In 1965 the National Weather Service began giving the probability of precipitation in forecasts across the country. Since then, Americans have heard the weather as a number: a 30 percent chance of rain, a 70 percent chance of snow.
The number means “the probability of occurrence of measurable precipitation at a point in a specified period of time.” In one study published by the National Weather Service in 1980, “a majority of the participants believed that a PoP forecast is an area forecast.”