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23rd May 2025
03:00pm BST
When you check the weather app on your smartphone, and it says '30 per cent rain', you assume that means there is a 30 per cent chance it will rain in that area, at that time.
That makes perfect sense. The scientists behind these forecasts can never be 100 per cent certain of rain, so they hedge their bets with percentages and then we are left trying to decide whether to take a brolly with us or not on our daily walk. However, that explanation is wrong.
Over the years social media users have offered up alternative theories about what the percentage of precipitation levels actually mean.
One TikTok user went viral after seemingly discovering that the percentage meant ‘that there's 100% chance it’s going to rain, and it’s going to be in 30% of your area.’
However, though slightly closer to the truth, that explanation isn’t 100 per cent correct either.
AccuWeather Assistant Chief Video Meteorologist Geoff Cornish weighed in with his own explanation.
“We don’t want anyone to overthink this,” Cornish said.
“Your probability of precipitation is the likelihood that you will receive measurable precipitation during the forecast timeframe.
“It is the probability that at least 0.01 of an inch of precipitation will fall on your rooftop if you live in the forecast area. That’s about enough rain to produce a small, underwhelming puddle.”
Cornish explained that two ingredients are built into that probability percentage; the confidence level in precipitation developing and also how widespread or spotty the precipitation might be.
The probability of precipitation has nothing to do with the length of time precipitation may fall or the intensity at which it could fall.
Cornish added: “Take it at face value. Your probability of precipitation is simply the chance of rain on a scale of zero to 100, if it's warm enough for rain, at some point in the forecast time. The same applies to snow, freezing rain or sleet.”
