1. skephu

    skephu Senior Member

    But aerodynamic contrails look in fact quite similar to exhaust contrails themselves:
    [​IMG]
    The part close to the wing is different, but then the turbulent region after about 50 m is hard to distinguish from an exhaust contrail. And I don't see why it couldn't form pendules. Skywriting smoke forms pendules as well.
     
    Last edited by a moderator: Apr 11, 2015
  2. Trailspotter

    Trailspotter Senior Member

    This image from http://journals.ametsoc.org/doi/pdf/10.1175/2008JAS2768.1 appears to show a mixed aerodynamic + (low density) regular contrail. Pure aerodynamic contrail looks like this:
    [​IMG]
    However, the wake vortices do define its shape, so it should be possible to see the Crow instability effects as well, if it persists long enough.
     
  3. skephu

    skephu Senior Member

    The paper shows it as an example of an aerodynamic contrail, and claims that the conditions excluded the formation of an exhaust contrail. So the paper discusses it as a pure aerodynamic contrail.
     
  4. Trailspotter

    Trailspotter Senior Member

    I would not be so sure. The conditions probably excluded the formation of a persistent exhaust contrail, however, a short exhaust contrail may still form at that distance from the plane.
    Actually, I've just observed such a contrail behind the same plane type (A340) as in the photo in the paper:
    IMG_3541.JPG
    IMG_3544.JPG
     
  5. skephu

    skephu Senior Member

    The legend to the figure says:
    And the text says:
    And also:
     
  6. skephu

    skephu Senior Member

    The goal of geoengineering is usually not to create clouds, but to create a more reflective layer in the stratosphere or higher. The lifetime of clouds is short, they are affected by the weather, and their effect on the energy balance is uncertain. The proposal in the paper I linked in #14 is an exception. But actually it does not propose to create more clouds, it proposes a method to reduce the amount of cirrus clouds, which contribute to warming. By spraying ice nucleants into some cirrus clouds, the ice crystals in them will be larger, and therefore they will fall faster. The result is that the amount of cirrus would be smaller, not larger. It's a technique to remove cirrus clouds from the sky.
     
  7. Trailspotter

    Trailspotter Senior Member

    The legend to the figure also says:
    That is, it is not the authors' observation.

    Yes, this takes place at probably longer distance from the plane, like in this image:

    Note that white turbulent plumes do not align with the engines, unlike the photo in the paper.
     
  8. skephu

    skephu Senior Member

    Here's a better copy of the photo in the paper. Do you find that the plumes align with the engines?
    [​IMG]
     
  9. Trailspotter

    Trailspotter Senior Member

  10. skephu

    skephu Senior Member

    It's not obvious to me at all. In fact, to me it appears that they align with the parts of the wings between the engines.
     
  11. Trailspotter

    Trailspotter Senior Member

    Well, the exhaust plumes are not ideal straight lines (see, for example, my photo in #4 above). But, because the wing aerodynamic contrail is shaped by the wake vortices, I would expect to see the vortex-induced plumes on the outline of the aerodynamic contrail, as in #7, rather than inside. I have not found the original, non-adapted photo used by the paper's authors, but think that it would look similar to mine.
     
    Last edited: Apr 11, 2015
  12. Mick West

    Mick West Administrator Staff Member

    I think the main reason an in-flight (as opposed to landing or maneuvering) aerodynamic contrail looks different is because of where it comes from on the wing - i.e. it leaves as a broad sheet, with very thin dense trails from the wingtips, this is unevenly modified by the shape of the plane, and possibly be engine exhaust gases.

    But much of it is then entrained in the two wake vortices, curling round to give you what looks like an exhaust contrail.

    So if your were spraying ice nuclei from points coincident with the engine, I'd think it would look like an exhaust contrail, unless the microphysics were such that the gap was much larger, or the optical density much finer.
     
