Okay, to get the first thing out of the way. I believe that there is a scientific explanation for CONtrails. It's the hot air that comes out of the jet engines. The air cools down and the water condenses because cold air can't hold as much water as the hot air can.

But here comes the thing where I can't get my head around:
We can observe the same behaviour at cold days when we breathe out. But in this sceneario the vapor vanishes within seconds whereas contrails stay in the air for several hours. Even more so the air velocity at high altitudes is significantly higher than at lower altitudes so the contrails should actually vanish faster. But it is reasonable to ask the same questions for clouds...