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Persistent Spreading Contrails over Minneapolis
I saw some interesting airplane contrails over Minneapolis today. They didn't dissipate quickly like I would expect them to. So since I am overly curious, I watched them for several hours. Here are some photos of what I saw.

12:53 pm, CDT
Most planes travel in the direction of the bottom of the photo to the top (east to west). Though I didn't see these planes as they flew over. These photos were taken northwest of the MSP international airport.

12:55 pm, CDT
This is a close-up taken at the same time as the photo above.

1:17 pm, CDT
The contrails have moved southeast, leaving a light haze behind.

1:35 pm, CDT
The contrails have moved southeast and started to dissipate. The haze is more noticeable in this photo.

1:51 pm, CDT
The contrails have moved more to the southeast. The sky is now just generally hazy. I am using the roof line to block the sun glare on the camera lens -- the glare is a little visible in the top right of the image.

2:03 pm, CDT
The contrails have not moved much more. Wispy clouds are starting to form.

3:53 pm, CDT
This picture was taken an hour and fifty minutes later, three hours after the first photo. Very distinct wispy clouds.

My Thoughts
I'm not really sure what I was looking at. Definitely airplane contrails. This seems like a pollution problem to me. Unless I am misunderstanding, these airplanes seem to have left some white haze behind in their exhaust. Maybe they are running really old jet engines that have become more inefficient and polluting over time?

Further Information
NATO has posted this study on jet engine pollution: Gaseous and Particulate Emissions with Jet Engine Exhaust and Atmospheric Pollution.

This NASA news release from 2004 cites the Journal of Climate, saying that cirrus clouds created by jet aircraft exhoust could be a significant cause of climate warming.
Here is a link to the actual journal article: Contrails, Cirrus Trends, and Climate. Journal of Climate, Volume 17, Issue 8 (April 2004).

There is some interesting information and ideas here, I was especially interested in the historical photos of contrails. Not sure how reliable this all is though: