Chance of Rain?

(credit: Meteorologist Ash Bray)

It’s common to hear from meteorologists, our weather apps, or even our weather-obsessed friends about the “chance of rain”. At one point in our lives, we have all scoffed, “Well, the weatherman said there was a ninety percent chance of rain today, but we didn’t see a single drop!” While ninety percent is quite high, there was a still a ten percent probability of no rain. Let’s break it down, shall we?

Meteorologists use a percentage to describe the chance of precipitation occurring at a given point. The National Weather Service defines the probability of precipitation, or PoP, as the probability of precipitation occurring at a given point. Mathematically, PoP is defined as: PoP = C * A – In this equation, “C” equals the confidence that precipitation will occur somewhere within the forecasted area, while “A” equals the percent of the forecasted area that will receive measurable precipitation, if any is to occur. For example, the NWS states, “…if the forecaster knows precipitation is sure to occur (confidence is 100%), he/she is expressing how much of the area will receive measurable rain. (PoP  = “C” * “A”, or “1” times “.4” which equals .4 or 40%). Most of the time however, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce a measurable rain over about 80% of the area, the PoP is 40%. (PoP = .5 x .8 = .4 or 40%).”

Predicting the chance of precipitation is difficult enough when we consider said chances at a given time, but what about the chance of precipitation on a given day? This can often become confusing, mostly because when we hear “high percent chance of rain”, we tend to perceive that as meaning it could rain all day long. While that could be true if dealing with widespread rain, this logic is flawed when dealing with pop-up storms and/or a thin band of storms in which it only rains for an hour or two, but a large area receives rain. Therefore, rain chances would technically be rather high (70% – 100%) despite it only raining for a few hours. In summation, the NWS’s take on the chances of precipitation, they define PoP as there being a given percent chance that precipitation that rain will occur at any point in the forecasted area throughout the entire forecasted period, often being 6 to 12 hours. This interpretation may seem daunting and raise additional questions. Realistically, most of us are only concerned with the chance of rain for our region, not a large swatch of land. For all we know, the area our weatherman is discussing could be 2 square miles around us or the entire county. That’s why many on-air meteorologists have multiple interpretations when it comes to PoP.

These interpretations have similar definitions but are not all the same. Steve Hamilton, an on-air meteorologist for The Storm Report, Inc., believes there are “several disconnects between weather presenters, the NWS, and consumers” in regard to PoP. Mr. Hamilton believes the primary reasons for these disconnects are due to most consumers “not understanding the technical aspects of PoP” and the fact “on-air meteorologists rarely have the time to explain PoP in a cognitive way during their short segment.” With multiple interpretations, the definition and understanding of PoP can become confusing. According to a study performed by the University of Georgia, there is inconsistency with the use of the definition of PoP within the meteorological community itself: “Respondents expressed a range of different definitions of PoP and were highly confident in the accuracy of “their” [own] definitions”. This study should concern the weather community simply because it demonstrates that the general misunderstandings about PoP does not solely lie within the listening audience. So, what can we do to solve this issue? Mr. Hamilton suggests that weather presenters “avoid using PoP in favor of similar terms or broad statements”. Instead, the weather presenter could “be more specific about when, where and how much precipitation can be expected within their [consumer] specific coverage area.” Perhaps the weather community as a whole should strive for a single industry standard definition of PoP, thus allowing us to better communicate and educate those around us.

©2018 Meteorologist Ash Bray