When your weather app shows a 70% chance of rain, it does not mean rain will fall across 70% of your local area, nor that it will rain for 70% of the day. According to weather experts, the figure comes from ensemble forecasting—a method that runs multiple weather simulations with slightly different starting conditions. If seven out of ten simulations predict rain, that yields a 70% chance.
How the Met Office Calculates Rain Probability
Grahame Madge of the Met Office explained that the percentage is the result of complex calculations using data from ocean buoys, land-based instruments, and orbiting satellites. This data is fed into the Met Office to create what is known as an ensemble forecast.
"Ensemble forecasting, basically, it's like running several weather forecasts with slightly different starting positions, and if 70% of those forecasts show rain, then that is where the 70% chance of rain will come from," Madge said. "Some people think it's by area or some other metric, but actually, if you run a forecast 10 times, and seven of those runs show rain, then that is where the 70% rainfall comes from."
Why Forecasts Never Reach 100%
Current forecasts are not entirely accurate, and Madge noted that apps typically show probabilities only down to around 5% or up to 95%. "Calling 100% in meteorology is not technically possible, because you can always get something that's just outside of the forecast," he said.
Long-time weather reporter and former meteorologist Crystal Egger agreed. "If the forecast says there's a 60% chance of rain, it means there's a 60% probability that your specific location will receive measurable rain during that period," she said. "It doesn't mean it will rain for 60% of the day or that 60% of the area will get rain."
Visualising Probability and Practical Advice
Egger suggests visualising probability with a group of ten people. "If all 10 are in the same predefined area and there's a 50% chance of seeing rain, you could visualise that as five out of 10 people seeing rain. With a 60% chance, it would be six out of 10."
She also stressed that weather prediction remains an inexact science. "You can have an 80% chance of rain and only get a few hundredths of an inch, or a 20% chance and get a significant downpour if a storm happens to move over you."
Madge offered practical advice for planning days out: "If the next day's symbol for the weather shows rain and you want the day out, just look below that and look at the hourly rainfall, because that will give you a good idea if there's a good usable part of the day. If it's only light rain, does that stop you? Could you be weather-ready and take an umbrella or raincoat for the children and wellies? Let's not let the weather be a block just because you see a rain symbol, it doesn't mean to say that you're gonna get rain at the particular time that you want to go out."
