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 percent sure that rain will occur and expects that, if it does occur, 80 percent of the area will get that rain? 50 percent of 80 percent is 40 percent, so the POP is 40 percent.