What do weather reporters mean by a percent chance of rain?
I don’t think they even know what they mean, but here’s the gist of it: Precipitation probability forecasts were developed by the National Weather Service in 1965. It works on a scale of 0 to 1. 0 means there’s no chance of precipitation (ppt) occurring, and 1 means it’s inevitably going to precipitate. However, when we see this scale used on the news, it has been converted to percentages. 0.6 is the same as 60%, 0.45 is 45% and so on. . . So what exactly does it mean when they say there is a 60% chance of rain? First of all, it must be understood that this percentage refers to a specific forecast area. i.e.) San Fernando Valley. Second, this percentage is valid only within a specific time frame; usually within a 12-hour time frame. Third, it is a percentage that says that AT LEAST 0.01″ of ppt. will fall at ANY POINT within the specific area within the next 12 hours (or whatever the time frame is.) Anything less than .01″ of ppt is considered trace ppt, and does not fall into the meas
