It is one of the biggest challenges we face in weather communication. On a recent tubing trip, I heard a woman lamenting about rain. She said there was only a 20% chance of rain so “why was it raining, those meteorologists always get it wrong.” I thought to myself, “it wasn’t 0% so why the complaints.” Studies and my own personal experience reveal that the public doesn’t understand the concept of % chance of rain, and it may contribute to misguided conclusions like “meteorologists are wrong half the time.” However, a new study by students and faculty at the University of Georgia suggests that it may not just be the public.
Figure courtesy of WCNC and Meteorologist Brad Panovich (@WxBrad on Twitter)
The UGA study published in the American Meteorological Society’s journal, Weather and Forecasting, examined whether the same variations in interpretations and meanings of Probability of Precipitation (PoP) exist among the professional atmospheric sciences community. Before reviewing the basic results, it is instructive to sample a few of the definitions.
“PoP = C x A where “C” = the confidence that precipitation will occur somewhere in the forecast area, and where “A” = the percent of the area that will receive measureable precipitation, if it occurs at all. So… 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” x “A” or “1” times “.4″ which equals .4 or 40%.)”
What may be more surprising to the public is that more commonly PoP is an expression of both confidence and area. If a forecaster is only 50% certain that precipitation will happen over 80 percent of the area, PoP (chance of rain) is 40% (i.e., .5 x .8).
Meteorologist Troy Kimmel has a detailed discussion of this meaning of PoP online. He points out that the PoP is made up of the following elements: likelihood of occurrence of measurable precipitation (one hundredth of an inch or 0.01) for any given point in a specific area for a specific time period. Yes, that is a mouthful. I think WCNC Meteorologist Brad Panovich summed it up best in a 2013 blog post entitled, “Why a 50% chance of rain usually means a 100% chance of confusion.” He explains the NWS definition but then goes on to explain how he uses PoP in Charlotte.
“For our area, I often say there is a 20% chance of rain and that means for any given point on the map. So if you stay in one spot all day your chance of rain remains 20%. The problem is people rarely stay in one spot on the map all day. So if you travel from home to work, school, the gym, the park, the grocery story or anywhere else you will be increased your chance of seeing rain. It’s like buying more raffle tickets each one you buy increases your chances of winning. For our purposes and as an example we will use a point on the map within 10 miles. So for every 10 miles you travel you will increase your odds of seeing rain by multiplying the chance of rain at each point.”
The UGA study surveyed 188 meteorologists and broadcasters. Key findings include:
- Respondents expressed a range of different definitions of PoP and were highly confident in the accuracy of “their” definitions.
- The variance in definitions were found to be related to how PoP was derived from weather model output statistics (MOS), application of 12-hour Pop over smaller time periods, or extrapolating a point PoP to a wider area.
43% of respondents said there is a very little consistency in the definition of PoP. Less than 10% felt that it has been used consistently. For these reasons, many expressed concern that PoP was of limited value because roughly 3/4’s of the population lacks accurate understanding of PoP.
The study, led by UGA Professor Alan Stewart represents the new frontier in meteorology, in part spearheaded by Weather and Society*Integrated Studies (WAS*IS), a grassroots effort to integrate more social science (a catchword for communication, psychology, perception, equity, policy, health, economics, and so forth) into weather applications and research. Castle Williams, one of the graduate students involved, who has background in geography, atmospheric sciences, and psychology says,
“While many studies have already examined the confusion associated with the Probability of Precipitation (PoP) from the public’s perspective, we really wanted to determine if this miscommunication originated from the atmospheric sciences community.”
This study, which consisted of faculty, graduate students, and undergraduates continues to make the case that even with a good technical forecast, it is bad if people don’t understand or consume it properly. Or as Williams concludes,
“It is evident that we need to work together as a community to define the PoP in order to establish a clear and consistent message involving the communication of uncertainty information to the general public.”