As I said a while back:
Originally Posted by
NoelEvans
The aeroplane disturbs moisture that is already in the air causing it to condense and form the contrail. Watch on days where there is a very dry upper atmosphere, You can easily see on those dry days how short the contrail 'tails' on aeroplanes are.
So we now agree!! Relative humidity is the defining factor for length of contrails.
Yes, the aeroplane emits a wee bit of water, but
not all of the water for those contrails that run from one end of the sky to the other.
(About the flight levels, being several hours apart would have been the factor, they would have been fairly similar flight levels.)
Out of interest, I have had a contrail forming behind me in a
Piper Aztec! Aeroplanes at different levels (up and down) told me. I tried to 'turn back' to see it, but as I was at a level that was humid enough for the contrail to form, the 'background' patches of cloud at the same level meant that my contrail was 'camouflaged' against them and I couldn't see it. The other aeroplane said that it was absolutely clear though and my wife has just this minute confirmed that as she was in one of those aeroplanes. (Just a wee bit of weather research that we did once where we learnt a thing or two about the atmosphere first hand...)