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  13. Mick West

    Mick West Administrator Staff Member

    This classic example is (I think) all aerodynamic, but looks like exhaust.
    [​IMG]

    In fact the three engines (or maybe just two engines and the body) seem to create gaps in the the trail (evapotrails? :) )


    With a four engined plane, it seems like you can see these gaps behind the engines, and the body:
    [​IMG]

    The inner gap seems to turn into what looks like an exhaust contrail, but the outer one does not, so I suspect the inner trail is just some kind of turbulent mixing that is not actually an exhaust contrail.

    This is now making me question other trails that I'd assumed were combined aerodynamic and exhaust trails.
     
  14. Trailspotter

    Trailspotter Senior Member

    You have annotated my close-up photo to point out lacking of the exhaust trails from outside engines, but they are apparent in a medium range zoom photo from the same series:
    [​IMG]
     
  15. Mick West

    Mick West Administrator Staff Member

    Interesting. Here I've overlaid a pure A340 exhaust contrail over your image (and black-pointed the sky for contrast). If one assumes that the exhaust contrails are not visible for quite some distance behind the plane, then you can see how they match.
    [​IMG]
     
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  16. Trailspotter

    Trailspotter Senior Member

    Thanks, this illustrates my earlier point that a short non-persistent exhaust contrail combined with the aerodynamic contrail can explain the photo in the paper. And I have taken a photo of A340 today with exhaust contrails forming at quite a distance from the engines. There has been no good conditions for long contrails today, all that I've seen were very short or non-existent.
     
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  17. Trailspotter

    Trailspotter Senior Member

    Here is an aerial close-up view of A340 aerodynamic contrail, similar to #2 above:
    [​IMG]

    I also have searched flickr for photos of A340 aerodynamic contrails to see what features come from the plane shape and what may come from the engine exhaust:











    And here is the original photo used in the paper:
    [​IMG]
    http://www.airliners.net/photo/Scandinavian-Airlines--/Airbus-A340-313X/0870588/

    Jeff Well also copyrighted another iconic photograph of A340 aerodynamic contrail:
    [​IMG]
    http://www.airliners.net/photo/Virgin-Atlantic-Airways/Airbus-A340-642/1088680/
     
    Last edited: Apr 12, 2015
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  18. skephu

    skephu Senior Member

  19. Trailspotter

    Trailspotter Senior Member

    Well, I would not use word "clearly" here. This image is notably different from other images of A340 aerodynamic contrails that I have seen so far. It is probably an HDR image, a composite of several different exposures enhancing some of the trail's features and reducing the others. Still, there are two types of linear 'trails', those that are parallel and those that slightly diverge behind the plane. You can repeat the @Mick West's exercise and overlay this image on an image of A340 regular exhaust contrail with the plane in the same or very similar orientation to see if they align with diverging trails. Here are a few examples from flickr:




    Also, I intent to do a similar exercise to compare aerodynamic contrails from the different photos collected above when I have time for it. But you are welcome to do meanwhile your own comparison of these photos to draw your own conclusions.
     
  20. skephu

    skephu Senior Member

    This is a two-engine plane:
    [​IMG]
    Here, it is indeed very clear that the trails are coming from the surfaces of the wings next to the engines. The engines themselves actually produce gaps in the trails.

    Also, we know that the conditions for aerodynamic contrail formation and exhaust contrail formation have little overlap, so a simultaneous appearance of aerodynamic and exhaust contrails is a rare occurrence.
     
  21. Trailspotter

    Trailspotter Senior Member

    Yes, this picture is similar to that I attached to #7 above. As has been noted already, aerodynamic contrail comes from the wing:
    And this is exactly what we see in these two pictures of aerodynamic contrails from two-engine planes.

    We have indeed discussed this before and my counter-argument is not rebutted. The conclusion about little overlap between the conditions for aerodynamic contrail formation and exhaust contrail formation resulted from the calculations made for medium size aircraft: B737, A320 etc. For heavier planes the overlap is probably substantial. If I recall correctly the results of my inspection of photos of aerodynamic contrails from the contrail spotting cites, about half of them showed co-occurrence of aerodynamic and exhaust contrails, mainly behind heavy planes:
    https://www.metabunk.org/threads/de...aying-aerodynamic-contrails.4454/#post-127166
     
    Last edited: Apr 13, 2015
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  22. skephu

    skephu Senior Member

    But as we see, there are ambiguous cases where it is not easy to decide whether some part of a trail is exhaust or aerodynamic just by looking.

