Contrails are condensation trails, or vapor trails, made by the exhaust of aircraft engines or wingtip vortices (quite a difference, don’t you think so?) which precipitate a stream of tiny ice crystals in moist, frigid upper air (thanks to Wikipedia). For some reason I believed that it’s a turbulent flow, but Tennekes and Lumley says that it’s just ‘born turbulent’ but if it does not diffuse after a while, than it doesn’t hold the diffusivity property of turbulent flow. So, it’s not turbulent. So what.