It’s one of the most misunderstood terms in all of weather: the chance of rain.
You hear it and see it in just about every weather forecast nowadays, but few people understand exactly what it means. Whenever people hear the chance of rain, I find that every person interprets it slightly differently.
So when we say there’s a 40-percent chance of rain, some people think that’s means 40-percent of the area will see rain. Some people think it means it will rain 40-percent of the time. Some people just think it’s the odds of seeing rain. The third option is closest to the truth, but it’s not the whole story.
What it is supposed to mean
You have to start with the real terminology which is Probability Of Precipitation or POPS for short. That’s what us weather geeks call it. It's the purest meaning of the term “chance of rain” used by most forecasters and The National Weather Service. It is a mathematical calculation using the forecasted coverage of the rain multiplied by the confidence in the forecast.
It's CHANCE OF RAIN = COVERAGE x CONFIDENCE
So I say tonight in Charlotte there is a 50-percent chance of rain. I am 100-percent confident that 50-percent of our area will see measurable rain of 0.01” or more. The amount isn’t factored in, but I’ll get to that later. So the equation looks like this.
50% (Coverage) x 100% (Confidence) = 50%
What if I think that 50-percent of our area will have rain, but I’m only 50-percent confident in that forecast?
50% (Coverage) x 50% (Confidence) = 25%
You can see here, even though I think the same area will be covered, I’m not as confident in the forecast.
There are times I am 100-percent confident in my forecast, but just 20-percent of the coverage area is expected to see rain. So even in this case, the chance of rain is 20-percent. So only when I am 100-percent confident in my forecast does the rain chance equal the coverage of rain.
Increasing the odds & how we really use POPS
(Note this does reduce the coverage weight, and this is how we use POPS on TV at WCNC)
Now that you know where the chance comes from I should also tell you how we use them on TV. For our area, I often say there is a 20-percent chance of rain and that means for any given point on the map. So if you stay in one spot all day your chance of rain remains 20-percent. The problem is people rarely stay in one spot on the map all day. So if you travel from home to work, school, the gym, the park, the grocery store or anywhere else, you will be increasing your chance of seeing rain. It’s like buying more raffle tickets each one you buy increases your chances of winning. For our purposes, and as an example, we will use a point on the map within 10 miles. So for every 10 miles you travel, you will increase your odds of seeing rain by multiplying the chance of rain at each point.
Below you see a simple grid. If you travel from point A, to point B then to point C you will increase your rain chances. You multiply each locations chance. So for every point you cross you have increased your chance of rain by 4-percent.
20% (Chance) X 20% (Chance) = 4%
After crossing 3 points, your total chance of rain now is 32-percent. Each point increases your chances by 4-percent that would be 12-percent added to the 20-percent for a total of 32-percent.
Chance doesn’t equal intensity or amounts
So can you have flooding if you only have a 20-percent chance of rain? Yes! The chance of rain is just that, the chance of seeing measurable rainfall which is 0.01” or more. There is nothing calculated into the chance of rain for how fast it falls or for how long. So yes, if you are the 20-percent that get rain it most certainly could be a flash flood, especially in summer. Then again, the chance of rain can be 100-percent and it could just be a few hours of drizzle everywhere. Also just because it’s raining where you are, doesn’t mean that chance of rain should be 100-percent.
So how do we use it on TV?
For the most part, we use the chance of rain as the odds of seeing rain during the forecasted times periods throughout the day at any given point on the map. For the most part, I use the chance of rain as my confidence in the possibility of rain. It is a safe bet to just use it as a scale. The higher the number, the better the chance is you will see rain on that day. Hope that helps clear it up the confusion to maybe about let’s say 40-percent.
Update and Feedback
Let me know how you interpret the chance of rain below in the comments. Maybe we can come up with a unified way that works for everyone. Would a 1-10 rain factor scale work better? Not sure but I am open to the discussion.