See the bump in the blue line on the chart at right? That represents the more than 8,000 Americans who listen to National Public Radio on their iPhones (rather than, say, their car radios) between 8 a.m. and 10 a.m. on a typical weekday morning — one of the more surprising results of an hour-by-hour study of listener habits released by NPR Wednesday.

“This is a unique pattern among NPR mobile platforms,” writes NPR data analyst Meredith Heard about the iPhone listeners, “and may indicate a strong use of that particular app during the morning commute.”

Heard speculates that some iPhone owners are streaming the station through their car stereo, something the latest research from Infinite Dial indicates 6% of U.S. cell phone owners do.

Although NPR is also available on Apple’s (AAPL) iPad, Google (GOOG) Android devices, and NPR’s mobile website, those platforms see more steady use until early evening. That’s when, as Heard puts it, “the mobile site and iPad app experience a bit of a second wind of traffic, perhaps as people get their last dose of news before bed.”

An earlier version of this story suggested that more people listen to NPR through the Web than on the radio. This is not true — far from it, as NPR publicist Emerson Brown made clear in a follow-up note:

“Radio continues to be the #1 way that people hear/experience NPR. 27 million people listen to NPR programming on the radio every week — a number which actually showed growth from the previous reporting period. Also, in just a quarter hour of radio (also known as AQH – a standard radio metric) NPR reaches more than 2.4 million average listeners during typical weekday morning drive time, which dwarfs all the other platforms.”

It’s all laid out in the NPR slideshow (Flash required) pasted below. You can read Heard’s analysis here.

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[Follow Philip Elmer-DeWitt on Twitter @philiped]