Most people wrongly understand rainy weather forecasts

April 15th, 2009 - 1:49 pm ICT by ANI  

Washington, Apr 15 (ANI): Only half the people understand what a forecast means when it predicts a 20 percent chance of rain, according to researchers at the University of Washington.

Susan Joslyn, a UW cognitive psychologist and senior lecturer, has revealed that the majority of 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.

“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.

She added: “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.”

In order to know about people’s understanding of the more familiar probability of precipitation, the researchers tested more than 450 Pacific Northwest college students in three experiments.

It was found that students wrongly perceived rainy weather forecasts, and that an explicit statement, such as there is a chance it won’t rain, could weaken the percent of time and area misconceptions.

The researchers said that a person, who thinks that a probabilistic forecast means that the weather event will occur (in some percent of the area or for some percent of the time), might be more likely to take expensive precautionary action than someone who realized that there was only a chance of that event occurring.

Joslyn added if the misunderstandings uncovered in this research exist among a college-educated group of students from the Pacific Northwest, where it frequently rains, then similar error probably occur in similar, or larger, numbers elsewhere among the general public.

According to the researchers, the errors are caused by the difficulty in making decisions when uncertainty is involved.

“In dealing with a forecast about rain people must simultaneously consider several hypothetical outcomes, their corresponding levels of uncertainty and their consequences. For some people it may be easier to commit to a single outcome, reducing cognitive load, and proceed as through the uncertainty has been resolved. In some cases they may not be aware of this simplification,” said Joslyn.

The research also has financial implications for forecast uncertainty and misinterpretations about such weather-related decisions as school closures, agricultural crop protection and highway and road clearing during storms.

The study has been published in the Bulletin of the American Meteorological Society. (ANI)

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