Meteorologists get this a lot.
“Just what the heck is a percent chance for rain, anyway? If there is a 30 percent chance, does that mean 30 percent of us will get rain or that it will rain 30 percent of the day?”
In fact, Meteorologists hear about this almost as much as: “It’s the only job in the world you can be wrong 90 percent of the time and still keep your job!”
And, for the record, no it isn’t. If I was wrong more than I was right (even barely), I would definitely be fired. More on that a bit later, though.
Yo PoPs, what’s the haps?
PoP is shorthand for the Probability of Precipitation. When we talk about PoPs we are talking about a grouping of the chance for rain.
You may hear a meteorologist say, “PoPs today are around 50 percent” during a conversation. That just means that, generally, across the area there is a 50 percent chance of rain.
PoP is a – mostly – mathematical equation. Here is a quick explanation from the National Weather Service in Peachtree City, Georgia:
PoP = C x A
“C” = the confidence that precipitation will occur somewhere in the forecast area
“A” = the percent of the area that will receive measurable precipitation, if it occurs at all.
Both “C” and “A” are expressed a a decimal. So, we can work backwards out of this, for fun! Let’s say, in the summer, we knew storms would develop in the afternoon, but there were only going to be scattered here and there. So we say a 40 percent chance for rain.
Well, the mathematical side of the forecast might look like this:
40-percent chance of rain = PoP
PoP = 1.00 (confidence) x .40 (coverage)
Or we can flip-flop it. This is what it would look like if we weren’t sure if it would rain, but it it did, everyone was going to get rain…
40-percent chance of rain = PoP
PoP = .40 (confidence) x 1.00 (coverage)
Or we can get extreme. This is what it would look like if there was very low confidence in widespread drizzle.
10-percent chance of rain = PoP
PoP = .10 (confidence) x 1.00 (coverage)
We can even flip-flop that! A forecast that there was very high certainty that only a few showers would develop
10-percent chance for rain = PoP
PoP = 1.00 (confidence) x .10 (coverage)
PoPs in context
Now that you see how PoPs are created, hopefully it helps explain how the forecast works. Hopefully it also helps explain why on a day when there is a 30-percent chance for rain, you may see rain. And why, on the next day, when there is a 70-percent chance, you may not.
The forecast wasn’t wrong, and meteorologists aren’t offering snake oil predictions. You just happened to be on the unfavorable end of the forecast.
This is also why when I’m on the air, I try to use words to explain percentages, too.
10% = Stray
20% = Few
30% = Isolated
40% – 50% = Scattered
60% – 70% = Widespread
70% – 100% = Likely
That’s not PoPs
Meteorologists will never refer to a rain chance as a percentage of the day it will rain. Generally, we will explain that differently. We will use hour-by-hour charts or timelines.
PoPs will also not tell you how hard it will rain, nor will it tell you how much rain will fall. A 40-percent chance for a thunderstorm doesn’t mean that less rain will fall than when there is a 100-percent chance for thunderstorms. Rainfall totals are dependent on other factors like storm motion, PWAT values, and the length of time rain will fall.
That’s how it is done! Now that you know how a forecast for rain is created, you can make better decisions about when to pack the umbrella!