Cool pic, but I was always under the impression that the effect was because of the speed of the (tips of the) props (faster than the speed of sound.)
Not condensation, but vortex(?)
Disclaimer: Not a physicist, nor do I play one on tv...
The propeller tips create low pressure which causes water to condense out of the air, not from water vapor in the engine exhaust. The cause is slightly different but the effect is the same. I guess you could call it a vortex trail.