Contrails

[DVR6K]

Hiya,

Have a question for all you knowledgeable types.

Firstly, can someone please tell me how to spell "knowledgeable" and secondly, under what atmospheric conditions do aircraft form contrails in the cruise? Was having a chat with a mate at work who said aircraft always form them but I have looked up many a time to see an aircraft nice and high but with nothing coming out the back.

Thanks!

[phoenix son]

DVR6K,

Firstly you spelt "knowledgeable" right (twice)...
Secondly, aircraft certainly DON'T always form contrails, I presume it depends on the air temperature at the cruising altitude(?)

Any takers?

PHX

[PPRuNeUser0172]

depends on air temperature and it is nothing to do with exhaust fumes as i have heard before now!!

Contrails is an abbrev. of condensation trails which are formed when the warm humid air out of the engine mixes with low vapour pressure air of a lower temp.

Bit like breathing out on a frosty morning

[SLFguy]

Wow! Now that's how to answer & explain.

Ty DS.

[HEATHROW DIRECTOR]

Now ask what "chemtrails" are Tee Hee!

[Spitoon]

IIRC one of the mil FLIPs had a graph which you could use to tell whether the aircraft would contrail.

[chiglet]

When I woz in the RAF [BOF] we had to add "persistant, or non-persistant on contrails.
They [usually] form from [about] 29000ft up to the tropopause [sp?], but a lot depends on temperature and humidity.
The "Contrails" over London during WW2, were "Vortices" [pressure trails off the wing tips] Also [most] Boeing a/c have contrails off the flaps on landing in damp/humid conditions
we aim to please, it keeps the cleaners happy

[Milt]

Condensation Trails

Most of the above explanations are wrong.

Con trails at high altitude are ice crystals. They only form in super saturated air which is usually within a band of air wanting to form cloud but needing condensation nucleii on which droplets form.

Along comes an aircraft with engines spewing out exhausts rich in those tiny things called nucleii. Hey presto - tiny ice crystals form around the tiny particles.

Fighter pilots hate the stuff as it exposes their position.

Injecting nucleii into a growing cumulus cloud by burning silver iodide can often trigger copious water droplets to make rain.

The con trails at low altitude also need air close to saturation with water vapour. The sudden lowering of air pressure at wing tips or around the fuselage reduces the air temperature and water droplets form almost instantly.

Most of us have given this condensation the name "Ectoplasm".

Not pleasant when it forms around your cockpit at low level.

[747FOCAL]

ahh milt. At over 130 mph there will be nothing forming on the wing and staying but ice. Water droplets that form in "dead" zones are just there.

[Milt]

747focal

With condensation trails NOTHING in the way of condensation stays with or on the aircraft. No airframe icing.

High level cons only occur at air temperatures well below freezing - hence what you see are ice crystals which form on the nucleii coming from the engine exhaust, piston or jet and some distance behind the aircraft.

Ectoplasm is a small cloud composed of small water droplets which form in the lowered air pressure regions around the aircraft and often formed in the wing tip vortices from loaded up wings.. Nothing adheres to the aircraft. The water droplets you see almost immediately reform into water vapour (invisible) as the host air resumes its original pressure and temperature.

[mini]

You sometimes see "vapour trails" off the rear wings of Formula 1 cars, is this the same thing?

[john_tullamarine]

There is a nice pic of vortex condensation in the Tech Log sticky URL links.

These can form from any force-generating surfaces (wings, flaps, LEX, VGs, car spoilers/wings, etc.) where a vortex is shed. Tornadoes, water flow down the plughole (although you aren't going to see the condensation thing in the bath), etc. also come to mind.

To maintain a vortex, the flow requires a moderate to significant pressure gradient reducing toward the centre of the vortex .. otherwise there is no pressure gradient to produce a force to make the flow move in an arc. Spin a glass of water and you will have a vortex and a surface shape which mimics the radial pressure distribution.

The flow will approximate adiabatic and, associated with the lower pressure, is a lower temperature. For a given amount of water vapour (humidity), the relative humidity is strongly dependent on temperature (see another sticky URL link) .. pressure drops ... temperature drops ... RH increases ... if RH gets to 100 percent, the vapour condenses and instant cloud. Same thing with, say, orographic type clouds, and similar cloud formations associated with vertical air mass wave flow.

[Volume]

look at this impressive picture (http://www.airliners.net/open.file?id=632840&WxsIERv=QWlyYnVzIEEzNDAtMzEzWA%3D%3D&WdsYXMg=SWJlcmlh&QtODMg=SW4gRmxpZ2h0&ERDLTkt=SW50ZXJuYXRpb25hbCBBaXJzcGFjZQ%3D%3D&ktODMp=SnVseSAyMiwgMjAwNA%3D%3D&BP=1&WNEb25u=SmF2aWVyIEd1ZXJyZXJv&xsIERvdWdsY=RUMtSEdW&MgTUQtODMgKE=TWFzc2l2ZSB3aW5nIGFuZCBzdGFiaWxpemVyIGNvbnRyYWl sIGRpc2FwcGVhcmluZyBmYXN0LCBvbmx5IHZvcnRpY2VzIGFuZCB3aW5nIHR pcCB0cmFpbCByZW1haW5pbmcuIFtDYW5vbiBFT1MgMzAwRCArIENhbm9uIDc wLTMwMCBETyBJUyBVU01d&YXMgTUQtODMgKERD=NDEyNw%3D%3D&NEb25uZWxs=MjAwNC0wOC0wNA%3D%3D&ODJ9dvCE=&O89Dcjdg=MzI5&static=yes&size=L) of an A340 contrail.

You can see several effects there:
The wingtip vortices prouce ´contrails´ although they add neither particles (nuclei) nor additional humidity to the air. So the pressure drop alone is enough to trigger condensation of oversaturated air. This contrails obviously are the most stable ones and survive longest.
The outer engines produce small contrails, that are ´corkscrewed´ around the wingtip vortices and survive quite long.
The inner engines add humidity and nuclei to the air, so passing the low pressure area under the horizontal stabilizer causes very strong condensation, but this one vanishes first.

Maybe some other people can give more explanations on the effects seen in this picture.

This (http://www.airliners.net/open.file?id=637287&WxsIERv=QWlyYnVzIEEzMzAtMjQz&WdsYXMg=U3JpTGFua2FuIEFpcmxpbmVzIChBaXIgTGFua2Ep&QtODMg=SW4gRmxpZ2h0&ERDLTkt=VHVya2V5&ktODMp=QXByaWwgMjAwNA%3D%3D&BP=0&WNEb25u=QmFpbGV5IC0gQWlyVGVhbUltYWdlcw%3D%3D&xsIERvdWdsY=NFItQUwq&MgTUQtODMgKE=Rmx5aW5nIG9uIHRvcCBvZiBhbiBvbGQgZGlzc2lwYXRpbmc gY29udHJhaWwuIEF0IGdyb3VuZCBsZXZlbCB0aGUgc3VuIGlzIGFscmVhZHk gc2V0dGluZy4gUXVpdGUgYSBzaWdodC4%3D&YXMgTUQtODMgKERD=MzIzNjA%3D&NEb25uZWxs=MjAwNC0wOC0xMQ%3D%3D&ODJ9dvCE=&O89Dcjdg=&static=yes&size=L) picture shows a contrail during atmospheric conditions, where they survive over hours. The trail originates mainly from the engines.

Here (http://www.airliners.net/open.file?id=599180&WxsIERv=Qm9laW5nIDc0Ny00NDY%3D&WdsYXMg=SmFwYW4gQWlybGluZXMgLSBKQUw%3D&QtODMg=SW4gRmxpZ2h0&ERDLTkt=SW50ZXJuYXRpb25hbCBBaXJzcGFjZQ%3D%3D&ktODMp=TWF5IDIwMDQ%3D&BP=0&WNEb25u=QmFpbGV5IC0gQWlyVGVhbUltYWdlcw%3D%3D&xsIERvdWdsY=SkE4MDc2&MgTUQtODMgKE=SW4gdGhlIGFpciB0byBhaXIgcmVmdWVsaW5nIHBvc2l0aW9 uLiAxMDAwIGZlZXQgYWJvdmUgYW5kIGhlYWRpbmcgdG8gVG9reW8u&YXMgTUQtODMgKERD=MTM3ODM%3D&NEb25uZWxs=MjAwNC0wNi0xNQ%3D%3D&ODJ9dvCE=&O89Dcjdg=&static=yes&size=L) you can see contrails caused by the engines and the horizontal stabilizer, note that the inner engines cause more condensation, while most probably producing the same thrust and therefor adding the same ammount of particles and humidity.

