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PRI's Environmental News Magazine

9/11 Contrails

Air Date: Week of May 31, 2002

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When the FAA shut down commercial air traffic for three days last September, researchers were able to test their theories about how jet contrails can influence weather. Host Steve Curwood talks with atmospheric scientist David Travis about his study.


CURWOOD: You probably remember the eerily quiet and clear skies of September 11th. For three days following 9/11, all commercial air traffic in the United States was grounded. And except for military flights, the sky was free of airplane contrails.

Contrails are those long white cloud-like plumes that jet engines leave behind. David Travis joins me. He’s an atmospheric scientist at the University of Wisconsin-Whitewater, who studies contrails and how they affect weather.

So Mr. Travis, as horrible as the events of September 11th were, they did give you a chance to check your contrail theories in the real world.

TRAVIS: You know, it’s not the way we wanted this opportunity to come about, obviously. And, I was even a bit hesitant to get into it initially. But, we figured this was the only time, we hope this will be the only time, this sort of thing ever happens.

And, it’s also sort of a convenient coincidence for us that we had rather fast weather patterns across the country during those few days. We had periods of clear skies. September 11th, for instance, was really unusually clear. And then following September 11th, we had increasing cloud coverage and moisture on the 12th and the 13th across the east. And this allows us to take out the argument that changes in the temperature range were simply a function of the weather patterns occurring. There was no persistent one type of weather pattern, either completely sunny or completely cloudy for three days. And that really helped.

CURWOOD: Now, how did you use this aircraft shutdown to figure out how contrails affect our everyday weather patterns?

TRAVIS: Well, what we’ve been doing for the last ten years or so is publishing a series of papers, and doing a series of research projects, and investigating the effects of contrails in a variety of ways. There have been studies that have done modeling types of analyses where we go and we look at contrails, and try to predict what kind of affect they would have. We’ve also used a variety of other circumstantial evidence types of arguments to basically point out that we believe that jet contrails, especially as their coverage increases, could be affecting the temperature range, just like what natural clouds do.

Basically, natural clouds, during the daytime block out sunlight. And at night, they trap the outgoing radiation. So they keep it cooler during the day, and they keep it a little warmer at night. And we’ve been arguing for a while now that jet contrails are enhancing that effect.

CURWOOD: So what did you find when you looked at the weather records for those three days in September?

TRAVIS: Well, we first started looking at satellite imagery to confirm our suspicions that there was a notable decrease in cloud coverage. And, we began initially studying that. But we decided to turn our attention more directly to the surface temperature observations. And we looked at about 4000 weather stations around the United States. And, we found, immediately, that we noticed an increase in the daytime temperature, most notably, for those three days across the country and a slight decrease in the nighttime temperature.

So in essence, we did see that there was an unusual increase in the temperature range. And we compared that to see how unusual it really was. Because of course, we get random variations in weather patterns all the time. And it could have easily been explained by that. We found that, indeed, the temperature range was the largest during those three days than we had seen in the past 30 years.

CURWOOD: Talk to me about some of the regional differences you must have seen.

TRAVIS: We speculated that, if jet contrails were really having an impact, we would see these changes the greatest in places where aircraft coverage is most frequent and specifically where contrails are most prevalent. And that’s places like the Midwest, the Northeast, and even the Pacific Northwest. These are areas where we get a lot of air traffic. But, the airplanes are not necessarily flying in and out of hubs a lot. They’re actually at cruising altitude above these areas where they’re a lot more likely to produce contrails. And what we found was, indeed, the temperature range increase was more than twice as large in these areas as it was for the rest of the country.

CURWOOD: Now, what difference does a degree or two make, do you think?

TRAVIS: Well, I know to the average person, and even myself, sometimes when I’m dealing with this data, and I see that there are changes of a magnitude of only 1 degree Celsius, or 2 degrees Celsius, in various studies I’ve done, initially, it sounds like it’s not very much.

But you have to remember a couple of things. One is that we’re looking at a change against 30 years worth of data. So, we’re not looking at simply a change that happened over those couple days, but we’re comparing it to the average over the past 30 years. And we’re also looking across a very large expanse. We’re looking across the entire United States. And so, a 1 to 2 degree Celsius change, which is basically the average of what we found in the temperature range, is actually quite substantial. And so, there are a variety of affects out there where we see reducing the temperature range as being a negative thing on natural ecosystems.

If you look at a lot of the global warming modeling studies that have been done, and those that are predicting huge increases in global temperature, and the ramifications that these increases are going to have on sea level rises, and so on, most of them are projecting just a few degrees increase in Celsius. So we’re not too far off in that effect.

[MUSIC: L’ALTRA, "Traffic," IN THE AFTERNOON (Aesthetic – 2002)]

CURWOOD: David Travis is an atmospheric scientist at the University of Wisconsin-Whitewater. Thanks for taking this time with us today.

TRAVIS: My pleasure.



Dr. Travis’ webpage with a link to the research paper

A contrail page from NASA">


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