A 30% chance of rain doesn’t mean what you think

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(NEXSTAR) – You turn on the local news in the morning or open your phone’s weather app and you see there’s a 30% chance of rain in the forecast.

You (quite reasonably) take that to mean there’s a 30% probability it will rain where you live and a 70% probability it won’t rain. Right?

Wrong. It also doesn’t mean it will rain 30% of the day.

The percent chance of rain (or snow or thunderstorms) is called the “Probability of Precipitation,” or PoP. The figure refers to the chance that the forecast area will see at least 0.01 inches of rain, according to the National Weather Service.

Here’s where things get even more complicated: What is the forecast area? That depends on where you’re getting your weather from.

As Nexstar’s KXAN explains, some meteorologists are giving out rain chances for a large area. KXAN is based in Austin, Texas, but covers weather across 15 counties. Just saying there’s a 50% chance of rain in their viewing area could mean some people see a lot of rain while other people three counties over see nothing. It could also mean everyone sees a little rain.

That’s why meteorologists will get more specific and give you information beyond just the top-line rain chance. KNWA in Arkansas breaks down its viewing area into ten blocks. From there, the meteorologist looks at weather models and determines how confident they are a block will see precipitation.

The meteorologist uses that data to do some math: (how confident they are there will be precipitation) X (the area they believe will see precipitation) = PoP (probability of precipitation) a.k.a. percent rain chance.

Say the models show 50% confidence there will be rain over 50% of the area. That would be 0.5 X 0.5, which leaves us with 0.25, a 25% chance of rain.

Bear with us for one more example:

Say there’s 100% confidence there will be rain, but it’s only for 80% of the area. That’s 1 X 0.8, which is an 80% chance of rain.

You see an 80% chance of rain in your weather app, and you think, “It’s almost certainly going to rain today!” But what if you live outside that 80% rain area? You may end up seeing no rain at all, but the forecast was still accurate.

And remember, even when you understand what the rain chance means, it doesn’t tell you two key pieces of information: how long it’s going to rain and how much rain in total will fall.


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