There is a 30% chance of rain on Saturday, how did they come up with that percentage.

Forecasts issued by the National Weather Service routinely include a "PoP" (probability of precipitation) statement, which is often expressed as the "chance of rain" or "chance of precipitation".

__An__

__example of this__:

ZONE FORECASTS FOR NORTH AND CENTRAL GEORGIA NATIONAL WEATHER SERVICE PEACHTREE CITY GA 119 PM EDT THU MAY 8 2008

GAZ021-022-032034-044046-055-057-090815-CHEROKEE-CLAYTON-COBB-DEKALB-FORSYTH-GWINNETT-HENRY-NORTH FULTON-ROCKDALE-SOUTH FULTON-INCLUDING THE CITIES OF...ATLANTA...CONYERS...DECATUR...

EAST POINT...LAWRENCEVILLE...MARIETTA 119 PM EDT THU MAY x 2008

.THIS AFTERNOON...MOSTLY CLOUDY WITH A 40 PERCENT CHANCE OF SHOWERS AND THUNDERSTORMS. WINDY. HIGHS IN THE LOWER 80S. NEAR STEADY TEMPERATURE IN THE LOWER 80S. SOUTH WINDS 15 TO 25 MPH. .TONIGHT...MOSTLY CLOUDY WITH A CHANCE OF SHOWERS AND THUNDERSTORMS IN THE EVENING...THEN A SLIGHT CHANCE OF SHOWERS AND THUNDERSTORMS AFTER MIDNIGHT. LOWS IN THE MID 60S. SOUTHWEST WINDS 5 TO 15 MPH. CHANCE OF RAIN 40 PERCENT.

What does this "40 percent" mean? ...will it rain 40 percent of of the time? ...will it rain over 40 percent of the area?

The "Probability of Precipitation" (PoP) describes the chance of precipitation occurring at

*point you select in the area.***any**
How do forecasters arrive at this value?

Mathematically, PoP is defined as follows:

PoP = C x A where "C" = the confidence that precipitation will occur

*in the forecast area, and where "A" = the percent of the area that will receive measureable precipitation,***somewhere****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%.)

*But*, 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 about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )

In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area.

Explaining "Probability of Precipitation": http://www.srh.noaa.gov/ffc/?n=pop

You can have your students multiple percentages and come up with their own weather forecast for the surrounding area for the next couple of days and see if the weatherman in your area are more accurate at predicting than you are.

Do you think you could put more real-world scenarios in your classroom? Is this a good example to start with in your room?

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