Vapour
 

Hi people, just a quickie, im PPL, IMC, and night rated pilot, currently enjoying flying the warrior and traumahawk, and have a question for you. when taking off in say a 767 you look out of the window over the wing, (cos im always in cattle class), and when the aircraft rotates there is a huge streamer of vapour off the top of each engine, can you tell me precisely what causes this ? i have seen passengers who didnt know that this is normal panic when they saw it , believing that it was smoke etc ! your explanation much appreciated, cheers.

Shed vortex from the nacelle chine. Quite common on most recent jets.

http://www.smartcockpit.com/operatio...Generators.PDF

... to do with the lower pressure air behind an obstacle to the airflow having a lower capacity to hold water, it thus condensing out to form a mini-cloud?

Vapour
 

The answer above is almost right, but it is the temperature, not the pressure that causes it.

If the air is nearly saturated (100%RH), then the dew point will be only slightly lower than the static temperature. As the air accelerates over the top of the wing both the static temperature and pressure will reduce according to Bernoulli's theorem. The air then may be below dew point and excess water will condense as visible water droplets (vapour is actually a gaseous state and invisible).

As the air decelerates behind the wing and returns to its former temperature and pressure, it will take a few seconds for the air to reabsorb the water droplets as vapour.