This (http://www.airliners.net/open.file?id=590207&WxsIERv=Qm9laW5nIDc0Ny00NDY%3D&WdsYXMg=SmFwYW4gQWlybGluZXMgLSBKQUw%3D&QtODMg=SW4gRmxpZ2h0&ERDLTkt=SW50ZXJuYXRpb25hbCBBaXJzcGFjZQ%3D%3D&ktODMp=QXByaWwgMjAwNA%3D%3D&BP=0&WNEb25u=QmFpbGV5IC0gQWlyVGVhbUltYWdlcw%3D%3D&xsIERvdWdsY=SkE4MDc2&MgTUQtODMgKE=MTAwMCBmZWV0IGFib3ZlIGFib3V0IDM1MCBtaWxlcyBub3J 0aCBvZiBNaWR3YXkgZW5yb3V0ZSB0byBUb2t5by4gVG9vayBhZ2VzIGFuZCB tYW55IGF0dGVtcHRzIHRvIGZpbmQgYSBwb2ludCBvbiB0aGUgYWlyY3JhZnQ gd2luZG93cyB3aGVyZSBhIGNyaXNwIHNob3QgY291bGQgYmUgdGFrZW4gYW5 kIHRoZSBlZmZvcnQgd2FzIHdlbGwgd29ydGggaXQuIFNlZW4gd2l0aCBhIGZ pZnRoIGNvbnRyYWlsIGNvdXJ0ZXN5IG9mIHRoZSBnYWxsZXkgYW5kIHRvaWx ldCBzaW5rcyBkcmFpbmluZyBvdXQgdmlhIHRoZSByZWFyIG1hc3QgZHJhaW4 u&YXMgTUQtODMgKERD=MTY2MDk%3D&NEb25uZWxs=MjAwNC0wNi0wMQ%3D%3D&ODJ9dvCE=&O89Dcjdg=&static=yes&size=L) one is quite funny, it shows a fifth trail, obviously not comming from the APU as it starts earlier. Seems to be not from the outflow valves, which can be seen in half open position, but should produce two trails. So it must be produced by the waste water drain mast.

Here (http://www.airliners.net/open.file?id=606470&WxsIERv=Qm9laW5nIDc0Ny00ODE%3D&WdsYXMg=QWxsIE5pcHBvbiBBaXJ3YXlzIChBTkEp&QtODMg=T3ZlciBUb3JvbnRv&ERDLTkt=Q2FuYWRhIC0gT250YXJpbw%3D%3D&ktODMp=SnVuZSAxMSwgMjAwNA%3D%3D&BP=0&WNEb25u=R2FycnkgTGV3aXMgLSBBaXJUZWFtSW1hZ2Vz&xsIERvdWdsY=SkEqKioq&MgTUQtODMgKE=VGhlIG1vc3Qgc3BlY3RhY3VsYXIgY29udHJhaWwgSSBoYXZ lIGV2ZXIgc2Vlbi4%3D&YXMgTUQtODMgKERD=MzIyNjE%3D&NEb25uZWxs=MjAwNC0wNi0yOA%3D%3D&ODJ9dvCE=&O89Dcjdg=&static=yes&size=L) only the low pressure triggers condensation, without any obvious effect of the engines. You can even see the 747 humpback pressure peak triggers a contrail !

Obviously contrails also develop in supersonic (http://www.airliners.net/open.file?id=425035&WxsIERv=QWVyb3NwYXRpYWxlLUJBQyBDb25jb3JkZSAxMDI%3D&WdsYXMg=QnJpdGlzaCBBaXJ3YXlz&QtODMg=SW4gRmxpZ2h0&ERDLTkt=SW50ZXJuYXRpb25hbCBBaXJzcGFjZQ%3D%3D&ktODMp=U2VwdGVtYmVyIDE2LCAyMDAz&BP=1&WNEb25u=SmF2aWVyIEd1ZXJyZXJv&xsIERvdWdsY=Ry1CT0FE&MgTUQtODMgKE=QXMgd2UgY3J1aXNlIHRoZSBOb3J0aCBBdGxhbnRpYyBhdCA zODAwMCBmZWV0LCBzaGUgaXMgaGVhZGluZyBiYWNrIGhvbWUgd2F5IGFib3Z lIHVzLCBsZWF2aW5nIGEgaHVnZSBjb250cmFpbC4gW05pa29uIENvb2xwaXg gNTcwMF0%3D&YXMgTUQtODMgKERD=MzAzODk%3D&NEb25uZWxs=MjAwMy0wOS0xOQ%3D%3D&ODJ9dvCE=&O89Dcjdg=MjEw&static=yes&size=L) flight. (*sniff* a view never to be seen again)

[palgia]

Milt

Most of the above explanations are wrong.

