People's Misperceptions Cloud Their Understanding Of Rainy Weather Forecasts

http://www.sciencedaily.com/releases/2009/04/090414130623.htm

ScienceDaily (Apr. 15, 2009) — If Mark Twain were alive today he might rephrase his frequently cited observation about everyone talking about the weather but not doing anything about it to say, "Everyone reads or watches weather forecasts, but many people don't understand them."
He'd do that because new research indicates that only about half the population knows what a forecast means when it predicts a 20 percent chance of rain, according to researchers at the University of Washington.

Writing in the Bulletin of the American Meteorological Society, the researchers said the confusion comes because people don't understand what the 20 percent chance of rain actually refers to. Many people think it means that it will rain over 20 percent of the area covered by the forecast or for 20 percent of the time period covered by the forecast, said Susan Joslyn, a UW cognitive psychologist and senior lecturer.

"When a forecast says there is 20 percent chance of rain tomorrow it actually means it will rain on 20 percent of the days with exactly the same atmospheric conditions," she said. "With the exception of the probability of precipitation, most weather forecasts report a single value such as the high temperature will be 53 degrees. This is deterministic because it implies that forecasters are sure the high temperature will be 53 degrees. But forecasting is probabilistic and 53 degrees is in the middle of the range of possible temperatures, say 49 to 56 degrees."