I hope all you guys know these are condensation trails. Literally it is just a cloud. I'm honestly shocked at the number of people that don't understand the concept.
The two major components of exhaust are co2 and water vapor. Take that hot exhaust and cool it off and what do you get? You get air that is past its saturation point. Voila, the water vapor condensates into clouds.
The same thing can occur due to the changing pressure of the air flowing over the wing. I'm not going to go into great detail but if you understand the concept of an aerofoil, then you know that the pressure above the wing is much lower than the pressure below the wing. Air at a lower pressure cannot hold as much water vapor and again condensation can form.
The condensation trails produced at high altitudes are generally from the engines and the condensation trails at lower altitudes are generally from the pressure changes over the wings due to the higher ambient humidity at the lower altitudes.
The formation of these trails is all dependent on temperature and humidity where the plane is flying.
The rate that the cloud disperses is going to depend entirely on how the air is moving around it. Very calm or very smoothly flowing air will allow it to last longer where as turbulence will tend to disperse it much more quickly.
People really do like to jump on the conspiracy band wagon when there are things they don't understand. Anybody seen the "crazy sprinkler lady" on youtube?
EDIT: As for the climate experimentation, I think people are misinterpreting that. All of the studies I have seen on the subject are trying to determine if/how the condensation trails effect the weather/climate NOT purposely producing clouds as part of an experiment.
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