It is indeed marginal. Whether an aircraft contrails depends on a range of factors, relative humidity being among the most significant, but also altitude/air pressure, air temperature, temperature and pressure of the aircraft exhaust (they do vary), proportion of water vapour in the exhaust gases, and engine power setting. Dissipation is also dependent on winds and, to some extent, turbulence if any of the air.

It is calculable by meteorologists. Although not really of importance to civil airliners, it is very significant to the military who may not wish to draw attention to their presence, hence the work done to understand whether contrails will appear on any given day.

It's not just jets. WW2 piston-engined bombers over Europe, even at only 25,000 feet, certainly contrailed on occasions, which is why this analysis started. You can see it from B-17s in the 1943-original film "Memphis Belle" (not the more recent Hollywood travesty).