Contrails are the result of both exhaust fumes (jet fuel is similar to diesel fuel) and the effects of extremely hot exhaust gases reacting to the significantly colder atmospheric air.
Contrails are due to the atmospheric conditions, temprature and wind ( still).The exhaust from the engine freezes quickly under certain conditions and above 30,000 feet, which you see as trail of smoke frozen. Some times due to fuel is injected at the end of turbine to reduce the landing weight. this can also cause white line behind aipcraft.
White smoke normally means you are burning oil.
oil getting by the rings causes white smoke and oil dripping out of the tailpipe.
white smoke can be either your car overheating, or smoke from an electrical issue
White smoke means head gasket, cracked head or equivelant.
blown headgasket
water
blue smoke is oil. white smoke is water. black smoke is unburnt fuel
Quality of fuel plays major role in smoke emission
smoke
condensation
It's the fuel from the engines.