Rebuttal post 2

At 2:59, narrator correctly describes high bypass turbofans engines, then goes on to say they are less conducive to making contrails.  Based on what?  His feelings?  Yes, the fan exhaust is lower than the core exhaust, which is then surrounded by a doughnut shaped layer of fan bypass air (one of the reasons turbofans are much quieter than turbojets.). The boundary layer between these 2 sources of thrust has less turbulence (and less velocity difference) than a pure turbojet exhaust into static air.  It could be argued this creates more visible contrails just as easily (any fluid dynamics engineers please chime in).