Dear Tom,
What does it mean when the forecast says there is a 50 percent chance it will rain for a given area? Does it mean there is a 50 chance it will rain in that entire area, or that 50 percent of that area will get rain?
— James Shaw
Dear James,
Neither of your statements is precisely correct. In weather forecasts, the probability of rain is a so-called “point probability,” the chance of measurable precipitation (at least 0.01 of an inch) at a specific location. Regarding a 50-percent forecast, the forecaster may believe the entire area will experience rain if it rains, but his confidence that rain will actually arrive is only 50 percent. Or he may believe scattered showers will definitely occur, but affect only 50 percent of the area. Regardless of the weather situation, the meaning for you is always the same: a 50 percent chance of rain on your head
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Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or: asktomwhy@wgntv.com
Weather Report is prepared by the WGN-TV Weather Center, where Tom Skilling is chief meteorologist. His forecasts can be seen Monday through Friday on WGN News at 11:30 a.m., 5:30 p.m. and 9 p.m.
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