Just to be nitpicky, If you can see it, its not water vapour any more. WV
is an invisible gas, surrounding us at all times to some extent (humidity). If you can see clouds, fog, the stuff in the PIA photo above etc., the WV
has condensed, by means of a drop in temperature and/or pressure, into visible moisture. Condensation.
One hears talk of vapour trails from high flying aircraft. It was WV
for a split second as it left the engine, and its actually ice crystals, as the condensation of the water vapour in the exhaust has gone through the liquid stage straight to ice, as its probably less than -45 degrees celsius out there. I always thought that was called sublimation, when it goes straight to ice, but I've never heard of "sublimation trails"!
If a aircraft isn't making trails, its not cold enough, or the pressure isn't low enough, but probably the WV
content (i.e. the humidity) isn't high enough, but the WV
is still there of course.
It doesn't matter to the engine whether its eats the WV
in visible form (clouds, fog,) or invisible. Its there just the same.
You'll see more condensation over wings in the morning, when the air's colder. It doesn't take much of a pressure drop to set it off, which of course happens as a matter of course as the wing generates lift. The humidity must be high as well, which is an expression of how close the air is to its maximum WV
content. 30% humidity, for example, means the air is holding 30% of the WV
it COULD hold for the existing temp. and pressure.
is also responsible for the wing tip trails you see, also on cool, humid mornings. The vortices off the wingtips or the outer rear corners of trailing edge flaps, cause the air to suddenly experience a huge pressure drop. The WV
again condenses to visible moisture and by definition isn't WV
any more, until it eVAPORates back into the air.
Get all that?!
Hopefully not too rambly.
Regards - Musang