It seems like every time the TV weatherman quotes ‘70% chance of rain’ on the day there isn’t a drop of water in the sky, and conversely every time the weatherman says there’s a 10% chance of rain the heavens open up and you get completely dumped on. This has been the status quo my entire life, and finally I have an explanation (see below) as to why this happens. For whatever reason I’ve just always assumed that weathermen overstate the likelihood of rain in certain parts of the country and understate it in other parts because they don’t want to be wrong and get sassed on by the public. As it turns out there’s actually some serious science behind ‘the % chance of rain’ quoted by meteorologists on the weather channel, and based on the explanation below I now know why.
This first part explains just what the ‘chance of rain’ actually means and the second part explains why it always seems to rain whenever the % is so low, so read/scroll on if you’re interested:
via Marshall Shepherd on Forbes.com:
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.
The UGA study published in the American Meteorological Society’s journal, Weather and Forecasting (and now available in Early Online Release) 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.
The National Weather Service (NWS) Peachtree City Website gives the definition of PoP as:
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).
He goes on to list several more formulas and definitions for PoP, all of which you can read by CLICKING HERE to head on over to Forbes, but the important findings most important finding from the University of Georgia study is the following:
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.
So not only does the general public not understand the ‘Probability of Precipitation’ but as it turns out A SHIT TON of weatherman and meteorologists either don’t agree or don’t understand what it means either. However, now that you’re armed with the knowledge of how to actually read the ‘% chance of rain’ you shouldn’t be surprised next time it pours when there’s only a 30% chance of rain. And if you’re still confused I suggest following that link above and clicking on over to Forbes!!!