How do I know if it's going to rain?

By Sachin Croker

More or Less, BBC Radio 4

* 28 May 2018

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Image copyright GETTY IMAGES

How does BBC Weather work out the chances of rain? The truth is stranger than you might think.

Earlier this year, the BBC Weather app and website were updated to include hour-by-hour forecasts.

Each breakdown tells you the percentage chance of it raining where you are at each hour mark, but what does a 40% chance of rain at 14:00 mean?

Is it a 40% chance it will be raining at that exact time, that it will rain for that percentage of the hour from 14:00 to 15:00, that there's a 40% chance it will rain continuously for the whole hour, or that that amount of your local area will experience rain?

If the whole day has small percentages of rain every hour, does that mean that it will definitely rain that day?

As counter-intuitive反直觉的 as it might seem, hourly predictions of the chance of rain actually refer to the 60 minutes preceding - not following - the time mentioned.

"We have to verify our forecasts," says Nikki Berry, senior meteorologist气象学家 at MeteoGroup, which provides the forecast analysis for the BBC's weather services.

"So, if you imagine putting a measuring jug out in your garden at 13:00, and you go and check it at 14:00, the 40% chance of rain at 14:00 is the chance of you having some rain in your jug at 14:00."

If you're heading out at 14:00, looking ahead to the forecast at 15:00 will give you some guidance for how the weather might play out.

Given the many types and intensities强度 of wet weather, how do forecasters choose which symbol to use?

"The probability of rain is one element," says Ms Berry, "but we're also trying to forecast the amount of rain - is it a high chance of a little bit of rain, or a low chance of a heavy downpour倾盆大雨?

"When we're creating the weather symbol that you see above the percentage chance of rain, that's not just linked to the percentage chance of rain - that's linked to the humidity, to the temperature, the wind, and whether the rain is convective对流引起的 - showery - or whether its large-scaled with a band coming in."

The good news for those wanting to enjoy the outdoors is that forecasts are getting better. In fact, says Ms Berry, they are as accurate over three days now as they were over only one day 20 years ago.

Words：

counter-intuitive反直觉的

counter-intuitive反直觉的

meteorologist气象学家

intensities强度

downpour倾盆大雨

convective对流引起的