    Surely it would not be very hard to do some calculations based on the parameters of larger airplanes. The formulas must be the same, only a few parameters have to be changed.
     
  23. Trailspotter

    Trailspotter Senior Member

    I've just scrolled recent photos at www.luchtzak.be . The October 2014 forum has five recorded cases of aerodynamic contrails: http://www.luchtzak.be/forums/viewtopic.php?f=25&t=54662

    Three of these cases involved three turkish B777, two are co-occurrences:
    [​IMG]
    [​IMG]
    and one pure aerodynamic contrail:
    [​IMG]

    There is one more B777, also probable co-occurrence:
    [​IMG]

    and one B757 that also looks like co-occurrence to me:
    [​IMG]

    The main parameter for aerodynamic contrails is the air pressure difference created between the top and the bottom of the wing, which must be greater for heavier planes. The greater the pressure drop on the wing, the more likely the formation of aerodynamics contrail. This can explain the observed frequent co-occurrence of aerodynamic and exhaust contrails for such planes.
     
  24. Hama Neggs

    Hama Neggs Senior Member

    OK, but why would that DIFFERENCE matter? This is just a layman's guess, but isn't it more to do with the lowering of pressure, above the wing, beyond a certain parameter?

    On the other hand, some of those trails don't seem to manifest until well behind the wing, as if some sort of "shock" of the two layers of air coming back together has caused the manifestation of the aerodynamic trail.
     
  25. skephu

    skephu Senior Member

    "Looks like" is not enough. I think those examples are debatable.
    Not necessarily. Heavier planes require more lift, but their wings are also larger, so even the same pressure difference will cause more lift.
     
  26. Trailspotter

    Trailspotter Senior Member

    The lowering pressure above the wing results in simultaneous lowering the temperature and can push the local humidity values well above supersaturation level to trigger spontaneous condensation. The greater the pressure drop, the more likely the formation of aerodynamics contrail. The size of the wing may also matter, as the larger the area of low pressure above the wing, the more ice crystals would form to nucleate the trail.
     
  27. Trailspotter

    Trailspotter Senior Member

    Co-occurrence in the B757 trail is debatable, because of its unfavourable orientation and illumination in all three photos.
    However, one of the B777 trails is photographed in three different orientations showing that the aerodynamic sheet from the wing and exhaust from the engines are separated in 3D:
    Here is the photos of its sister plane making 'pure' aerodynamic contrail:
    Compare the last photos from each series where both planes are in similar orientations. The 'turbulent' trails in the first series start near the plane's tail and align with the engines.

    If you go through that contrail spotting site, you will find 'non-debatable' examples of co-occurrence for four-engine planes.

    It is true that larger planes have larger wings, but a quick look in Wikipedia for the parameters of different Boeings show that maximum takeoff weight increases by a greater factor than the wing area. For example, B737 classic has the wing area of 105.4 m2 and maximum takeoff weight of 68,000 kg, whereas these parameters for B747-8 are 554 m2 and 448,000 kg, respectively.
     
    Last edited: Apr 14, 2015
  28. Hama Neggs

    Hama Neggs Senior Member

    That was my point, as opposed to: " ...the air pressure difference created between the top and the bottom of the wing"
     
  29. Trailspotter

    Trailspotter Senior Member

    I think this is a correct scientific term. it is used in the paper cited in the beginning of this thread (#2):
    Screen shot 2015-04-14 at 08.12.21.
    http://journals.ametsoc.org/doi/pdf/10.1175/2008JAS2768.1
    The pressure difference is proportional to the temperature decrease below the ambient value.
    The Figure 2 also illustrates my point above that the wing size (width) probably matters as it defines the size of the area where initial condensation occur:
    Screen shot 2015-04-14 at 11.49.33.
     
