Why do jets leave a white trail behind them.

Hi everyone,

Me and a friend are wondering why jets leave a white trail behind them. My
guess is that it's moisture being compressed as it passes through the
turbine (because some leave a single trail, while others leave two separate
trails). My friend thinks it's because of moisture being cooled as it
passes over the wing (Bernoulli effect).

While flying a bonanza I have seen a small white trail extending from the
wingtip for a little while right after I break through the clouds
(descending).

Is either one of us right?

AliR.