Answering TikTok: What does a '30% chance of rain' actually mean?

Mark Puleo
·2 min read

What does a 30% chance of rain actually mean?

Don't overthink it, meteorologists say.

The question was sparked by a viral video that has received millions of views this week, originating on TikTok by user @sydjkell, who said she was "today years old" when she found that her long-held beliefs about precipitation probabilities may have been wrong.

"I thought, 100%, that when they said there was a 30% chance of rain, that it meant that there was a 30% chance that it was going to rain," she said in the video, which went viral on TikTok and Twitter. "I never, ever, ever knew that it meant that there's 100% it's going to rain, and it's going to be in 30% of your area."

Is that actually the case? Well, not so fast.

The video sparked plenty of conversation on social media, so AccuWeather Broadcast Meteorologist Geoff Cornish weighed in this week with his own video explaining how meteorologists use precipitation probabilities and how they should be interpreted by the public, and his expertise should clear up any confusion.

"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."

Cornish said the concept is plain and simple: "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," he explained.

Cornish went on to explain that there are two key ingredients baked into that probability percentage: confidence level in precipitation developing and also how widespread or spotty the precipitation might be.

In this Thursday, Jan. 30, 2020, file photo, the Union flag is reflected in a puddle during an event called "Brussels calling" to celebrate the friendship between Belgium and Britain at the Grand Place in Brussels. (AP Photo/Francisco Seco, File)

Multiple days out, uncertainty regarding a storm's track can influence the probability percentage. On the other hand, he said, on a summer day with isolated thunderstorms, the more limited aerial coverage can play a role in determining that percentage as well.

Cornish also wanted to make sure viewers understand that the probability of precipitation concept has nothing to do with the length of time precipitation may fall. It also is not meant to address the intensity at which precipitation could fall.

"We want it to be easy," he said. "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." And, he added, "the same applies to snow, freezing rain or sleet."

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