What is the Meaning of Probability of Precipitation?

Posted by Kyle Brookings on Sunday, August 20, 2017
black and-white, clear, cool
When we look at a weather report we often see the term "probability of precipitation", but what does it mean?

First off, what is precipitation? Well precipitation is any form of water - liquid or solid - falling from the sky.

The "Probability of Precipitation" (PoP) describes the chance of precipitation occurring at any point in the forecast area.

So if there is a "40 percent" chance of rain, it means it will produce measurable precipitation over an area.

Mathematically, PoP is defined as follows:
  • PoP = C x A where "C" = the confidence that precipitation will occur somewhere in the forecast area, and where "A" = the percent of the area that will receive measurable precipitation, if it occurs at all.
  • So... in the case of the forecast above, if the forecaster knows precipitation is sure to occur ( confidence is 100% ), he/she is expressing how much of the area will receive measurable rain. ( PoP = "C" x "A" or "1" times ".4" which equals .4 or 40%.)



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