People are only just realising what a ‘40% chance of rain’ really means

It’s a natural part of being British to spend a good chunk of your day looking at the weather forecast – but it turns out loads of us don’t know what one key indicator means.

It’s no secret that this country has a propensity for rain, however, the “percentage chance” in the forecast is something frequently misread here in the British Isles.

In fact, the measure for the likelihood of rain is even read differently here than how it is in the US, making things even more complicated.

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Across the pond in the States and Canada, the “chance of rain” makes reference to a calculation looking at both how likely it is it will rain and how widespread that rain will be over a given area.

The answer to this equation is called the “probability of precipitation,” or “PoP,” which is a multiplication of forecasters’ confidence rain will happen, and the the amount of area they think it will cover.

It’s given as a percentage by presenters and weather sites over there, but here in Blighty, it's a little different.

In fact there is no single, universal method for meteorologists worldwide, with different techniques preferred depending on where you are on the globe.

The UK's Met Office doesn’t use the PoP equation because it can get confusing – and even misleading

A woman has taken to TikTok to express her utter shock at the PoP system used in the US, which is different to what is used in the UK.

User sydjkell took to the site in 2021 to express her astonishment at the variation and has since gone viral in a big way.

She said: “I thought that when they said there's a 30% chance of rain that it meant there was a 30 per cent chance that it was going to rain.

“I never ever ever knew it meant that there's a 100% [chance] it's going to rain and it's going to be in 30% of your area.”

This isn’t quite right, because if you were to have a 30% chance of rain using the PoP system, that percentage could be reached in a number of ways.

Because it's a calculation, it could be reached by a 100% (1) chance and 30% (0.3) coverage as discussed.

It is true that 1 multiplied by 0.3 is 0.3, or 30%, but 30% can be reached too with a 0.5 chance multiplied by 0.6 coverage.

To make things even more complex, in the UK it’s done differently again, something that was cleared up by the Met Office, who made their own video.

According to the Met Office’s experts: “The percentage of rain means the chance of rain at that time for that location.

“So 60% means a 60% chance of rain, 40% chance of dry.

“There are different ways of expressing percentages, and in the US they do use the area one, but generally not here in the UK.”

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