For many, meaning of rain forecast is cloudy at best
— -- When your local weather forecaster announces that there is a 30% chance of rain tomorrow, not everyone knows what that means.
Some think it means 30% of an area will get rain. Others think it will rain for 30% of the day. In fact, of all the forecast terms used by meteorologists, this remains one of the most baffling to the public.
Some people don't understand that the forecaster simply means there's a 30% probability it will rain at some point during the day. Susan Joslyn, a senior lecturer in the psychology department at the University of Washington in Seattle, and colleagues have been studying such confusion.
"It's well-known that people have a lot of trouble understanding this," Joslyn says. Her studies of more than 450 Pacific Northwest undergraduates found that although "probability-of-precipitation" forecasts have been used since the late 1960s, many people completely misunderstand them.
Most troubling is that Joslyn's research sample was of college students in the oft-rainy Northwest; confusion probably would be worse among the general public across the country.
Joslyn says the confusion arises because people are trying to simplify the information: "We try to take the simplest possible explanation and automatically think that it's area or time. We try to make it a deterministic forecast: that it will rain 30% of the time or in 30% of the area."
Why? "This may be part of a general human tendency to avoid the complications of incorporating uncertainty in the decision process by ignoring it or turning it into certainty," Joslyn and colleagues wrote this year in the Bulletin of the American Meteorological Society.
Ultimately, does the confusion really matter? As Joslyn and her co-authors note in the paper, the consequences of carrying around an unnecessary umbrella for a day may be small, but "for other weather-related decisions, the cumulative costs of unnecessary school closures, road treatments or crop-protection procedures could be substantial."
