What exactly causes vapour trails?

[Capt Hollywood]

Can someone please explain exactly how the vapour trails that you see behind jets at flight levels are formed?

Always behind curious about that one!

Cheers.

That was supposed to say always 'been' curious about that one!

[HEATHROW DIRECTOR]

There's a significant opinion, mainly in the US, that they're caused by chemicals being sprayed on the unknowing population beneath. A neighbour of mine, writing to the local newspaper, declared that they are polluting exhaust fumes from jets... which merge together and form the dark, overcast, clouds which we see so often in the UK.

I don't recall mention of either of these reasons when I studied Met!

[BlueEagle]

Warm, moist air being flung out into extremely dry and cold air.
Warm, moist air then has to vapourise and becomes visible, (usually). It is all to do with the ability, or non abilitiy, of cold, dry air to absorb warm moist air. Basic physics really.

[Capt Hollywood]

Thanks BlueEagle, not quite sure where the 'moist' air comes from though. I understand the concept as in Met where warm moist air is pushed up into the atmosphere, cools and so condenses and forms water vapour (clouds). But all the engine is doing is compressing the air, heating it and throwing it out the back! Where does the moisture come from? There is probably an obvious answer here that I'm missing, baring in mind I don't go anywhere near flight levels in my helicopter!

[BlueEagle]

The exhaust from the jet engine is, comparatively speaking, moist since it comprises the residue of the burnt fuel, much more moist than the ambient air at those altitudes.

[john_tullamarine]

If I scratch my head from a long time ago, the complete combustion of a hydrocarbon in the presence of sufficient oxygen produces carbon dioxide and water. In the real world this generally is not achieved completely and there are lots of other combustion products, carbon monoxide, soot plus various products which come from impurities in the fuel (in this case).

However, there is LOTS of water vapour pumped out the back of the engine ....

[Capt Hollywood]

Thanks John,

Exactly the kind of explanation I was after. Thanks also to Blue Eagle, all inputs appreciated.

[mattpilot]

not so fast....

there are 2 different kind of contrails airplanes can leave behind.

1) condensation contrail
-as described above

2) aerodynamic contrail
- The reduced pressure produced by air flowing over the wing causes the air to cool. This cooling may supersaturate the air


How can you tell the difference? Aerodynamic contrails disappear quickly - meaning they are very short compared to the longer condensation contrails

[BigGreenPleasureMachine]

To confuse the issue further, how come if the wake turbulence from the wing is strong enough to knock a buzzard from a sh*t wagon, it doesn't seem to disturb the nice, regular contrail pattern we see on those lovely sunny days?:cool:

Answers on a postcard please.....

[mattpilot]

uhm, because contrails constist of water vapor and not solid matter?


If you get a close look at one of those contrails, you'll see rotation in them. At least with the aerodynamic contrails.

Infact, the reason the aerodynamic contrails dont last long is because, and i qoute from my meteorology book, "this type of trail usually disappears quckly in the turbulent wake of the aircraft"

:cool:

oh, and before i forget,

the wake turbulence sinks at a rate of about 900fpm.

This means, and i'm speculating now :), that the turbulence can't get into the condensation contrail, thus its not "disturbed"

[Duke of Burgundy]

BGPM - Think of it in terms of the wake from a ship. Turbulent to be in but slow moving relative to its environment.

When aircraft trail condensation plumes from their wing tips, the condensation takes place in the low-pressure core of the vortex generated by each wing. The two, normally invisible, counter-rotating vortices trail behind the aircraft. The exhaust from the engines is swept into this rotation, which explains why contrails always appear as a pair of rolls of condensation, regardless of the number of engines.

Anyone heard of a Distrail? If the aircraft is flying above a layer of thin cloud, the vortices may force warmer air down from the higher level. This can be sufficient to disperse the cloud beneath the path of the aircraft, so forming a Distrail.

Finally just to exhaust everything I know about this subject. An aircraft flying through supercooled water droplets may initiate freezing in the cloud, producing a clear channel when the ice crystals fall out into the lower air.




:confused: :confused:

[ORAC]

No!! No!! It's all lies!! Believe in the Chemtrails!!

Chemtrails (http://www.carnicom.com/contrails.htm)

:D :D

[BELHold]

I noticed the other day two wide bodies, looked like 767s following more or less the same track,and very close together heading towards the Belfast VOR and from what I could tell more or less at the same level. The AC in front was trailing like mad yet the one behind had no trail at all.

Most of the other AC that day were also trailing.
How come the second AC had no trail?

[mattpilot]

well, my only guess on that one is that they weren't on the same altitude. Couldn't explain it any other way :confused:

[152captain]

vapour trail (http://www.airliners.net/open.file/239080/M/)

[Capt Hollywood]

Awesome photo 152captain. Not too often you see photos from above.

Thanks to everyone, this is my first post, quite amazed at how quickly the replies came in!

Cheers.

[Whoopedo_STALL]

When i was coming home from Italy in June, I was in a britannia boeing 767-300 taking off from Pisa. I was near the window over the wing and noticed a strange vapour trail coming off the top of the engine as the aircraft rotated quite quickly. At first i thought something was leaking out of the engine but then realised it was only some sort of vapour trail coz it vanished really quickly. :)

apparantly its quite common and it shocks some passengers into thinking the engines about to blow up. :p

[lunkenheimer]

Heathrow D-
Your neighbor isn't totally daft-it seems that contrails do indeed sometimes merge into overcast-see this:
www.atmos.berkeley.edu/news/cohen_jul2002.article

[Vim Fuego]

Out of interest, is there any way of knowing the altitude at which an aircraft will begin to leave contrails?

[Duke of Burgundy]

Vim - Yes the likelihood or otherwise of contrails is forecastable and depends entirely on atmospheric conditions on the day.

Upper air soundings are taken by radio-sonde balloons at regular intervals from many locations and the resultant readings enable weather forecasters to determine the characteristics of the atmosphere up to very high altitudes and hence predict the types of cloud formation, from what altitude clouds will form etc. etc.

Hope this helps.

DoB

[mattpilot]

from what i read, a good average is 30000ft