LITTLE ROCK, Ark. — There has been a lot of confusion behind percentages and how people may interpret it when it comes to rain chances.
If you hear your local meteorologist talk about having a 60% chance of rain, do you normally think there’s a 60% chance that it will rain where you live?
To understand percent chance of rain, we need to understand how rain is forecasted.
The National Weather Service uses “PoP,” or probability of precipitation, when talking about the chance of precipitation in a specific forecast area. The PoP represents both confidence and area. It is defined as the probability that a select area will receive at least 0.01” of rain.
The formula PoP = C x A is used to express percentages, where the variable “C” represents the confidence that precipitation will occur somewhere in the forecast area and variable “A” represents the percent of the area that will receive measurable precipitation.
Let’s say that a forecaster is 90% confident that rain will occur over 70% of the forecast area. “C” is 0.9 and “A” is 0.7. Multiply them together and you get chance of rain percentage of roughly 0.6 or 60%.
PoP values vary depending on which one of the variables the forecaster focuses on.
Remember this the next time you hear there’s a 20% chance of rain in the forecast, but it’s raining a lot where you are. People aren’t staying in one spot all day, so traveling increases the chances you’ll end up seeing rain.
The way chance of rain is communicated to the general public is dependent on how a forecaster chooses to explain or interpret it.