What does the percentage of rain mean on weather forecasts and the iPhone app?

What does the percentage of rain mean on weather forecasts and the iPhone app?
What does the percentage of rain mean on weather forecasts and the iPhone app?
London’s faced some pretty torrential rain of late (Picture: PA)

The British weather can be unpredictable at the best of times, shifting from baking sunshine to torrential rain within a matter of days or even hours.

So it’s good to know that we can rely on the forecasts – on TV, online or even on our phone’s weather app – to let us know if we should take an umbrella with us when we’re setting out.

You’ve probably noticed, though, that when it comes to rain, it’s predicted by way of a percentage.

Just what does that percentage mean when it’s on forecasts – and on the app on your phone?

What does the percentage of rain mean on weather forecasts and the iPhone app?

Scarlett Moffatt recently caused a bit of a stir on TikTok when she suggested that a ’30 per cent chance of rain’ listed on a weather app meant that rain would fall in 30 per cent of the surrounding area.

However, that’s not actually true – as the percentage refers to the chance of rain falling in a specific region, rather than how much rain might actually fall.

Weatherman Alex Deakin said in a TikTok: ‘Certainly for the Met Office app, the percentage of rain means the chance of rain at that time for that location.

iPhone weather app showing weather in London
That percentage doesn’t mean how much of your area can expect rain (Picture: metro.co.uk)

‘So 60 per cent means a 60 per cent chance of rain, 40 per cent chance of dry.

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

‘So if you’ve got the Met Office app, that’s one less thing to worry about; mind unblown.’

The Met Office have gone into further detail on their website, explaining the probability of rain when viewed in a percentage form.

‘In weather forecasting, suppose the Met Office says that the probability of rain tomorrow in your region is 80%…wat they are saying is there is an 80% chance of rain occurring at any one place in the region, such as in your garden.

If it doesn’t rain in your garden tomorrow, then the 80% forecast wasn’t wrong, because it didn’t say rain was certain. But if you look at a long run of days, on which the Met Office said the probability of rain was 80%, you’d expect it to have rained on about 80% of them.

‘The Met Office checks their probability forecasts against what really happened in this way, and they do pretty well.’

So now you know…


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Author: Caroline Westbrook