Con trails at high altitude are ice crystals. They only form in super saturated air which is usually within a band of air wanting to form cloud but needing condensation nucleii on which droplets form.


First of all let me say that this is one of those things to which there is no short, sweet and correct answer. Cloud physics is one of the most poorly known areas of meteorology, where there is still a lot of unknown. I personally understand very very little about it, but I can tell you it gets extremely complicated if you really want to know the correct answer. So in a way, we all have the wrong answer... I will try to give my 2 cents but I welcome and encourage any corrections.

Having said that, and specifying that I am by no means an expert in this so I could very well be wrong, I think that what you wrote above is not exactly correct.

Specifically, I'm not sure about contrails forming only in supersaturated air. In fact, it depends on what you mean by "supersaturated air". If you mean air that has more than 100% RH, then I don't think there is ever a place on the planet where the RH can remain above 100% for extended periods of time (and certainly not in cloud-free air). I was taught that the only times RH can go above 100% is inside certain clouds, particularly convective clouds, and that it doesn't last very long.

It is my understanding that contrails form just like any type of mixing fog. In other words, we take two parcels of unsaturated air (ie. non-cloudy air, with RH<100%), mix them together, and just like magic, we get a cloud (a saturated parcel). In this respect, I think the best analogy was given by Dirty Sanchez. I think the "breathing out on a frosty morning" analogy is spot on. In that example we take air out of your lungs (which is very moist, but not saturated) and mix it with with the outside air (also not saturated) and create saturation (water droplets). In the aircraft, we take the engine's exhaust gasses (unsaturated) and mix it with non-cloudy air and form a cloud.
How is it possible for two unsaturated parcels to mix and create a saturated one? Well, its because the clausius-clapeyron explession is exponential in nature while the resulting temperature of the mixed parcels is dependent on linear weighted arithmetic averaging of the absolute temperatures of the two parcels being mixed. A more intuitive explanation is this:
When you mix two parcels of air of different temperatures, the temperature of the resulting mixture will be the average of the two parcels. For example if I mix 1kg of air @ 300K with 1kg of air @ 600k, I will get 2kg of air at 450k (300+600/2). Note that you need to use the absolute temperature in kelvins.
So we agree that when the exhaust gasses mix with the outside air, the resulting temperature will depend on the initial temperatures as well as the ratio of masses you are mixing.
Now for the more complicated part. The clausius-clapeyron expression dictates the relationship between temperature and saturation vapor pressure. Saturation vapor pressure is nothing more than the partial pressure exerted by water vapor when the air is saturated. One might expect the vapor pressure to be directly proportional to temperature (ie. if the temperature doubles, the vapor pressure doubles) but this is not the case. The relationship is exponential (if you're anything like me, you will ask why this is the case. It can be back-tracked all the way to Maxwell-Boltzmann statistical distribution of kinetic energies, which will affect the evaporation rate, and which is far from linear).
So the vapor pressure increases very rapidly with increasing temperature. This creates an exponential curve when graphed. For a good diagram, look here (http://www.env.leeds.ac.uk/envi1250/lectures/lect2.html) . On that diagram you will see that at both points C ands D, the air is not saturated. But when the two are mixed together, their temperature averages to point E, which is saturated.

So this is in a nutshell how contrails form. You do not need saturated air (think about it, if you are in saturated air, you're in a cloud already!) and super-saturated air is kinda hard to find (and it would most certainly be in a cloud). (we're always talking in relation to water) But you do need air that is fairly moist. If the air is too dry, the contrail will not form.
The addition of water vapor by the engines is of fundamental importance. And there's a lot of it coming out too. I did a little number crunching and at stoichiometric mixtures, combustion should produce amounts of water equal to 1.42 the amount of fuel burned (by mass).

At the temps found at cruising altitude, spontaneous nucleation should not be a problem. This means the water vapor condenses into liquid, then freezes into ice crystals. We also get deposition of water vapor directly onto ice crystals.

