What does a 30 percent chance of rain actually mean on the weather forecast?

Ever look down at your phone and see the weather forecasting “30 percent chance of rain”?

Well, it turns out that for many people, those percentages don’t quite mean what they thought they did.

You might assume a 30 percent chance of rain means there is a 30 percent chance that it will rain in your area.

But what it actually means is a combination of a) how high the chance of rain is over any given area, and b) how much of the area it is expected to cover.

“The probability of precipitation forecast is one of the least understood elements of the weather forecast,” the US National Weather Service explains.

“To summarize, the probability of precipitation is simply a statistical probability of 0.01 inch [0.025 centimeters] or more of precipitation at a given area in the given forecast area in the time period specified.”

It comes after an account on X/Twitter, posted a viral picture of somebody saying: “Am I the only one who thought 30% chance of rain meant it’s a 30% chance it’s gonna rain?”

What actually happens is as follows. To calculate the precipitation probability, forecasters multiply how much of an area they believe will receive this much rain, by how confident they are in their prediction that rain will fall in that area.

The NWS gives the following examples, which both lead to a prediction of a 40 percent chance of rain via very different weather scenarios:

“(1) If the forecaster was 80% certain that rain would develop but only expected to cover 50% of the forecast area, then the forecast would read ‘a 40% chance of rain’ for any given location.

“(2) If the forecaster expected a widespread area of precipitation with 100% coverage to approach, but he/she was only 40% certain that it would reach the forecast area, this would, as well, result in a ‘40% chance of rain’ at any given location in the forecast area.”

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