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Uploaded:2017-07-29
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Your friendly local weather person says there's a 10% chance it will rain today, so you throw on your flip-flops and head out to enjoy a beautiful day. Next thing you know, you're running through puddles, trying to get out of a torrential downpour. Bad luck or bad weather person?

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Sources:
https://www.weather.gov/ffc/pop
https://weather.com/news/news/heres-why-it-rained-when-your-forecast-said-10-percent-20130816
https://www.forbes.com/sites/marshallshepherd/2015/11/27/do-you-or-your-meteorologist-understand-what-40-chance-of-rain-means/#e0adca725d56
http://journals.ametsoc.org/doi/abs/10.1175/WAF-D-15-0058.1
(00:04) Have you ever felt like you need a Ph. D. in meteorology to decipher the daily weather forecast? Like that 50% chance of rain thing. What does that mean, exactly? Does it mean it'll be raining 50% of the time today? Or that there's a 50% chance that it'll rain at some point today?

(00:18) Meteorologists call this number the probability of precipitation. According to the National Weather Service (NWS), this percentage is supposed to tell you the chance of precipitation occurring at any one spot in the area covered by the forecast. They define it with a mathematical formula: the probability of precipitation (PoP) equals C times A. (That is, PoP = C × A.)

(00:34) C is the meteorologist's level of confidence that precipitation will occur somewhere in the forecast area, and A is the percentage of the area that's expected to get measurable precipitation (which is defined as 0.25 mm in the USA). That's usually enough for a couple little puddles to form.

(00:49) If your weather person is 100% confident that 25% of the forecast area is going to get rain, that's a 25% chance of rain. But if they're 50% confident that 50% of the area is going to get rain, that's also a 25% chance of rain. This means that the more you move around, the more likely you are to run into that part of the map that has a chance of getting rain at some point.

(01:07) If you're still a little confused, don't feel bad. This is the official NWS definition of the PoP, but even meteorologists admit they aren't consistent about it. A 2015 survey of 188 meteorologists and broadcasters found that many people calculated PoP in slightly different ways. Different specialists had different ways of turning the outputs of their forecast models into percentages.

(01:28) They also varied in how they generalized from the chance of rain in one very specific spot to a wider area, and in how they generalized a prediction for half a day to a more specific time. What they all had in common, though, was a lot of confidence that their answer was the right one.

(01:41) Bottom line? Predicting the future is hard. And the PoP could be better defined by meteorologists and better understood by the public. But if the forecast says 10% chance of rain and it rains, your meteorologist wasn't wrong; you don't need to call them and complain. It's just that with the current conditions, you happened to draw the short straw.

(01:59) Thanks to all our Patreon patrons who keep these answers coming! If you want to learn more about weather science, check out our videos where I explain why rain has a smell.