“Tonight, we’ll see cloudy skies with a 40% chance of rain or snow”. We have all heard this phrase said by one of the 10/11 meteorologists but there’s a better chance most people are confused on the exact meaning of what that percentage really means. I’ll try and clear the skies of any doubt that may be developing.
The percentage is the Probability of Precipitation or as we in the biz say, “PoP”. And most people perceive it as… it will rain for 40% of the day…or over 40% of the area. It actually describes that precipitation could occur at any point in our viewing area on days that had similar weather conditions. There is a mathematical equal that forecasters use to calculate the percentage.
PoP = C x A
Where, C = the confidence that precipitation will occur somewhere in the area, and A = the percent that our coverage area will receive measurable precipitation, if any does occur.
So, I’ll use the 40% chance of rain or snow example above to show how the math works out. If we are completely confident the precip is going to happen (C = 100%) for 40 percent of the coverage area (A = .40), the math would look like this…
PoP = 1 x .4
Conversely, it could also happen that we are not as confident that we would get rain/snow (C = 50%) but that the majority of the area could see it (A = 80%), then the math would work out this way…
PoP = .5 x .8
PoP = 40%
Either way the math works, we will give more specific details on where and when our viewers can expect see anything Mother Nature throws at us. I hope this will break up the fog bank of confusion on this topic.
Meteorologist Tony DeGrand