just a couple of probs with that post
I don't think you need either temp below zero, or super saturated air.
Certainly supersaturated air will help, but you can get contrails at an ambient RH of less than 100% because you put more water into the air (and thus can rise the local RH to above 100%).
If the air is supersaturated you do not need to add any water - just add condensation nuclei.
Non supersatured air will also form contrails behind wingtips/flaps etc because the wing changes local RH.
The temp is also irrelevant. You can get contrails at above 0 degrees. Relative humidity is the important thing.
You can get condensation about the wing tips of aircraft near the ground on hot balmy days in poor weather (RH basically almost 100%). You are less likely to get contrails behind an engine in these conditions because the exhaust is hot (so lowers local RH). At altitude the exhaust quickly cools.