What does 50% Chance of Rain Really Mean?

If I had a dollar for every time someone told me, “It must be nice to get paid to be wrong all the time”….I’d have a good looking savings account! My job also focuses on the reconstruction of past weather events, rather than forecasting…but that’s another topic altogether. A recent Tiktok (a video-sharing social media app, for anyone who is unfamiliar with it) reminded me of one of the biggest communication problems meteorologists face…and why so many people might think we are “wrong all the time.”

The video, which began with the question, “What’s something you found out late in life that you should have known earlier but just didn’t?”, went on to claim 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.” The response was…100% confusion, skepticism, and everything in between! Everything from, “I found that out…right now” to “I thought it was how heavy the rain was….like 100% max rain and 30% was drizzle” all the way to “I’m a meteorologist and this isn’t true.”

Whew! This video and recent research from the University of Georgia suggested that the public isn’t alone in their mixed views of the concept of Probability of Precipitation (PoP)…even some meteorologists have different viewpoints! So much so, that some broadcast meteorologists don’t even use percentages in their forecasts to the public anymore. To better understand this, let’s take a deeper look at the meaning of PoP.

The National Weather Service defines PoP in the following way: PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measureable precipitation, if it occurs at all. So… if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = “C” x “A” or “1” times “.4” which equals .4 or 40%.). Another way to explain this, is that if there is a 50% chance of rain in 80% of a given area, there is a 40% PoP.

This means that PoP is an expression of both confidence and area. If a forecaster is only 50% certain that rain will occur over 90% of the area, then the PoP is 45% (because POP in this instance would equal 0.5 x 0.8).

The UGA study I referenced above surveyed 188 meteorologists and broadcasters and found that respondents expressed a range of different definitions of PoP, and that each person was highly confident in their definition. The expression given above (PoP = Precip X Area) is one of the most common, while another common interpretation of PoP focuses on a specific point instead (PoP = Precip X Point).

Nearly half of those involved in the survey also felt there was little consistency in the definition of PoP, and the study concluded that it was evident that we need to work together as a community to establish clear and consistent messaging involving the communication of uncertain information. However, to be clear, none of the interpretations of PoP reference the intensity, amount, or duration of precipitation. You can still see flooding with a 30% PoP, and there could also be a day with 100% PoP that results in little accumulation (such as a day when meteorologists are anticipating a few hours of drizzle everywhere). Planning a beach day and see an 60% chance of rain? Dive deeper – don’t let it ruin your fun at first glance! It could mean that there is a high likelihood of a 30-40 minute storm, versus an all-day event. This is why context is critical when consuming any kind of information – especially a weather forecast.

Fortunately, even if everyone isn’t on the same page of what PoP means, they’ve likely adapted their own internal definition of it. If you see an 80% chance of rain, you’ll likely remember to grab a rain jacket or umbrella on the way out the door, regardless if you think the 80% is referring to how much rain your area will receive or how long it will last. A quick glance at the forecast for PoP is generally all people need to know if they need to grab their rain gear, but if you have plans outdoors, be sure to take a closer look at the forecast. Though I don’t recommend the weather app that comes pre-installed on your phone, there are several good weather apps with forecasts created by meteorologists, and your local broadcast meteorologist will also have the full run-down of anticipated weather conditions. Your local National Weather Service also publishes detailed forecast discussions as well, if you want to dive in even more.

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