Weather Update: Polar Vortex, Summer Edition

Air Date:
Heard On The Larry Meiller Show

Larry Meiller visits with the Weather Guys and finds out whether that was really the polar vortex bringing cooler temperatures earlier this summer. Plus, how to interpret the probability of rain on a given day.

Featured in this Show

  • What Does A 20 Percent Chance Of Rain Really Mean?

    NPR has been doing a series about probability and what it means in a variety of fields and situations. One of the topics it recently covered was what it exactly means when a weather forecast predicts a 20 percent chance of rain on a given day.

    “A lot of people misinterpret that,” said Steve Ackerman, the director of the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin-Madison and a professor of atmospheric sciences.

    Ackerman explained that when a forecast says that there is a 20 percent chance of rain in a particular location, it means that if there were 100 days with identical conditions, it would rain on 20 of those days.

    Despite those identical conditions, Ackerman said there are tiny factors that determine whether or not it will actually rain.

    Jon Martin, a professor of Atmospheric and Oceanic Sciences at UW-Madison, agreed.

    “That one thing that makes the difference might be hard to detect in the forecast office, and therefore (makes it hard) to specify where it might happen,” Martin said.

    Martin added that conditions in Madison on Sunday night before were a perfect illustration of this.

    “It was obvious that there were going to be scattered thunder showers,” he said. “What’s not obvious is whether your backyard is going to be underneath one of them during the course of their four-hour or so life cycle in the afternoon.”

    Since it is impossible to say with certainty that all locations are sure to get rain, the forecast has to express that variable.

    “So, it’s best to say, ‘If I have 100 such days, there’s no way that on 20 of them your backyard won’t see some of that rain,’” said Martin.

    Ackerman noted that Madison has had a lot of scattered showers in the forecast recently.

    “Unfortunately, they’re missing my yard,” he said. “And I need water on my flowers!”

Episode Credits

  • Larry Meiller Host
  • Judith Siers-Poisson Producer
  • Steve Ackerman Guest
  • Jon Martin Guest

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