What does a 60 percent chance of precipitation mean?

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What does a 60 percent chance of precipitation mean?

The probability of precipitation (fondly known as PoP) has been part of weather forecasts since the late 1960s, and is the only forecast element that includes a probability. Unfortunately, there is confusion about the exact meaning of a “60 percent chance of precipitation.” Part of that confusion, we are sad to say, reflects discrepancies in interpretation and application among meteorologists.

PoP does not mean

    • the percent of time precipitation will be observed over the area; or
    • the percentage of meteorologists who believe precipitation will fall!

The most common definition among meteorologists is the probability that at least one one-hundredth inch of liquid-equivalent precipitation will fall in a single spot. Here’s a good way to grasp a PoP of 60 percent: If we had ten tomorrows with identical weather conditions, any given point would receive rain on six (60 percent) of those days. And rain would not fall on four of those days — on any given point.

Remember: this PoP is a forecast for 10 potential tomorrows, not a forecast for the next 10 days! Although this may be confusing, weather forecasters have no problem thinking of tomorrow’s weather as a group of potential tomorrows.

One thing you can take to the bank: as the PoP increases, precipitation grows more likely.

Steven A. Ackerman and Jonathan Martin are professors in the Department of Atmospheric and Oceanic Sciences at UW-Madison, are guests on the Larry Meiller‘s WHA-AM radio show the last Monday of each month at 11:45 a.m.