Alright, which is simpler? The government is exposing its own citizens to various chemicals by spraying them out of jet engines in an incredibly inefficient manner, leaving the chemicals to travel randomly by wind without any control whatsoever as to who or what the chemicals affect, or "chemtrails" are simply a thermodynamic result of (as highimpact said) spraying hot exhaust in a cold environment. Hey what happened the last time you had a shower? Did your mirror fog up? Hey, that's condensation from the hot water vapor on your cold mirror! Oh wait, those form clouds in the sky! That's probably the reason why "chemtrails" do not disappear.

As a scientist, I know you never perform an experiment without having a hypothesis and control. What is their control here? I mean, its possible they spray chemtrails in some select cities but not in others, but there are many, many people claiming to have seen them in many cities. If the conspiracy is big enough that you have to say the program is just too big to comprehend, its probably just a pattern in the mist- the brain is excellent at picking out patterns, and we often pick out these patterns in random nonsense.

But hey, this is America, go toke up and believe what you will, as long as you don't impose those beliefs on others (too bad it never works that way)