What do forecasters mean when they say things such as, “There is a 30 percent chance of rain”?
The National Weather Service says, “The probability of precipitation is the likelihood of measurable precipitation (0.01 inches or greater) for a specified forecast time period, and occurring at any point for which the forecast is valid.” A reasonable English translation of this bit of government speak is: The probability gives the odds of any one place in the area covered by the forecast getting wet, whether it’s from rain or snow. The 0.01 inches or greater comes from the fact that any less rain or water from melted snow or ice can’t be measured. If the bottom of the rain gauge is wet, but the water isn’t deep enough to measure, that’s called a “trace’ and really doesn’t count. A forecaster assigns a precipitation probability that shows his or her confidence in the forecast, how much of the forecast area is likely to have precipitation, and low long the precipitation is expected to last. The important point is that the odds are for the rain or snow to fall on any place in the area co
