I watched three contrails today (there weren't many).
The first was a long contrail that lingered for a long, long time.
Some hours later, the next was very short. It disappeared a short way behind the aeroplane.
A few hours later another one was very long, but did not linger nearly as long as the first.
All were high, on trans-Atlantic tracks (I checked the last one, it was a KLM Cargo 747 going to Chicago).
Were they all emitting water differently? Was the second one emitting very little water? How?