Re: Aircraft condensation trails

There are two explanations. Either or both may be significant, depending on 
the exact conditions.

The first and simplest is that the trail may form from water vapour from the 
aircraft's own exhaust. Remember that burning any petroleum based fuel 
produces quite a lot of water as well as carbon dioxide, e.g.

C9H20 + 14 O2 ---> 9 CO2 + 10 H2O

As the exhaust cools, ice crystals will start to form from it (and hence a 
cloud or 'vapour trail') because the very cold air at the height where the
aircraft is operating cannot hold so much water vapour.

The second effect is a more subtle one. Sometimes air can become 
supersaturated with water vapour. If the air is very still, and has just 
risen slowly and cooled, it gets to an unstable state where normally ice
crystals should have started forming. But those ice crystals have a hard 
time getting started. It requires something special, like a bit of dust,
some charged particles, or some turbulence, to get them going. A passing
aircraft can provide any or all of these. So a trail of ice crystals marks
its passage.

I think the first effect is the more common and the more important as far as
aircraft are concerned. Vapour trails of the first kind will tend to slowly
disperse, while those of the second kind will rather tend to be followed by
a gradual cloud build-up. (I know that on sunny summer days in Britain you
sometimes seem to get persistent vapour trails that do neither!) Vapour 
trails of the second type might potentially be formed by balloons or gliders
as well as aircraft, while the first type definitely requires some sort of
motor that is burning fuel.

It is interesting to note that the second type of effect was used in the
Wilson cloud chamber, which was historically used for detecting the paths
of the charged particles that were emitted in radioactive decay.

John.