What Does It Really Mean When There’s A 20% Chance Of Rain? It’s Complicated
March 15, 2021BruceDayne
My chaotic energy knows no bounds, and thus I rarely check the Weather app. Usually, I find out it’s raining in real time, or if someone tells me to expect showers during small talk. I suspect that most functioning adults peek at their weather app of choice sometime between waking up and leaving their house in the morning, if only to determine whether they should grab an umbrella on their way out the door. And while it would seem that being informed is the more responsible choice, as it turns out, many people completely misinterpret one of the most widely read weather stats: how likely it is to rain. Take that, “functioning” adults!
I first caught wind of the actual meaning of the Percentage of Precipitation (PoP), as it’s known in meteorological circles, on TikTok. A video went around that claimed that a 30% chance of rain doesn’t mean that there’s a… 30% chance that it might rain that day. Instead, it means there’s a 100% chance it will rain, but said precipitation will only cover 30% of a given area. Cue the pandemonium. Who would have assumed that was the correct interpretation? People felt lied to. But for every viewer expressing dismay, another expressed skepticism. How could we be sure this random TikToker had it right? (Spoiler: They didn’t.)
The TikToker — who didn’t respond to Refinery29’s request for comment by press time — was incorrect, but the truth is, PoP is tricky to define. I asked Castle Williams, PhD, a social science contractor supporting National Oceanic and Atmospheric Administration’s Research’s Weather Program Office, to explain PoP to me as though I was a precocious fourth grader. He said that would be “a really difficult ask.”
@sydjkell Reply to @spice.hound The original ##ChanceofRain video w added ##closedcaption for ##deaf and ##HoH viewers!! ♥️ (And those who scroll TT w sound off😂)
♬ original sound – Syd
Dr. Williams wasn’t trying to insult my intelligence. In a study he conducted with the University of Georgia, he found that meteorologists don’t agree on how the percentage is calculated. In fact, there are five different commonly used definitions for PoP among meteorologists alone. No wonder the average weather app user is confused.
One of the five definitions in Dr. Williams’ study is similar to the reasoning behind the TikTok: It’s based off of the idea that PoP = C x A. In that case, “C” means the confidence that it’ll rain somewhere in the forecast area. And “A” means the percent of the area that will receive a measurable amount of rain, if it occurs at all. So a 50% chance of rain in 80% of a given area = a 40% PoP. But Dr. Williams says this isn’t the most widely accepted definition among meteorologists.
His preferred definition is this: “[PoP measures] the likelihood that you’ll see rain at any given point within the forecast area within a 12 hour time period,” he tells me. His interpretation is more in line with the common understanding of PoP — not the TikTok.
Mindy Beerends, Meteorologist-in-Charge at the National Weather Service Center Weather Service Unit in Indiana, has a more technical definition: “It’s the chance of precipitation, which we define as 100th of an inch of rain falling at a particular point over a certain period of time,” she says. Beerends’ explanation falls in line with the official NOAA definition of PoP, and both also seem to debunk the viral TikTok.
While the video that’s been circulating recently is new, its so-called revelation isn’t. My research turned up similar “I was today years old when I learned…”-style factoids on social media and mentions of PoP misinterpretations in old articles stretching back to the early 2000s. Some, like the TikTok, claim that a 30% chance of rain refers to the proportion of a given area that will receive rain. But another misreading is that the PoP actually means you’ll be seeing rain for 30% of the time in a given forecast area over a certain period. Both are wrong. Apologies in advance if I just shattered your world view…
A lot of PoP miscommunications happen because meteorologists and apps give us the percentage without a “reference class.” That’s when you provide context: “A percentage in a PoP forecast is most useful when the percentage is accompanied by a point of comparison that anchors the intended meaning of the forecast,” he says. “For example, if I say there’s a 20% chance of precipitation for your area tomorrow in the afternoon or evening, it’s unclear what that 20% is referring to. By adding a reference class, such as, on one in five days like today, there will be rain in your area, it provides context and anchors the probability information.”
But the thing is, even if you personally interpret PoP incorrectly, Dr. Williams says that if it’s working for you, it’s no big deal. One of the reasons there’s not a major push to help people understand PoP (or to get all meteorologists on the same page) is because there aren’t major negative consequences to misinterpreting whether it’ll rain. “The worst that happens is you don’t bring your umbrella or raincoat,” he says. “We have more dangerous hazards that are taking the stage.” In other words, meteorologists, by and large, are more focused on tracking tornadoes, hurricanes, and storms that threaten people’s lives. And most people are getting along fine even when they misunderstand PoP.
“Even though people aren’t understanding or can’t define what the PoP means, they’re using the percentage correctly when they take protective action,” Dr. Williams says. If you see there’s 80% chance of rain, you’ll probably dress appropriately, even if you think the 80% is referring to how much land will receive rain, or how long the rain will last.
“We all have our own internal definition of what PoP means, and we’ve been using it for as long as we’ve been using weather forecasts,” Dr. Williams says. “We’ve all personalized PoP and we all have our own threshold and our own gauges of when to take an umbrella and when not to.” Basically, we don’t question our understanding of PoP because it’s been working for us. That is, until someone on TikTok comes along and rains on our PoP parade.
Still, both Dr. Williams and Beerends say they’d like to see more be done to educate folks about PoP, whether it means an asterisk within the weather app with information going into detail about the stat and how it was calculated, or learning about it in more detail in schools. “It’s something we should be doing some more public education around,” Beerends says. “It’s important to have people be aware of what that percentage means, so it can help them make informed decisions so they can be safe and prepared”
I asked Dr. Williams why he thought that this PoP conversation was becoming so popular now. Could it be a reaction to all the uncertainty we’ve faced amid the COVID-19 pandemic? Are we desperate to know the certain truth about something, even if it’s just our understanding of the weather? He said it was a good hypothesis. “In the last year, uncertainty has been heightened and we’ve become ‘uncertainty fatigued,’” Dr. Williams says.
Regardless, if your interpretation of PoP has been working for you — meaning you almost always have an umbrella or raincoat on hand when it begins to rain — there’s no need to readjust your perception. But if the whole mysterious mess is infuriating to you, take a page from my book and ditch the tyranny of the weather app altogether. Dancing in the rain is more fun than coming prepared anyway.
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