I would say the contrails are caused by as you said the air saturated with humidity at some extent, on that one the temperature is probably not far from the dew point, and mainly because, I think, on the pic it's pulling quite much in ressource, making it having enough G to create that effect. The evaporation temperature of water and any gaz is reduced with pressure, so when you pull G the vapor turns back to liquid water where the pressure is high. See fighters for example.
