Cuchculan
Let us not rush into any weird conspiracy theories here. Some things can be explained away. But some people simply don't want to listen. They like their theories. So won't believe the real truth. Which is as follows.
Jet engines spew out very hot air. And, because water vapour is one of the byproducts of the exhaust, the air is also very humid. However, high in the atmosphere where these jets fly, the air is typically very cold -- often lower than -40 degrees Fahrenheit. Additionally, the atmosphere up there is often of low vapour pressure, or the force exerted by a gas on the surrounding environment. When a jet engine is spewing out hot, humid air into an atmosphere that is cold and has low vapour pressure, the result is condensation. The water vapour coming out of the engine quickly condenses into water droplets and then crystallizes into ice. The ice crystals are the clouds that form behind the engine. This is why the streaks are called contrails, short for "condensation trails." To help explain it, scientists liken it to seeing your breath on cold days. You may have noticed that puffs of breath dissipate quickly on drier days. The same is true of contrails: When the atmosphere is more humid, the contrails linger, but when the atmosphere is dry, the contrails disappear more quickly.