When we check our trusty weather app or listen to a forecast, and there’s precipitation in the outlook, we’ll see a percentage of rain. A simple number tells us the chance of seeing rain for the day.
However, it’s easy to misinterpret what this number represents. If you’ve ever found yourself confused by the rain percentage meaning in the forecast, don’t worry, even meteorologists who come up with these forecasts can’t agree on exactly how to calculate the number.
At least some of this confusion can be attributed to a viral TikTok video that claimed that the percentage of rain you see on TV and in your weather app doesn’t actually predict how likely rain is to occur but rather what fraction of a given area is expected to see rain. It stated that a 30% chance of rain means that there’s a 100% chance it will rain in 30% of the forecast area. Unfortunately, it’s not quite that simple.
While there is some disagreement, any meteorologist will tell you that the TikTok video didn’t quite get it right. As you may remember from your March Madness bracket, predicting the future is hard, and meteorologists aren’t always sure if it will rain. So, the forecaster’s level of confidence in rain falling must be considered in the equation.
With that in mind, let’s find out what the percent chance of rain—referred to by meteorologists as the Probability of Precipitation (PoP)—really means.
What Does the Percentage of Rain Mean?
According to the National Weather Service (NWS), the official definition of the percent chance of rain—AKA the Probability of Precipitation—is the statistical probability that a given point in a forecast area will receive at least 0.01” of precipitation in the specified time period. The probability is determined by multiplying two factors—the forecaster’s certainty that rain will fall in the area and how widespread it is expected to be.
Here’s How You Calculate the Percent Chance of Rain
Meteorologists calculate the Probability of Precipitation (PoP) using a simple formula: PoP = C x A. Where “C” stands for the confidence a forecaster has that rain will occur somewhere in the forecast area, and “A” stands for the percentage of the area where they expect a measurable amount of rain to occur.
By multiplying these two factors (and then moving the decimal point two places to the left), a single number will give a forecaster’s best estimate of how likely you are to see rain.
For a more visual explanation, watch the video below by SciShow.
What Does a 30% Chance of Rain Mean?
Using the formula above, we can see how this number might change in different scenarios. As the TikTok video has suggested, if a forecaster is 100% sure that rain will occur but that it will only cover 30% of a given area (PoP = 100% x 30%). You’ll have a 30% chance of rain.
But the TikToker got it wrong by suggesting that this scenario is the only way of getting a 30% chance of rain. You could also have 60% confidence that rain will occur over 50% of the area. In that case, PoP = 60% x 50%, which also equals 30%.
The rain percentage on your weather app refers to the chance you will experience a measurable amount of precipitation (0.01”) in the given period. Meteorologists determine the Probability of Precipitation by multiplying their forecasting confidence in rain occurring with the expected areal coverage of rain.
While some meteorologists disagree on how to calculate the number, the discrepancies aren’t enough to make a difference in our daily routines.
After looking at weather percentages for long enough, most of us have a pretty good idea of how high that number would need to be to have to grab an umbrella or postpone an outdoor activity, especially if we live in one of the rainiest cities in the US.
That said, at least now, when you look at a forecast or a radar app, you’ll know exactly what the percentage of rain means. To learn more about how forecasts are created and weather forecast accuracy, read our article on the most accurate weather apps.