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As Benjamin Franklin wrote in 1789, “In this world, nothing can be said to be certain, except death and taxes.” For everything else, there is an inherent degree of uncertainty. We don’t often come face to face with quantitative probabilities in our everyday life, save for one: probability of precipitation (PoP).
A seemingly simple concept, PoP is present in most of our daily routines as we check the weather before getting dressed. You may not explicitly realize it, but we all have personalized thresholds for the PoP at which we will choose to bring an umbrella or cancel an outing. For some, a 60 percent PoP warrants carrying an umbrella; for others, only 80 percent or higher. Unfortunately, most of us don’t have an accurate idea of what PoP truly means, even though most of us are certain we do!
A 60 percent PoP does not mean that 60 percent of an area will receive precipitation. It also does not mean that it will rain for 60 percent of the time period, and it does not mean that a forecaster is 60 percent confident that it will rain. So, what does it mean?
As defined by the National Weather Service, a probability of precipitation (PoP) is the “chance that at least 0.01 [inch] of rain will fall at the point for which that forecast is valid over the period of time given.”
So, a 60 percent PoP means that when these meteorological conditions occur in this area, 6 times out of 10, there will be at least some rain. That’s what PoP is. But how is it calculated?
PoP = Confidence x Coverage
To find the PoP for a given area over a given time period, we take the confidence of the forecaster and  by the area that will be affected by the precipitation. Say I was 100 percent confident that 50 percent of Cleveland would receive at least 0.01″ of precipitation tomorrow, the PoP would be (1 * 0.5 = 0.5) 50 percent. Now, if I was only 80 percent confident in my prediction that 50 percent of Cleveland is getting wet tomorrow, the PoP would be (0.8 * 0.5 = 0.4) 40 percent!
If you stay in the same spot all day (like I did writing this article), then a 40 percent PoP means you have a 4 in 10 chance of being rained on. But, if you move around within an area, or between areas, your probability of encountering rain increases. As Brad Panovich, Chief Meteorologist at WCNC Charlotte , “It’s like buying more raffle tickets. Each one you buy increases your chances of winning.”
If you were mistaken about PoP until today, count yourself among good company. Weather forecasts have been available to the general public in the United States since the late 1960s, but in studies, between Ěý˛ą˛Ô»ĺĚý of respondents defined PoP wrong, even when they were meteorologists! A  that incorrectly taught people the untrue percent-of-land PoP definition certainly hasn’t helped things.
It turns out that even those sort of wishy-washy terms meteorologists use to describe weather  “scattered flurries” or “isolated showers” have fairly strict definitions too. The National Weather Service uses  to communicate the degree of certainty in a forecast: “slight chance,” “chance,” and “likely.” There are also particular qualifiers to convey the portion of the area that will be affected: “isolated/few”; “widely scattered”; “scattered”; “numerous”; or “occasional/periods of.”
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At least in Canada, the term “risk” as in a “risk of thunderstorms” indicates a 30–40 percent chance of said weather occurring. Fun fact, also in Canada, a PoP of 50 percent is , because it seems .
So, to those who previously found themselves cursing the local weather forecaster for never getting it right, hopefully, this article helps explain that your own lack of knowledge was more likely at fault than theirs. Believe it or not, weather forecasts have actually been getting more and more accurate with time. In 1972, a National Weather Service forecast made three days before was off by an average of . Forty years later, it was down to . In the late 1980s, when trying to predict where hurricanes would make landfall three days in advance, the National Hurricane Center missed by an  350 miles. Now the  is only about 100 miles.Â
Now, that’s not to say that meteorologists can’t be biased. Many weather agencies previously biased their forecasting toward more precipitation than will actually occur. This so-called “wet bias” meant that for years when the Weather Channel predicted a 20 percent PoP, it actually rained only roughly 5 percent of the time. It’s unclear if the wet bias is still influencing forecasts today.
Take a look at the chart below. It shows the percent confidence on the top and the percent area on the side. By multiplying these together we get the PoP at the intersection of the two. The chart is symmetric. For example, if your confidence is 90 percent and the area affected is 50 percent, the PoP equals 45 percent. If the values were opposite, the PoP would still be 45 percent. This means that even though there are multiple ways to arrive at each PoP, in the end, they mean similar things.
A 40 percent PoP can be arrived at by multiplying 100 percent and 40 percent or 50 percent and 80 percent. Therefore, a 40 percent PoP could mean that it is:
- Absolute certainty (100 percent) that some (40 percent) of the area will receive rain, or
- Quite likely (80 percent) that half (50 percent) of the area will receive rain, or
- Somewhat possible (40 percent) that all (100 percent) of the area will receive rain, or
- Possible (50 percent) that most (80 percent) of the area will receive rain.
In the end, all these basically mean “you probably won’t need an umbrella, but it’s not a bad idea.”
Similarly, you’ll notice that to get a PoP of 70 percent or above, one of either the confidence or area must be greater than 70 percent. Regardless of whether that’s the result of being very sure it’ll rain over more than half the area, or being fairly sure it’ll rain over the entire area, what matters is that you remember to close your bedroom window.