Once the ice contrail is formed, one of three things can happen.

1. The contrail starts disappearing more or less rapidly (how rapidly it disappears will determine how far behind the aircraft it will remain visible for)

2. The contrail remains the same size. (theoretically possible, but in practice very unlikely)

3. The contrail starts growing and keeps persisting, becoming a widening cirrus cloud.


Depending on atmospheric moisture, the contrail will behave in one of the three ways listed above.

When the contrail disappears its because the atmosphere is not humid enogh to sustain its growth. In theory, the contrail "should" not have been formed in the first place. But since the exhaust gasses are not perfectly mixed with the outside air, they can "get away" with condensing for a short while. As unsaturated air is entrained in the contrail vortex and as small eddies create enough turbulent mixing, the contrail sublimates and therefore disappears. This is also what happens in the "breathing out on a frosty morning" analogy.

Now for the really interesting case, when the contrail grows to become a relatively large cirrus. This happens when the atmosphere is very humid, and the vapor pressure is at or above the saturation vapor pressure with respect to ice.
Thats because the clausius-clapeyron graph comes in two flavors: one for liquid water and one for ice. They have roughly the same shape, and are very similar, but not the same. (this is fortunate for the survival of the planet, without this small but important difference, it would be a whole lot harder for rain to form!) At any point in the graph, the saturation vapor pressure over ice is lower than for over water. What this means is that water vapor can deposit onto the ice crystals even at relative humidities below 100%, while the parcel is still unsaturated with respect to water. This means the ice crystals grow just by attracting water vapor molecules like magnets. The vapor pressure over the ice crystals will be lower than ambient vapor pressure so there will be a pessure gradient force driving them towards the ice. All this can, and does, happen at RH below 100%.


Milt also brought up the point of al the aerosols the engines produce. I'm sure they help in the condensation and ice nucleation process, however I don't think anything interesting would happen if all you did was dump combustion by-products at those altitudes in clear air. I think you really need to add the waver vapor. I might be wrong though.


So, to answer the original question, its the relative humidity that determines whether or not contrails form or not, and what their fate is.
Some days they form, some days they dont, in the same way that during some winter days your breath will condense and become visible, while on others it will not.
Why do contrails remain visible while your breath disappears? Because contrails are made of ice crystals which can exist and grow even at RH under 100% while your breath is made of water droplets. I was told by several people who visited Siberia, Greenland and Antartica that it gets so cold there, that the breath will actually freeze too and last longer suspended in the air.:uhoh:

Sorry about the long post. Hopefully it wasn't too boring.:bored:

palgia


PS. edited to correct typos. Awesome pics volume! Interesting to see the low-pressure contrails beginning over the wing while the engine-contrails starting only a cetain distance behind the aircraft, after enough mixing and heat exchange tood place for condensation. In the case of dissipating contrails, the "thickness" or "density" of the cloud first increases and then decreases as we move away from the aircraft. Thats because in real life the turbulent mixing is constantly changing as the exhaust gasses are being "diluted" through entrainment. If you look at the graph in the link I provided, for a dissipating engine contrail, the mixture of gasses would "travel" from point D to point C as it gets diluted and sublimates away, with the "thickest" contrail occurring at point E.

[Milt]

Volume - a great contribution - fascinating pics.

palgia - yours is worth much more than your 2 cents.

Was hoping a knowledgeable meteorologist would join us.
Just love to have any of my misconceptions reliably corrected.

Most of my knowledge on the subject came from upper atmospheric research on condensation nuclei as they may affect worldwide weather.

Flew a Dr Telford from Melbourne University to FLs 400+ with a contraption into which a measured amount of outside air could be introduced. Around the base of the contraption was a tray of saturated sugar solution. About 2 minutes after introducing outside air sugar crystals could be seen growing as a result of the nuclei in the air. These crystals were subsequently counted as a measure of the number of nuclei per sample.

Later had some experience with rain making by burning silver iodide within growing cumulus clouds. It worked with clouds which were due to rain anyway. Took about 30 mins after seeding so one had to go upwind by 30 mins if you wanted it to rain on a target.

Regret now not having burnt the silver iodide in high RH air at altitude. Reckon I could have done some fantastic sign writing by turning the burner on and off. Maybe someone will try it sometime. Using GPS one could do some fancy lettering.

Remember you read it here first.