Jet Trails

[zcottovision]

What exactly are the jet trails that we all see up in the sky left by big jets?

I ask because I was watching the progress of what might have been a 767/777 out of my window today. The trails would run for a few miles, and then stop, and then about a few miles later, they would start again. Why would this be?

[N505AF]

There water vapor

[zcottovision]

OK....and how do they get there? Are they like exhaust gases from the engine, and can the pilot turn them on/off?

[SilverFox441]

Contrails form based on the water content of the air and it's temperature. That's why they sometimes start and stop...the air changes temp or water content.

Contrails form when the  water vapour in the exhaust recondenses as it cools.

[Ivan]

Is it possible to make engines which have very small or no contrails...
it's said the MiG-25 doesn't make contrails

[Woodlouse2002]

Stealh aircraft don't leave vapour trails. Some thing to do with the jet nozzles and letting the water recondense before it leaves the aircraft I think. Although i'm probably wrong.

[loomex]

Stealh aircraft don't leave vapour trails. Some thing to do with the jet nozzles and letting the water recondense before it leaves the aircraft I think. Although i'm probably wrong.

You are very close. Both the F-117 and B-2 have the exhaust coming out over the top of the aircraft. This serves two purposes. The exhuast is not easily seen from the ground and both aircraft have some sort of heat absorbing material to cool the air( like the space shuttle). I read that the B-2 also has a system that injects a coolent into the exhuast. I am not sure of this, but it sound like a good idea

[Ivan]

The F-117 has cold air intakes just before the exhaust, and the exhaust is tilted somewhat upward, probably giving the exhaust air some extra time to cool down.