Doesn't have much to do with altitude so much as air pressure, condensation (air temp) and humidity.
But.... At higher altitudes, condensation from the decreased pressure will increase the wingtip vortex, but for our game the majority of contrails would come from engine exhaust...

I wouldn't mind this, it would be pretty cool. But with contrails do we also start to model wake turbulence? That would be even better cooler! Lose control, go into a flat spin from the wash!
