Why do jets leave a white trail behind them.

AliR,

Is either one of us right?


Your friend, partly. There are atmospheric conditions when the pressure
change over the wing or at a wing tip is enough to make the water in
the air visible. But they are rare and don't make long contrails. The
long contrails from jets you see are because the exhaust contains small
particles (soot and other stuff) which cause the water vapor in the air
to condense on them where it wouldn't condense without them.

--
Thomas Borchert (EDDH)