Comment: Here are some:

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In post: Riot Theory
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Here are some:

http://www.thothweb.com/article-print-2367.html
Read this whole article. It is really good.

http://www.flickr.com/photos/7300722@N04/466997638/
These are good examples, others to the right of the main one.

http://www.squidoo.com/chemtrailcontrail
Good site for information.

I've seen the round or at least horseshoe trails recently from pictures here in the US, but I'll have to keep an eye out for them and post when I see them.