A condensation trail, also called a vapour trail, forms at altitudes above 25,000 feet in temperatures below -40 degrees when engine exhaust condenses into ice crystals, creating an artificial cirrus cloud. Water is produced by hydrocarbon burning engines in about the same quantity as fuel consumed, and in the right conditions, this extra addition of water into the air pushes the water vapor past the saturation point, and condensation occurs. It takes a moment to happen, which accounts for the contrail appearing a short distance behind the aircraft, rather than immediately. Contrails can also be caused at high altitudes by the extreme low pressure areas created by wingtip vortices, which reduce the temperature enough to condense the existing moisture in the air. I bet you're glad to know that.
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