    Last edited: Apr 14, 2015
  30. Ian Jacobs

    Ian Jacobs New Member



    Very interesting discussion. Thank you for using my image.

    I agree that mixed trails are uncommon. It has taken me some years to photograph what I believe to be a mixed trail.

    .

    The image on the left was taken as the plane climbed away from a distant airport (Suvanaboom in Thailand). The mixing of the two trail types disappeared some 5-10 seconds later as the plane climbed into cooler air. (Right hand image).

    Best regards to all.

    Ian
     
    Last edited: Apr 16, 2015
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  31. Mick West

    Mick West Administrator Staff Member

    Thanks Ian, I think your photo illustrates a key distinguishing feature of the exhaust component, the angular portion aligned with the engines.
    [​IMG]
     
  32. Ian Jacobs

    Ian Jacobs New Member

    That is what I thought ... I live in Bangkok (tropical location) and see about one aerodynamic trail for every few hundred or so engine trails. This is the only trail I can identify as being mixed out of some 20 or so aerodynamic trails. It was present for less than a minute and could easily have been missed.

    Thanks for the prompt response.

    The image is one of a set of contrails of various types on flickr. If anyone is interested feel free to browse or download. https://www.flickr.com/photos/jacobs_ian/sets/72157646270623530/

    Ian
     
    Last edited: Apr 16, 2015
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  33. Trailspotter

    Trailspotter Senior Member

    Thanks for the info. I was going to ask you how many aerodynamic trails have you photographed before.

    Also, I think that the frequencies of observations of aerodynamic and mixed trails may depend on geographic location and plane types. Do you have an estimate of the fraction of heavy planes in your observations of aerodynamic trails?
     
  34. Ian Jacobs

    Ian Jacobs New Member

    I am not sure what exactly you mean by "heavy" planes. My observations lead me to think that two and four engine air liners flying in the right conditions produce aerodynamic trails. Relatively low cruising altitude seems to be required. If I see one aerodynamic trail I am likely to see others. On one occasion many, that persisted for hours morphing via pendule trails to cirrocumulus bands with fibratus and virga. (Image 3 in the album linked above). Thin wingtip trails seem to occur mostly on their own at low altitudes but I have one set that morphed to an exceptionally thin persistent trail at altitude.
     
  35. Trailspotter

    Trailspotter Senior Member

    I mean four engine planes and large two engine planes, like B777 or A330 (I think this is the plane in your last picture here), but for simplicity you can limit your estimate only to four engine ones.
     
  36. Ian Jacobs

    Ian Jacobs New Member

    ... given that definition .. all of my observations of aerodynamic trails have been from large "heavy" planes. That is not surprising since smaller executive jets and the like only fly over here at relatively low altitudes on approach or departure from the domestic airport that is some 15 km from my location at home and at work.
     
  37. Trailspotter

    Trailspotter Senior Member

    Thanks again. I gather from your reply that there are not many B737s and A320s flying over your place. These and similar medium size planes are very common in my location in the UK.
     
  38. Trailspotter

    Trailspotter Senior Member

    I think that you are probably mistaken in your assumption of the plane still climbing after takeoff from an airport in Thailand. The plane livery matches that of Gulf Air A330 (A97-KF) flying non-stop between the Bahrain and Manila airports:

    The return flight (GF155) passes over Bangkok at day time:
    View attachment 12338
     
  39. Ian Jacobs

    Ian Jacobs New Member

    My thought on the planes origin was an assumption. Since the images were taken in Pathumthani and the
    time, flight direction and livery match you are likely correct. The observation stands .... Indicating changing local atmospheric conditions at altitude.

    Many thanks ... You are remarkably well informed.

    Best regards

    Ian
     
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  40. Trailspotter

    Trailspotter Senior Member

    You are welcome. I agree with you on the local changes of atmospheric conditions being the most likely cause of this mixed contrail.

    Best regards,

    Alexey, a.k.a Trailspotter