When the weather forecast calls for "20 percent chance of showers," what does that really mean? Is it definitely going to rain on 20 percent of the forecast area, or is there a 20 percent chance of rain everywhere? Or does it mean something else entirely?

When the weather forecast calls for "20 percent chance of showers," what does that really mean? Is it definitely going to rain on 20 percent of the forecast area, or is there a 20 percent chance of rain everywhere? Or does it mean something else entirely?

— G.S., Medford

There are lots of bad jokes about journalists doing math (especially percentages), so we'll hope we get this right, G.S.

For thunderstorms and other intensely localized weather phenomena, a 20 percent chance means there's a one-in-five likelihood of a T-storm at any particular site in our local forecast area, whether it's Medford, Prospect or Wimer, says Marc Spilde, a forecaster at the National Weather Service's Medford office.

Looking at the other side of the coin, that means there's a four-in-five chance that you won't get torrential rain, limb-snapping winds or hail big enough to dent your car roof. Pretty good odds, eh?

The percentage thing works a little differently during winter, when storm fronts are large and tend to affect the forecast area as a whole. A 20 percent chance of rain or showers during a winter storm would mean a one-in-five chance of measurable precipitation (defined by the Weather Service as at least .01 inch) across the forecast area.

Got it? There'll be a quiz Friday.

Send questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by e-mail to youasked@mailtribune.com. We're sorry, but the volume of questions received prevents us from answering all of them.