Why Aircraft with Jet Engines Form Cloud Trails

[wavingerwin]

At higher altitudes in the atmosphere, why do aircrafts (particularly fighter aircrafts) with jet engines leaves some kind of white cloud trailing behind?

It doesn't disappear for a reasonable amount of time.
What is this?

Thanks!

 

[Danger]

There are aeronautical experts here who can give you more information, but the basic idea is that they leave a 'cloud' of water vapour as part of their exhaust. Unless significant winds are involved, the vapour trail persists for quite some time.

 

[negitron]

The above is basically correct; however, conditions have to be right for cloud formation in order for the vapor trail to become a visible cloud--you don't see these contrails, as they are called, behind every plane. The tiny, tiny droplets which condense out of the jet's exhaust form a tenuous mist but if the conditions are right (basically air saturated with water vapor and a temperature below the dew point), these tiny, tiny droplets become nucleation sites for even more atmospheric water to condense upon then, POOF, you've got a visible cloud trail.

 

[Naty1]

You can likely get some good analogous insights from reading about fog formation here on earth. Fog is bascially clouds at ground level. Forecasting when it will appear or not is tenuous at best: we know the conditions, but guessing when the right conditions will be present is a crapshoot.

Until recently I used to boat summers in Maine; the one time I could be almost positive of fog all day was when the national weather service would declare "Fog will disspate by 10AM". As soon as I heard that, I changed any plans and would plan to remain at anchor all day.

 

[DaveC426913]

Until recently I used to boat summers in Maine;

'Boat' is a verb?

the one time I could be almost positive of fog all day was when the national weather service would declare "Fog will disspate by 10AM". As soon as I heard that, I changed any plans and would plan to remain at anchor all day.

Despite what it may seem to readers, that doesn't necessarily mean they got it wrong. It simply mean they were predicting for the general (mostly land) area. You, being on the water, would have to factor that in.

Unless you were specifically listening for the marine forecast...

 

[negitron]

'Boat' is a verb?

http://www.answers.com/boat This is not a new usage by any stretch.

 

[Danger]

'Boat' is a verb?

Actually, yes. 'Boating' is an accepted verb to describe the action of messing about with boats. To take part in such activity is to 'boat'.

edit: I see that you sneaked in ahead of me, Negitron. Nice link.

 

[DaveC426913]

Yeah. I'm familiar with it as an adverb at least. "I went boating." It just sounds funny to say "I like to boat."

 

[Hurkyl]

At higher altitudes in the atmosphere, why do aircrafts (particularly fighter aircrafts) with jet engines leaves some kind of white cloud trailing behind?

If you want to do some searching on your own, those are called contrails.

 

[negitron]

But for God's sake, steer clear of any sites that talk about "chemtrails."

 

[wavingerwin]

If you want to do some searching on your own, those are called contrails.

The above is basically correct; however, conditions have to be right for cloud formation in order for the vapor trail to become a visible cloud--you don't see these contrails, as they are called, behind every plane. The tiny, tiny droplets which condense out of the jet's exhaust form a tenuous mist but if the conditions are right (basically air saturated with water vapor and a temperature below the dew point), these tiny, tiny droplets become nucleation sites for even more atmospheric water to condense upon then, POOF, you've got a visible cloud trail.

Thanks! now that i know what the proper name is, i can do some search and reading myself

 

[negitron]

Incidentally, in case it wasn't immediately evident, "contrail" is a contraction of "condensation trail."

 

[Born2bwire]

But for God's sake, steer clear of any sites that talk about "chemtrails."

Or Woodpecker Grids. Good Lord...

 

[russ_watters]

It is worth pointing out that jet plane exhaust is mostly water...

 

[mgb_phys]

It is worth pointing out that jet plane exhaust is mostly water...

But the visible contrail is mostly from atmospheric water, the exhaust water just provides a nucleation source.

 

[russ_watters]

But the visible contrail is mostly from atmospheric water, the exhaust water just provides a nucleation source.

I think the first part is right, the second part isn't. The added moisture pushes the air further above saturation. I think this may be a nitpicky point, though and I'm not too concerned about it either way.

 

[J_Cervini]

Air (the gas) contains water vapor. When this rapidly gas travels from high to low pressure it expands which condenses out the water vapor. The high pressure is ahead of the airfoil (wing or rear stabilizer) - the low pressure is after. When atmospheric conditions are correct, this difference in pressure allows the water vapor to condense after the airfoil.

