FEBRUARY 23, 2021 UPDATE: There is a new viral tiktok video floating around the internet where a person is attempting to explain how “a chance for rain” really works.
Because it is the internet, people are taking it and running.
Despite many people believing what is being said, sadly, the video is not accurate. It is a bit like that State Farm ad from, like, 2012:
And, while I’m glad there is so much interest in the topic, I am bummed out that the person explaining “what a chance for rain means” isn’t explaining it accurately.
So, Let’s take a look at how “a chance for rain” is really calculated and what it may mean for you…
ORIGINAL:
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!”
The first part is a pretty straight-forward question, with a semi-complicated answer. The second part? Well, for the record, this isn’t a job where one can be wrong frequently and still be employed. If I was wrong more than I was right (even barely), I would definitely be fired.
The confusion is a mixture visibility and misunderstanding about what a forecast actually means. 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 with other meteorologists. But when talking to someone outside of the meteorological world, that same meteorologist might say, “there is a 50-percent chance for rain” instead.
That just means that, generally, across a given area there if you asked 10 random people what the weather did that day, that five of them would day “it rained.”
But that doesn’t mean this is PoP equation is offering a forecast for rain coverage. PoP is a 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 as a decimal.
So, we can work backward out of this, for fun! We will start with an easy one. Let’s say, in the summer, we knew storms would develop in the afternoon – it was a guarantee! But, there were only going to be scattered here and there, covering 40-percent of the area.
So meteorologists would 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 a meteorologist wasn’t as 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 spotty drizzle.
12-percent chance of rain = PoP
PoP = .35 (confidence) x .35 (coverage)
We can even flip-flop that! A forecast that there was very high certainty that everyone sees rain!
81-percent chance for rain = PoP
PoP = .90 (confidence) x .90 (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 that 30-percent chance drenched your yard. Or why, on the next day, when there was a 70-percent chance, you stayed dry.
The forecast wasn’t wrong, and meteorologists aren’t offering snake oil predictions. You just happened to be on the unlikely side 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
While the words above aren’t a perfect way to describe the math in words, I hope that it is a good start.
That’s not PoPs
Now that we know what PoPs is, let’s talk about what it is not.
- 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. That is determined by updrafts and downdrafts within each individual storm.
- PoPs will not 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 PoPpin
That’s how it is done! And so when you hear a 30-percent chance for rain and see rain, it isn’t a bad forecast… You just got the 100-percent of the 30-percent. Or if there was an 80-percent chance for rain, you may have been in the 20-percent of the area that wasn’t anticipated to see the rain.
Now that you know how a forecast for rain is created, you can make better decisions about when to pack the umbrella!
