This might sound complicated, and that's because weather deals in probabilities, and probabilities often require complicated math. The good news is that the average person doesn't need to get out a pencil and paper every time they read the forecast. According to the National Weather Service, if you see a 40 percent chance of rain, "there is a 40 percent chance that rain will occur at any given point in the area." Explore how we predict the weather in the videos below.
Here's What A Chance Of Rain Really Means
In 2015, The Washington Post polled its readers on their knowledge of weather forecasts. Most readers believed that if the forecast calls for a 60 percent chance of rain, that means that it will rain across 60 percent of the area tomorrow. This is a common belief, but it's only half right. In reality, the chance of rain, or probability of precipitation (POP), is based on a mathematical formula that takes the forecaster's confidence into account. That formula is as follows:
POP = Coverage x Confidence
To use a very simple example, if a forecaster is 100 percent confident that 40 percent of a given area will see measurable rain, the POP is 40 percent. Of course, 100 percent confidence almost never happens in science, so the formula is often more complicated. What if a forecaster is 50% sure that rain will occur and expects that, if it does occur, 80% of the area will get that rain? 50 percent of 80 percent is 40 percent, so the POP is 40 percent.
How To Understand The Weather Forecast
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from The Royal Institution
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from It's Okay To Be Smart