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% chance of rain some people think that’s means 40% of the area will see rain. Some people think it means it will rain 40% of the time. Some people just think it’s the odds of seeing rain. The 3rd 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. In the purest meaning of the term “chance of rain” used by most forecasters. The National Weather Service definition is as follows.

The “Probability of Precipitation” (PoP) simply describes the probability that the forecast grid/point in question will receive at least 0.01″ of rain. So, in this example, there is a 40 percent probability for at least 0.01″ of rain at the specific forecast point of interest!

The National weather service and forecasters use this mathematical calculation to come up with the chance of rain for a given spot. The equation uses the forecasted coverage of the rain multiplied by the confidence in the forecast for a point on the map or area.

The Equation is this–>   CHANCE OF RAIN = COVERAGE x CONFIDENCE

So I say tonight in Charlotte there is a 50% chance of rain. I am 100% confident that 50% 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% of our area will have rain, but I’m only 50% 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% confident in my forecast, but just 20% of the coverage area is expected to see rain. So even in this case, the chance of rain is 20%. So only when I am 100% 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% 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%. 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 increased 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. This is also very important if you check the forecast for your house but are traveling somewhere else. The chance of rain for your house might not to be the same as where you are going. So always take that into consideration.

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 location’s chance. So for every point, you cross you have increased your chance of rain by 4%

20% (Chance) X 20% (Chance) = 4%

After crossing 3 points, your total chance of rain now is 32%. Each point increases your chances by 4% which would be 12% added to the 20% for a total of 32%.

The rain chance doesn’t equal intensity, amounts, or duration:

So can you have flooding if you only have a 20% 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% that gets rain it most certainly could be a flash flood, especially in summer. Then again, the chance of rain can be 100% 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%. This is really important when looking at an app. Do you know how the app is using rain chances and an 80% chance doesn’t mean a washout? It just means a good chance of rain at some point in the day, like a 30-40 min storm out of 24hrs which would not cancel my plans. Which if you are going to the beach this is like every day so be cautious changing plans based on just an app forecast of rain. You should ask a human meteorologist if it’s just a quick pop-up shower or storm or something more.

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 time 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. The thing to remember even a 20% chance means you could see rain and it could be a downpour but you still have an 80% chance of it being dry. If I’m playing golf I will chance a 20-50% esecpailly if I know it’s scattered and won’t last long. I can work around it. Though if I just poured a concrete patio or have something outside that cannot get wet at all I might change my plans for a 20-30% chance. It all depends on your threshold for getting wet or not and how long you need it to be dry. I hope that helps clear up the confusion to maybe about let’s say 40%. 🙂

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.