Vapour....trails that is

[KennyR]

It's easy, not a matter of physics, but rather, chemistry.

Hydrocarbon + oxygen --> carbon dioxide + water

The water vapour comes from the combustion itself.

[KennyR]

I think the simple fact is that in cooler air the water vapour becomes supercooled upon leaving the engine. As it hits the jetstream formed by the plane, it drops out of vapour and forms liquid droplets - giving the misty white haze that you see from jets, and occassionally, scraped from the air by wingtips.

[KennyR]

JaXanim wrote:
At this point, the numbers look astronomical. I'll put them up when I check them again. The vapour rate of a 747 (4 engines) is just mind boggling!

Yes, they are. Some scientists are also starting to believe that jets can cause changes to weather patterns because of what they do up there.