Vapour Trails
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Vapour Trails
I have a question about vapour trails/contrails.
Having spent some time in Europe, I have noticed that there are almost always aircraft making vapour trails - these are more pronounced in winter, in my experience. However, in Southern Africa, one hardly ever sees vapour trails. I know that this is most likely due to the fact that we have relatively little high altitude jet traffic, but am still interested.
I wonder if anyone could explain the vapour trail phenomenon - why do they form, in which conditions are they seen, and whether altitude has any effect on them at all - i.e. is there an altitude below which they never form?
Thanks in advance for the help - sorry if this has been asked before.
Tx
Mike
Having spent some time in Europe, I have noticed that there are almost always aircraft making vapour trails - these are more pronounced in winter, in my experience. However, in Southern Africa, one hardly ever sees vapour trails. I know that this is most likely due to the fact that we have relatively little high altitude jet traffic, but am still interested.
I wonder if anyone could explain the vapour trail phenomenon - why do they form, in which conditions are they seen, and whether altitude has any effect on them at all - i.e. is there an altitude below which they never form?
Thanks in advance for the help - sorry if this has been asked before.
Tx
Mike
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.. while claiming no specific expertise, I suggest that the latitude differences and associated temperatures .. might have a material effect ...
.. not to mention the mind control chemical considerations, of course ....
.. not to mention the mind control chemical considerations, of course ....
).
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Good Grief!
Sorry I asked. I had no idea that they were so dangerous! (tongue Wedged firmly in cheek!)
I cannot believe that people are so dumb! Next thing you know, people will claim that aircraft are used to 'seed' clouds so they disappear, taking away the rain from poor farmers in drought stricken parts of Southern Africa!
Conspiracy theories are simply a plot by the government to control the minds, actions and day to day life of innocent civilians and (of course) members of the NRA! (Must get my tongue out of my cheek...)
M
(Edited since I can't spell - MUST be a government plot!)
Sorry I asked. I had no idea that they were so dangerous! (tongue Wedged firmly in cheek!)
I cannot believe that people are so dumb! Next thing you know, people will claim that aircraft are used to 'seed' clouds so they disappear, taking away the rain from poor farmers in drought stricken parts of Southern Africa!
Conspiracy theories are simply a plot by the government to control the minds, actions and day to day life of innocent civilians and (of course) members of the NRA! (Must get my tongue out of my cheek...)
M
(Edited since I can't spell - MUST be a government plot!)
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Interesting question, very interesting answers!
I reckon that the number of aircraft flying would make it more likely that there is an aircraft flying in the right atmospheric conditions to leave a con-trail. I may be completely wrong as I have never been to either Africa or Europe, but I'll take a shot. The exhaust from the jet contains a certain amount of water, it is held as vapour in the immediate jetstream but as we all know cold air can't hold as much water as hot air so as it enters the approx. -50C degree air it cools and if that air happens to be near saturation anyway the atmos says " no thankyou, can't take any more, out you go in the form of condensed water vapour(cloud)".
Maybe there is more humid air in Europe? Maybe it's just colder so can't hold the water as well, maybe neither but the number of a/c flying make the odds of a/c being in those conditions higher?
Would like others to comment as I am interested in con-trails purely because they look cool and I'd like to know if I am in the right ball-park.
I don't think there is an altitude below which they never form, if you cool the air quick enough and there is enough water in it a trail will form, at least that is what I assume after seeing trails coming off F-18 wings just at the point of rotation, the point where angle of attack is increased and therefor the speed of air over the wing is increased , lower temps etc.waffling now......might crank up the C182 and do some trials....
cjam....out.
I reckon that the number of aircraft flying would make it more likely that there is an aircraft flying in the right atmospheric conditions to leave a con-trail. I may be completely wrong as I have never been to either Africa or Europe, but I'll take a shot. The exhaust from the jet contains a certain amount of water, it is held as vapour in the immediate jetstream but as we all know cold air can't hold as much water as hot air so as it enters the approx. -50C degree air it cools and if that air happens to be near saturation anyway the atmos says " no thankyou, can't take any more, out you go in the form of condensed water vapour(cloud)".
Maybe there is more humid air in Europe? Maybe it's just colder so can't hold the water as well, maybe neither but the number of a/c flying make the odds of a/c being in those conditions higher?
Would like others to comment as I am interested in con-trails purely because they look cool and I'd like to know if I am in the right ball-park.
I don't think there is an altitude below which they never form, if you cool the air quick enough and there is enough water in it a trail will form, at least that is what I assume after seeing trails coming off F-18 wings just at the point of rotation, the point where angle of attack is increased and therefor the speed of air over the wing is increased , lower temps etc.waffling now......might crank up the C182 and do some trials....
cjam....out.
