What does “percentage of rain” actually mean? 

Imagine you’re planning a picnic in the park. You’ve got the basket packed and the blanket ready to go, but the local forecast says there’s a 40 percent chance of rain. Should you cancel your plans? Does this mean it’ll rain 40 percent of the day? Or does it mean that 40 percent of your geographic area will get rained on? Well, not exactly. When your local weather forecaster mentions the chance of rain, they’re actually interpreting the Percentage of Precipitation, or PoP. Simply put, the PoP is the statistical probability that there will be at least 0.01 of an inch of rain in a given area at a given time. Basically, the likelihood that it’ll rain at all. PoP is a unit of measurement—and there’s some math involved in figuring it out. The exact PoP formula is as follows: “P,” the portion of the forecasted area expected to receive rain, multiplied by “C,” the forecaster’s confidence that rain clouds will form and move into that specific area, equals the PoP. So if a meteorologist is 80 percent sure that rain will develop in 50 percent of your town, their prediction will be a 40 percent chance of rain. If the meteorologist is 40 percent sure that rain will develop in 100 percent of your town…that’s also a 40 percent chance of rain. Confused yet? Don’t worry. The most important thing to understand is that if the forecast predicts a 40 percent chance of rain, that means there’s a 40 percent likelihood of rainfall in that area. Does knowing the PoP mean you’ll accurately predict the weather every single time you check the forecast? Definitely not. In fact, the percentage of precipitation is one of the least understood concepts in meteorology because it represents a probability, not an exact scientific truth. That 40 percent chance of rain might just mean that your picnic plans will be perfectly safe and dry—but we’d bring an umbrella just in case.