What Most People Think a 20% of Rain is

 

Two answers and they are both wrong. It’s fascinating for me to see that most people in the United States view a weather forecast as deterministic, or if there is a rain percentage on the map, it definitely will rain. In other words, people interpreted 20% as it will rain 20% of the time or that 20% of the viewing area will get wet. The people conducting this study speculated that the general human tendency is to avoid complications of incorporating uncertainty in the decision process by ignoring it or turning it into certainty. Another example showed that people living in floodplains appeal to perceived cyclical patterns to predict the size of the next flood, rather than regarding it as uncertain.

The correct answer is if there is a 20% chance of rain today it may rain. In my experience, I’ve found that people in Southeast Texas view the chance as so small that it simply won’t rain. Most of the e-mails I receive from people upset about the forecast are on days with a 20% chance of rain and it does rain. The quote is usually, “You said it wouldn’t rain.”

Social science studies like this are important because it shows the correct message isn’t getting to the viewer. We as forecasters have to do a better job communicating the forecast. But the biggest problem is the forecast itself. The reason we use probabilities is because the future is uncertain. An 80% chance of rain means I feel strongly there will at least be a minimum amount of rain. As Frank Billingsley likes to say, “You only know what you know.”

To view past weather quiz answers click here:

Past Weather Quiz Answers

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3 Responses

  1. I think a lot of the confusion may come from Doug Johnson, when he was doing the weather for channel 2, stating quite clearly that a forecast of 20 percent chance of rain was a prediction that 20 percent of the area would receive rain. So you might understand why there is a reluctance that such an icon could be mistaken about that. The question on whether the standard has changed between then and now is an interesting one. And one that could be elaborated on.

    ~J

  2. Interesting. I always assumed that the percentage meant exactly what weather forecasters were implying….that there was a 20% chance it might rain (not the amount or location of the rain).

    Because Mother Nature is always changing, weather men and women have a very difficult job. Do they control the atmosphere? No. What they CAN certainly do, though, is use computer models and their broad base of scientific knowledge to help us understand it better. I think that meteorologists like Anthony Yanez are outstanding at what they do, and are not only relatively accurate, but also very educational. I truly appreciate their service and seeing their smiling faces every day – no matter WHAT the weather may have in store for us! 🙂

  3. 20% chance of rain means maybe within the general region a few raindrops must fall. What bothers me most is you call your weather reporting “severe” ? It makes it sound like we only have weather in Houston that causes damage. How about toning it down a bit to umm… “sensational” weather reporting…

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