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There is a difference between the "contrail" of an F18 taking off or an F16 making a sharp turn. These contrails are formed because of a local higher air pressure due to the manouvring which compresses the air.
The contrails left by airliners/fighters enroute is indeed because of the vaporized water/fluid coming out of the engine at colder tempratures. The lowest I have seen them was on a clear over Germany at FL240, near Ramstein airbase when a 737 passed underneath us. Spoiling some of the nice surroundings.
The contrails left by airliners/fighters enroute is indeed because of the vaporized water/fluid coming out of the engine at colder tempratures. The lowest I have seen them was on a clear over Germany at FL240, near Ramstein airbase when a 737 passed underneath us. Spoiling some of the nice surroundings.
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basically there are 2 types of contrails. Condensation contrails and aerodynamic contrails. Both have been described by coastrider. You can tell the difference by the length of the contrail. Shorter contrails are aerodynamic ones and the longer ones are formed by the engines.
Most condensation contrails appear as you approach FL300.
Most condensation contrails appear as you approach FL300.
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.. a minor correction to CoastRider26's explanation.
The local aircraft condensate occurs due to a significant pressure drop either
(a) inside vortex flow
(b) where there is a sharp change in flow direction
(c) transitioning to subsonic flow through a normal shock in supersonic flow.
Associated with the pressure drop is a temperature drop (adiabatic flow cooling). As the temperature drops, the relative humidity increases. If the RH reaches 100%, then the water vapour condenses and we "see" the centre of the vortex.
Examples are seen in the inflow to a propeller disc at takeoff in rain, shed vortices from the end of flaps, in the wingtip vortex, shed vortices from leading edge extensions (eg Hornet), the leading edge vortex on a delta (Concorde etc), supersonic aircraft "flying" through a shock wave cloud, etc....
The local aircraft condensate occurs due to a significant pressure drop either
(a) inside vortex flow
(b) where there is a sharp change in flow direction
(c) transitioning to subsonic flow through a normal shock in supersonic flow.
Associated with the pressure drop is a temperature drop (adiabatic flow cooling). As the temperature drops, the relative humidity increases. If the RH reaches 100%, then the water vapour condenses and we "see" the centre of the vortex.
Examples are seen in the inflow to a propeller disc at takeoff in rain, shed vortices from the end of flaps, in the wingtip vortex, shed vortices from leading edge extensions (eg Hornet), the leading edge vortex on a delta (Concorde etc), supersonic aircraft "flying" through a shock wave cloud, etc....
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It's all down to the temperature and humidity. The military regularly brief contrail altitude as they help the enemy see you. Occasionally you can see dark contrails above white ones if someone flies level at the same altitude as a thin layer of cloud. This is caused by the exhaust precipitating water vapour out of the cloud and forming ice crystals which fall under gravity.
Philosophical question to keep you awake on a long night flight - are contrails black or white in the dark?
Philosophical question to keep you awake on a long night flight - are contrails black or white in the dark?
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Apologies for being a bit thick here - but I'm still not 100% in the picture. 
I understand the concept of aerodynamic contrails and the basis of cjam's explanation of condensation contrails. The bit I am unsure about (for condensation contrails) is to where the extra water vapour comes from...
The story as I understand it so far...
The air in front of the engine at -50 degrees has a RH of x% (less than 100%). It passes through the engine and exits at a much higher temp which would allow it to hold more vapour invisibly (but the vapour was invisible at -50 degrees anyway). It then cools to its original temp but the RH is now 100%. Since aviation fuel contains no water (I would hope not anyway), where has the extra moisture come from?
look forward to your assistance / abuse
I understand the concept of aerodynamic contrails and the basis of cjam's explanation of condensation contrails. The bit I am unsure about (for condensation contrails) is to where the extra water vapour comes from...
The story as I understand it so far...
The air in front of the engine at -50 degrees has a RH of x% (less than 100%). It passes through the engine and exits at a much higher temp which would allow it to hold more vapour invisibly (but the vapour was invisible at -50 degrees anyway). It then cools to its original temp but the RH is now 100%. Since aviation fuel contains no water (I would hope not anyway), where has the extra moisture come from?
look forward to your assistance / abuse
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Since aviation fuel contains no water...
Leaving the rest to those of you who live this stuff or have more time than I right now to find the details!
Dave
Edited to note that there is always the possiblity that jet engines do NOT produce water vapor, but rather, well, you know...
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ahhh yeah...I think I will leave the chemical breakdown to someone who knows, a flash version of one H banging into two O's I guess. all I know is that water is a by-product of the combustion process.....
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That's answered it in enough depth for me - never good at chemistry at the best of times.
Still, at least I don't have to resort to "..I don't know - maybe all it's engines are on fire" when my little brother asks again
Still, at least I don't have to resort to "..I don't know - maybe all it's engines are on fire" when my little brother asks again
