MELISSA BLOCK, host:
The next time you're delayed by rain or snow at an airport, consider this: It's possible that an airplane actually caused the bad weather. NPR's Jon Hamilton reports on the surprising things that can happen when plane meets cloud.
JON HAMILTON: In 2007, a plane full of weather scientists flew through a very odd snowstorm near Denver International Airport. The storm was unusual because it left only a narrow stripe of snow leading from the runway. One of the scientists on the plane was Andy Heymsfield from the National Center for Atmospheric Research. He says the team realized that the snow was following the exact flight track of two turbo-prop aircraft that had taken off just a few minutes earlier.
Mr. ANDREW HEYMSFIELD (National Center for Atmospheric Research): The quite amazing thing was that their flight track actually produced about an inch of snow at the ground.
HAMILTON: Heymsfield says he and his colleagues were pretty sure the planes had made that snow fall, but how? Research in the 1980s found that airplanes, especially those with propellers, can cause ice crystals to form when they fly through certain clouds at just the right temperature.
So it seemed plausible that planes could trigger a little snow. And Heymsfield found evidence that they often did. But as he used satellites and airport data to study the phenomenon, he realized that a single aircraft could also cause snow to fall for miles around.
Mr. HEYMSFIELD: So the thing that piqued my interest was how the heck do you go from a little line that an aircraft makes and produce something that's many kilometers across.
HAMILTON: Heymsfield and his team found an answer by studying precisely what happens when an airplane flies through a very special type of cloud. The cloud has to contain water that's below freezing temperature but hasn't frozen yet. When an airplane comes along, its wings and propeller cause pockets of air to expand. That expansion reduces the air temperature, pushing it below a critical point where ice crystals start to form.
Then a sort of chain reaction takes place, causing the ice crystals to radiate outward, sometimes for miles. And ultimately, the crystals fall out of the sky as snow or rain, sometimes leaving behind a visible hole in the cloud layer. It's called a hole-punch cloud.
Mr. HEYMSFIELD: Oh, they're unbelievable looking. It looks like if you took a white sheet of paper and used a hole punch. It just - you know, you wonder how the heck could something like this happen.
HAMILTON: Heymsfield says his research suggests these clouds form surprisingly often at certain airports after airplanes take off.
Mr. HEYMSFIELD: Of course they take off into the wind. And so that snow, which would be generated, would then go over the airport.
HAMILTON: So you could have the airplanes cause their own weather delay.
Mr. HEYMSFIELD: Oh, absolutely.
HAMILTON: But most of the time, the extra flakes don't make much difference, and the cloud formation that's left behind is of more interest to cloudspotters than travelers.
Mr. GAVIN PRETOR-PINNEY (Founder, Cloud Appreciation Society): It can look like a very dramatic and unusual cloud formation.
HAMILTON: Gavin Pretor-Pinney founded the Cloud Appreciation Society and is the author of "The Cloudspotter's Guide." He says that before people understood how hole-punch clouds were made by planes, they were sometimes mistaken for the exit path of a flying saucer.
Mr. PRETOR-PINNEY: You see photographs of them, and you do think: ah, there's something strange there. That's not natural.
HAMILTON: But Pretor-Pinney says the fact that they're also not supernatural does not make them any less attractive or intriguing.
Mr. PRETOR-PINNEY: Each time one understands a little bit more, one engages a little bit more with them, one pays a little bit more attention to them, and for me that means one's a little bit more aware of their beauty and value.
HAMILTON: The new study appears in the journal Science. Jon Hamilton, NPR News.
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