The other point about this is that when the source of pressure change is
removed, the air is essentially unchanged from its equilibrium state and the
water goes back into its gaseous form. However, the contrail from an engine
is adding substantial amounts of water to the atmosphere, hence this
contrail lasts substantially longer and decays only through dispersion..

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news:3J29i.3772$jW6.3719@trnddc01...
It's not just jets. WW2 bombers, B17s and the like, also left contrails
when they were on high altitude runs. Combustion produces water vapor,
which under the right conditions condenses into a cloudlike trail of
visible water droplets.

Contrails (water vapor condensation) can also form in wingtip vortices or
propeller tips especially when pulling high G loads (high angle of attack
accelerated turns) IIRC they happen in these contexts as the low pressure
area over the wing (or prop) grows and the low pressure air can't hold as
much water vapor as higher-pressure air can, so the relative humidity
increases eventually to 100% of capacity at which point the H2O vapor
condenses out as droplets.

Mike