 

[negitron]

When atmospheric conditions are correct, this difference in pressure allows the water vapor to condense after the airfoil.

This is not the usual mechanism for contrail formation, which is as previously noted. The particular phenomenon you refer to is called a wingtip vortex (even thought they often form behind the control surfaces of airfoils, not just at the tips).

http://en.wikipedia.org/wiki/Wingti...rtices_due_to_water_condensation_and_freezing

 

[mgb_phys]

I think the first part is right, the second part isn't. The added moisture pushes the air further above saturation.

I thought you also got contrails from wingtip vortices and so on

I think this may be a nitpicky point, though and I'm not too concerned about it either way.

Who are you and what have you done with the real Russ?

 

[russ_watters]

I thought you also got contrails from wingtip vortices and so on

In the wiki, it says that contrails from the wingtip vortices are not the most common type.

Who are you and what have you done with the real Russ?

If you reread the post that you were first responding to, you'll see you were responding to something I didn't say. I'm not interested in arguing about something I didn't say and the particulars of the mechanism of contrail creation just don't interest me that much.

 

[Phrak]

Commerical jet contrails.

http://earthobservatory.nasa.gov/IOTD/view.php?id=7161"

 

[Rants]

I've seen a visible trail of mist or vapor or something forming while sitting near the wing of a commercial aircraft. It appeared that the speed and or temperature of the metal wing interacted with moisture in the atmosphere to create a trail of "smoke" behind the wing. I've been on about 40 flights the past 10 years or so and have only witnessed this phenomenon once or twice.

 

[DaveC426913]

I've seen a visible trail of mist or vapor or something forming while sitting near the wing of a commercial aircraft. It appeared that the speed and or temperature of the metal wing interacted with moisture in the atmosphere to create a trail of "smoke" behind the wing. I've been on about 40 flights the past 10 years or so and have only witnessed this phenomenon once or twice.

It is more likely a consequence of the change in air pressure from the trailing shock wave. Air temp might be very near dew point. Shock wave probably pushed it over.

 

[russ_watters]

It is more common in high performance aircraft, when pulling g's. The pressure above the wings drops a lot and that causes vapor to condense. http://contrails.iit.edu/History/PartnersInFreedom/F-16/p150.html

 

[Danger]

I was quite surprised, being a fixed wing private pilot, the first time that I saw a CF-18 pull a high-alpha turn at an airshow. The thing was barely moving (not much above stall, and almost standing on its tail), and the whole arse end was enveloped in a condensation cloud. There was also that neat series of pictures that Ivan (or was it Integral?) posted showing a huge ring of condensation just ahead of the tailplane of an F-16 as it broke mach. Looked like the bird was flying through a doughnut.
I didn't think to mention that form of the phenomenon in my first response, because I thought that the OP was referring to the trails seen at high altitude behind commercial liners.

 

[Cyrus]

It is more likely a consequence of the change in air pressure from the trailing shock wave. Air temp might be very near dew point. Shock wave probably pushed it over.

That must be one fast airliner to create a shock wave.

 

[DaveC426913]

That must be one fast airliner to create a shock wave.

All planes create shock waves of some degree. They don't have to approach or exceed Mach 1 to do so.

Um... don't they?

Correct me of I'm wrong. Your userpic has caused me to question my conviction...

 

[negitron]

Shockwaves are only caused by supersonic events. All planes (and everything else that passes through the air) create waves, of course, but these are not shockwaves.

 

[Cyrus]

All planes create shock waves of some degree. They don't have to approach or exceed Mach 1 to do so.

Um... don't they?

Correct me of I'm wrong. Your userpic has caused me to question my conviction...

You are right that the transonic wings have regions of critical flow for the speeds airliners travel. These regions end well before the trailing edge, which is usually where the wingtip trails form, so I'm not entirely convinced this is the source of what's going on.

 

[Borek]

I always thought that it is not water in exhaust gases that provides nucleation sites, but soot from the not burnt fuel.

If you have ever seen plane fyling diretly in your direction (or directly from you) you have probably seen that exhaust gases are brown - that's a soot I am talking about. It is not visible when the exhaust is seen from the side, that's practical application of Lambert-Beer law