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Definition of Probability of Precipitation
The National Weather Service defines "probability of precipitation" as the chance that at least 0.01 inch will fall in a given location within an area or region. A forecast of "60 percent chance of rain" means that there is a 60 percent chance that the location will see at least one hundredth of an inch. Probability of precipitation does not measure the intensity of rainfall or how long it will occur.
Formula for Probability of Precipitation
The probability of precipitation is calculated by multiplying the forecaster's confidence -- 0 percent to 100 percent -- that measurable rain will fall somewhere in the forecast zone by the percentage of the zone expected to see rain if there is rain. If a forecaster is absolutely certain, or 100 percent confident, that rain will occur somewhere in the area, and predicts that rain will cover half -- 50 percent -- of the zone, the probability of precipitation is 50 percent.
Descriptive Terms
Forecasters may describe the uncertainty and coverage of rainfall with terms assigned to particular probabilities. A 10 to 20 percent chance of rain means a "slight chance" or isolated or widely-separated areas of precipitation. At 30 to 50percent, a forecaster predicts a "chance" or "scattered" precipitation. Rain is "likely" or showers are expected to be "numerous" when the chance is 60 to 70 percent. A forecast may say "rain" or precipitation is "highly likely" when the chance is 80 to 100 percent.
Forecast Periods
Probabilities of precipitation are issued for particular forecast periods. The National Weather Service defines "today's" forecast as a period from sunrise to sunset for the current day. A forecast for "this afternoon" covers noon to 6 p.m. "This evening" refers to the period from 6 p.m. to sunset. "Tonight's" forecast predicts the weather from sunset through sunrise. The period from the following day's sunrise to sunset is "tomorrow's" forecast.