So... What Is A (PoP)

By: Bob Trihy
By: Bob Trihy

PoP in weather terms means Probability of Precipitation. For a lot of folks it's just plain confusing.

Often times you'll see a forecast something like this:

This Afternoon...Mostly cloudy with a 40% chance of showers and thunderstorms. Highs in the lower 80s.

The 40% is known as a (PoP) or probability of precipitation. This is a confusing term...even to a lot of meteorologists.

So...does this mean it will rain 40 percent of the time?  Will it rain over 40 percent of the area? Well..kind of.

The official definition of a PoP describes the chance of precipitation at any given point in the forecast area.

So it's math time.

PoP = C x A where "C" is the confidence that precipitation will occur somewhere in the forecast area and where "A" is the percent of the area that will receive rain measurable precipitation, if it occurs at all.

Most of the time..the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur..and expects that if it does occur, it will produce measurable rain over 80% of the area. The PoP is 40% (PoP = .5 x 8...which equals .4 or 40%.

So after all that...this is the correct way to interpret the forecast. There is a 40% percent chance that rain will occur at any given point in the area. But of course if it's raining where you are it's 100%.

Personally I'm not crazy about the (PoP).  But then again it is nice to see it in the forecast pages and the extended. Because after all... when you see a chance of rain at 80...90..or 100% percent... you know chances're gonna get wet.

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Comments are posted from viewers like you and do not always reflect the views of this station.
  • by Whezza Location: Martin County on May 19, 2009 at 08:18 PM
    Bob, I am still LOL, what a great respose! But I still want to know the difference between scattered and isolated thunderstorms........still seems like same thing to me......still lol.
  • by Bob on May 18, 2009 at 04:03 AM
    You got it all wrong.. PoP is soda in the northern states.
  • by Whezza Location: Martin County on May 17, 2009 at 02:31 PM
    Ok, I get the PoP....if i look outside or got outside and it is wet....then the PoP is 100% at that given point. Now, please explain the difference between Scattered thunderstorms and Isolated thunderstorms. Seems the same to me, of course, i am still kinda stuck on PoP. Thanks!

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