Probably the most common question that I get on a day to day basis, is "What's the chance for rain?" Now, they're not asking whether I think it will rain or not, they are looking for that "percent number" that we put on our forecasts. As a joke, I could always go with 50%...it might, or it might not! But that's not how it works.
While the definition of the rain chance is pretty simple, the interpretation of rain chance varies from person to person. To set the record straight, the percent value, normally rounded to the nearest 10%, is the chance that the atmosphere will produce at least 0.01" of rain in any location of the geographic area that the forecast represents during a particular time period.