatened. It is also why the lowest triggers we see, 0.1mm at one venue on Hewitt Street, are set deliberately low: the venue wants its reward to go live on almost any wet morning, not just a proper downpour.

2mm: enough to notice, not enough to change your day

On 26 August 2026, the forecast for 87 Hewitt Street came in at 2.006mm for the day. That is a bracket 24 of our 293 checks landed in, 8%. Two millimetres is a morning where you would put a coat on without thinking about it, pavements go dark and stay that way for a while, and anyone stood outside for more than a few minutes notices. It is not the kind of rain that closes anything down.

5mm: proper rain

Three days later, on 29 August 2026, the same venue's forecast reached 5.035mm, one of 19 checks in our 2 to 5mm band (6.5% of the total). This is where rain stops being background weather. An hour of it outdoors is genuinely uncomfortable without a coat, ground starts to hold puddles rather than just shine, and it is the kind of day that would make most people think twice about an outdoor event without cover.

9 to 13mm: a wet one

At the top end of what we regularly see, Emmanuel Church in Didsbury had forecast totals of 8.576mm on 11 June and 12.695mm on 27 August. Only 9 of our 293 checks, 3%, reached 5 to 10mm, and just 3 went higher still, into the 10 to 15mm band. A day like that is a washout in the ordinary sense of the word: standing water on anything that is not draining well, outdoor plans genuinely at risk, and the kind of rain people plan their day around rather than shrug off.

The wettest morning in our data: 17.5mm

The single highest forecast total we logged in this period was 17.533mm, for 27 Blossom Street in Ancoats on 27 August 2026, the only check in our whole run to clear 15mm. That is a genuinely heavy day, the sort that makes the news for flooded roads rather than just damp pavements. It is also a useful anchor: everything else in this list sits well below it, which is itself the point. Most "rain" in a British forecast is a long way from that top end.

What this means for a trigger

This is exactly the judgement a Rainy Refunds trigger asks you to make when you buy cover for an event. A low trigger, similar to the 0.1mm and 1mm levels some of our Rewards venues use, pays out on almost any wet forecast, including the kind of drizzle that would not stop most people going out. A higher trigger, up in the 5mm or 10mm range this piece has walked through, only pays out on a forecast that would visibly disrupt an outdoor day. Neither is right or wrong, it depends on how much rain would actually spoil your event for you.

Whatever level you pick, the mechanic is the same one behind these numbers: at an agreed check time, Rainy Refunds compares the Met Office forecast for your venue and time window against your trigger, and if the forecast meets or passes it, your payout is sent automatically, no claim to make and nothing to argue about afterwards. Rainy Refunds policies are underwritten by Tokio Marine HCC; Rainy Refunds Limited is an Appointed Representative of Stubben Edge and Partners Limited, authorised and regulated by the Financial Conduct Authority.

If you have an event coming up and these numbers have helped you picture what a given forecast actually means, see how a Rainy Refunds trigger is set for your event in the Rainy Refunds app.