What does “80 percent chance of rain” mean?

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Dear Tom,
If there is an 80 percent chance of rain today in a certain area, does that mean 80 percent of the area will get some rain? Or does it mean that there is an 80 percent chance there is any rain in that area?
Debbie Goddard, Plainfield
Dear Debbie,
Questions similar to yours come up frequently, so let’s address the issue again. An 80 percent chance of rain (or of any other kind of precipitation) means the weather forecaster believes there will be an eight in ten chance (or 80 chances out of 100) of measurable precipitation (0.01 inch or more) in the area under consideration during the time interval that is specified in the weather forecast (for example, “today” or “this afternoon”, etc). Ideally, a forecaster would prefer to issue a “zero” or “100 percent” chance of precipitation in a weather forecast, but the state of the “forecasting art” rarely allows that degree of certainty.


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