When the weather forecast calls for a 20 percent chance of rain, does that mean it will rain on one-fifth of the forecast area, or what?

When the weather forecast calls for a 20 percent chance of rain, does that mean it will rain on one-fifth of the forecast area, or what?

— Bob J., via e-mail

It's a little more scientific than that, Bob. When meteorologists say there's a 20 percent chance of rain, they mean that rain will fall on 20 percent of the days that have those particular atmos-pheric con-ditions. That's one in every five, or pretty good odds you can leave your umbrella at home if you're a betting man.

By the same method, an 80 percent chance of rain means that rain will fall on four days out of every five with similar conditions — a strong likelihood you're going to get wet.

Here in the Rogue Valley, where the sodden Northwest meets California's sunny Mediterranean climate, we have lots of 40- and 60-percent days between October and March.

We also get a few of those 100 percent days, usually in November, December and January, when you don't need a weatherman to tell you it's gonna be wet, and stay wet.

On the other end of the scale, there's summer in Southern Oregon, when the probability of rain rarely rises above zero between the Fourth of July and Labor Day.

Interestingly, you're not the only person who's unsure about forecast terms. Researchers at the University of Washington recently tested more than 450 college students and found many of them were confused. Some thought a 20 percent chance of rain meant it would rain on 20 percent of the forecast area; others, that it would rain 20 percent of the day.

Only about half of the students got it right — that rain would fall on 20 percent of the days with similar conditions.

The test results were published in the February 2009 issue of the Bulletin of the American Meteorological Society.

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