Answer to question from Wikipedia:
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Condensation from decreases in pressure
Main article: wingtip vortices
The wings of an aircraft cause a drop in air pressure in the vicinity of the wing. This brings with it a drop in temperature, which can cause water to condense out of the air and form a vapour trail or contrail. This effect (the
Prandtl-Glauert singularity), is more common on humid days, and can be seen on fighter jets performing high energy maneuvers, during shuttle launches, on the expanding surface "bubble" of nuclear explosions, or on airliners during takeoff and landing. Additionally, the area around a turbo-fan intake will be at a lower pressure than the surrounding air, and may result in a condensation fog forming there during high power settings.
These effects are compounded with the other explanation of contrails, which is the water vapor produced by the combustion of jet fuel. High altitude contrails are seen directly behind the one or two jets, and with aircraft with four jets, such as the Boeing 747, there are four contrails. The vapor trails caused by the first mentioned effect are usually seen at low altitude where the ambient humidity is higher, and they follow the wings rather that the jet engines.
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Hope that helps in understanding. Looks like the high energy maneuver is the cause.
Thanks again for the comments.
Jeremy