Short ones that disappear immediately. (Contrails)
Very long ones that hang in the air for hours. ("Chemtrails"--whatever those really are)
The trails that hang around for hours often turn into "normal-looking" clouds or a pearly haze. Occasionally they disperse completely.
Trails that turn into clouds usually result in rain within about 24-48 hours.
Trails that turn into a pearly haze and screw up the air quality for the rest of the day.
The long trails are often created in rows or a grid pattern--very neatly done. I've watched planes do this over the course of a morning. They are not flying somewhere on a normal flight pattern.
What I don't know, because I cannot observe this, is what the long trails are made of and why they are being dispersed. I think most of the time it's to seed clouds and create rain, in which case these "chemtrails" are probably "mostly harmless".
Either way, there's nothing I can do about it directly aside from look at the sky and go "hmm". If it's chemicals, then I'm gonna have to trust in my own body's ability to clean garbage out of it.