On a sunny day, look up and you may see a contrail, which is a normal vapor trail that dissipates as quickly as a jet passes. Then there are chemtrails, which hang up there all day and spread out. How the hell can people not notice these things?
I know (or at least claim to know) many things, but one thing I'll never understand is why you chemtrail folks persist in seeing me as a likely convert to the cause. I may be full of shit, but that doesn't mean I want to be full of your shit.
I'm no apologist for the status quo, and I'm perfectly prepared to believe the military-industrial complex is evil enough to do what chemtrail theorists claim. I'm just not willing to believe they're that smart.
I understand you've seen evidence supporting the chemtrails theory. It was probably the first evidence you ever looked at closely, and naturally, you were impressed. But remember, Orly Taitz has evidence the president was born in Kenya. Those guys in Utah had evidence of cold fusion. Erich von Däniken (remember him?) had evidence that aliens built the pyramids. Evidence is cheap.
Besides, if you're looking for a meteorological conspiracy to expose the excesses of global capitalism, may I humbly suggest global warming? It's got end-of-the-world sex appeal, plenty of real science on its side, and doesn't require anything of its conspirators beyond garden-variety human greed, stupidity and short-sightedness.
As for the varying durations of contrails: The ones that run through warm, dry air evaporate quickly. The ones that run through cool, wet air stick around for a while. Car exhaust (or even your own breath) will do the same thing, dissipating quickly in dry conditions and hanging around longer on cold, rainy days.