In article ,
wrote:

Highly simplified explanation:

Technically, contrails are formed at altitude from exhaust water
vapor and what you see at low altitudes are vortices usually from
wing tips.

As things like wings move through the air they cause a pressure
drop around them which in turn decreases the air temperature.

If the temperature drop is enough to get below the dew point, the
water vapor condenses any you get visible "fog" for lack of a better
term.

You can also watch them form off the prop tip under humid conditions.
Several posters have put up some photos of C130's, Corsairs, etc over on
the binaries group. I have been in a C170 in central Florida and watched
it happen.