Conspirationists call these "chemtrails"

A few days ago I discovered on XKCD this crazy "chemtrail" conspiracy theory: trails left by aircraft would actually be chemical or biological agents deliberately sprayed at high altitudes for purposes undisclosed to the general public in clandestine programs directed by government officials. (Wikipedia).

 

So this is the kind of photo they use as a "proof". Wow... These conspiracy theory fanatics sure have a LOT of imagination. Well, at least they gave me an idea for the title for a change. :)

Done