What Does the Percentage of Rain Mean — Because It's Not What You Think

And here we thought that if you see a 20 percent chance of rain, it just means there's a 20 percent likelihood those drops will fall.

Melissa Willets - Author

Apr. 25 2024, Published 12:01 p.m. ET

woman talking on phone in the rain, holding umbrella
Source: Getty Images

If you see the percentage of rain in a forecast or on your weather app, chances are you don't really know what it mean. But you aren't alone! In fact, most people — even professionals — misunderstand this piece of information, which admittedly, helps us plan our activities and ultimately stay dry.

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Read on to learn what percentage of rain really indicates, but be warned: It's far more involved than you may have thought.

And then, spread the word.

woman with curly hair stands under umbrella and wears trench coat, looking at her phone
Source: Getty Images
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So, what does percentage of rain mean?

It's one of those things that many of us misunderstand: the percentage of rain.

So here's the real deal: According to the National Weather Service, a percentage of rain, or, as the professionals call it, the probability of precipitation (PoP), is a statistical probability of 0.01 inch of rain falling in a given area.

And here we thought that if you see a 20 percent chance of rain, it just means there's a 20 percent likelihood those drops will fall on you!

But there's more to it than that. The National Weather Service offers this example: If there's a 40 percent probability of rain, you are not being told that this is the percentage of your area in which it will rain. It also doesn't mean that for 40 percent of the time during the forecasted period, it'll rain.

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So what does it mean then? The percentage is reached via an equation, actually. For those of us who hate math, this is bad news. But hang in there.

You see, to get the answer, you multiply a forecaster's level of confidence that it will rain at least 0.01 inches over some period of time by areal coverage of precipitation that is expected.

You then move the decimal point two places to the left. Got that?

Here's another example the NWS shared to help explain percentage of rain. If a forecaster is 80 percent certain rain will develop but in half of the area, you'll get a 40 percent chance of rain.

Indeed, it's a bit more complex than you may have imagined! But at least now you can understand what the percentage actually tells you.

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You aren't alone if you don't get the science of the percentage of rain.

In 2021, a viral TikTok proclaimed that if there's a 30 percent chance of rain, there's a 100 percent chance that 30 percent of your area will receive rain. But this is incorrect. Go back to the equation to see why.

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Just don't beat yourself up if your brain is tied in knots over the percentage of rain.

A lot of people aren't sure what it means, actually! As Jamie Dyer, a climatology and meteorology professor at Mississippi State University, told USA Today, "Even professional meteorologists seem to misunderstand it sometimes."

Well, then what hope is there for the rest of us?

Good thing we have weather apps to help (confuse?) us.

Or, good thing we aren't all meteorologists who truly need to understand this.

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