#WxGeekSpeak: Probability of Precipitation

Every week, the blog introduces you to a technical term from the American Meteorology Society’s “Glossary of Meteorology”. Welcome to #WxGeekSpeak!

What does a 20% chance of rain really mean? The video above settles the long debate about probability of precipitation.

Full definition of probability of precipitation: The forecast of >0.0254 cm (0.01 in.) of liquid equivalent precipitation at a specific point over a specific period of time. This specific term expressing the uncertainty inherent in a precipitation forecast originated concurrently with the nationwide implementation of a probability of precipitation forecast program by the Weather Bureau (now the National Weather Service) of the United States in 1966 and the statistical guidance supporting the program that continues today.

The term should not be confused with such concepts as the probability of precipitation over some portion of an area or the probability of precipitation over all of an area (larger than a point). If a POP of x percent were issued for an area, such as a county, that would mean the POP was valid at each and every point in the area. When alternate definitions are used [e.g., probability of precipitation > 1 cm (0.394 in.)], the event being forecast needs to be clearly specified.


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