Jul 23, 2014
Originally published on July 23, 2014 10:36 pm

Many listeners and readers felt a concise explanation of "a 20 percent chance of rain" was missing from this story about weather forecasts and probability, so we followed up with two meteorologists.

From meterologist Eli Jacks, of the National Oceanic and Atmospheric Administration's National Weather Service:

"There's a 20 percent chance that at least one-hundreth of an inch of rain — and we call that measurable amounts of rain — will fall at any specific point in a forecast area."

And from Jason Samenow, chief meteorologist with The Washington Post's Capital Weather Gang:

"It simply means for any locations for which the 20 percent chance of rain applies, measurable rain (more than a trace) would be expected to fall in two of every 10 weather situations like it."

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ROBERT SIEGEL, HOST:

It's time now for your letters. This week we've been exploring how well we understand probabilities. Yesterday I asked what it means when we hear there's a 20 percent chance of rain.

AUDIE CORNISH, HOST:

The question was put to a mathematician and a social scientist at the National Weather Service. But many of you weren't satisfied. Nancy Gold of Cambridge, Massachusetts, was among those left confused. She writes, does it mean that it will definitely rain in only 20 percent of the general area? That there's a one-in-five chance that it will rain in any one spot or what? Instead it turned out that there was apparently a 20 percent chance that you'd tell us. Dang.

SIEGEL: Well, we went back to the National Weather Service today. Here is meteorologist Eli Jacks.

ELI JACKS: There's a 20 percent chance that at least 100th of an inch of rain, and we call that measurable amounts of rain, will fall at any specific point in a forecast area.

CORNISH: I forecast that just 20 percent of you will be happy with that answer.

SIEGEL: And that's why we also checked back in with Jason Samenow, meteorologist with the Washington Post's Capital Weather gang. And he came up with this definition. It simply means for any locations for which the 20 percent chance of rain applies, measurable rain would be expected to fall in two of every 10 weather situations like it.

CORNISH: And then there was this letter from Jeffrey Riel of Leonex, Massachusetts. He writes, how 20th century of you - no one has to live with that uncertainty anymore. Mr. Reil explains, by following the satellite tracking of storms, weather on the NOAA or similar website or using a phone app such as RadarNow, I always know whether it will rain where I am, at what time it will arrive, its strength and its duration.

SIEGEL: He ends by writing, so I always know with 100 percent certainty whether it's going to rain on my parade or not. I am sorry to rain on yours.

(SOUNDBITE OF SONG, "DON'T RAIN ON MY PARADE")

BARBRA STREISAND: (Singing) Who told you you're allowed to rain on my parade. I'll march my band out.

CORNISH: Well, rain or shine keep sending your letters. Go to npr.org and look and contact on the bottom of the page.

(SOUNDBITE OF SONG, "DON'T RAIN ON MY PARADE")

STREISAND: (Singing) Your turn at bat, sir. At least I didn't fake it, hat, sir. I guess I didn't make it. But whether I'm the rose of sheer perfection. A freckle on the nose of life's complex. Transcript provided by NPR, Copyright NPR.