Weather apps show precipitation as a percentage chance 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. Rain percentage forecasts can be confusing, but don’t worry, even meteorologists who come up with these forecasts can’t agree on exactly how to calculate the number. But science holds the key.
With that in mind, let’s find out what the percentage chance of rain—referred to by meteorologists as the Probability of Precipitation (PoP)—really means.
What the Rain Percentage Doesn’t Mean
Some of this confusion can be attributed to a viral TikTok video, claiming the percentage of rain forecast doesn’t actually predict the likelihood of rain but rather what fraction of an area that is expected to have rain.
The video mistakenly assumed that a 30% rain forecast means 30% of the area will get 100% rain. Despite the confusion, any meteorologist will confirm that the TikTok video didn’t get it right. Predicting the weather is hard, and even meteorologists aren’t always sure if it will rain. So, the forecaster’s interpretation of data must be considered when deciding if rain will fall.
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 specific area in a forecast zone will receive at least 0.01 inches 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.
👉 Related reading: How To Read A Weather Map
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.
“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 for a percentage value), gives the 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 / 100). You’ll have a 30% chance of rain, which translates as a 30% chance that the area will receive 0.01 inches of rain.
The whole area doesn’t have to receive 0.01 inches of rain. If the forecaster’s confidence is 100% only 30% of that area has to receive rain for the forecast to be accurate.
The TikToker got it wrong by suggesting that the percentage of rain can only be achieved when the forecaster’s confidence is 100%, focussing on the area as the measure of rain. A forecaster could also have 60% confidence for rain over 50% of the area. In that case, PoP = 60 x 50 / 100, which also equals 30%.
Final Thoughts
The rain percentage on your weather app refers to the chance you will experience a measurable amount of precipitation (0.01 inches) in the given period.
Meteorologists determine the Probability of Precipitation by multiplying their forecasting confidence that rain will occur with the expected rain coverage. Disagreements on how to calculate the number doesn’t significantly affect our daily routines.
If you know the weather in your area, you will know when to grab an umbrella or cancel outdoor activities, especially if you live in one of the rainiest cities in the US.
When you see the forecast or a radar app, you’ll know exactly what the percentage of rain means. Learn more about how forecasts are created and weather forecast accuracy, by reading our article on the most accurate weather apps.
