1. Mick West

    Mick West Administrator Staff Member

    Jay posted a link to this video:

    Which suggests that part of the reason that people have been noticing more contrails since the late 1990s is partly due to the introduction of more efficient high-bypass engines with "cooler and wetter" exhaust, which is more likely to produce contrails, as the exhaust is more likely to freeze before it's fully mixed in the ambient air.

    The video is nicely done, and in large part it follows the classic paper on the subject

    Experimental Test of the Influence of Propulsion Efficiency on Contrail Formation, Schumann and Busen, 2000.


    The gist of which is:

    Now this provides a great answer to "what changed in the late 1990s". However I've never really liked to use it, because I've never been clear as to the exact nature or magnitude of the change. For one thing it's obviously been a very gradual change, over a decade or more, and not the sudden change that people claim. So in that sense it's not really different to the gradual rise in air traffic.

    For another, the actual amount in increased contrail cover seems to be quite small. The observed difference in formation altitude in the above experiment is just 260 feet, or 650 in ideal conditions. Future increases in engine efficiency might push this to 2,300 feet. But then the paper goes on to say about that:

    So this very large increase from 0.3 to 0.5 will only increase contrail cover by 20%. This is a lot from a radiative forcing perspective, but does not seem like much in terms of how it might affect the perceptions of individuals. Also it implies that the much smaller increase in efficiencies we've seem with high-bypass engines would result in an even smaller increase in contrail cover.

    In summary, while changes in engine efficiency have certainly increased the number of contrails, the increase seems to have been pretty small, simply magnifying slightly the gradual increase in air traffic. While it's a nice point to make, I don't think it's very accurate to blame the chemtrailers perceptions of a sudden change in the late 1990s on the introduction of more efficient engines.

    I think the more accurate reason for this perception is simply an individual tipping point, where either they notice the contrails on a particularly favorable day, and hence pay attention to them, or they hear about the "chemtrail" theory, and start looking for them. Unfortunately this reason is much harder to communicate - so it's tempting to use the "new engines" theory. I just don't think it's entirely accurate.

    I'll do some more research to try to determine the actual magnitude of the change in contrail cover from engine efficiency, and would appreciate any pointers.
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  2. Jay Reynolds

    Jay Reynolds Senior Member

    Schumann U., Influence of propulsion efficiency on contrail formation, Aerosp. Sci. Techn., 4
    (2000) 391-401.

    Busen R., Schumann U., Visible contrail formation from fuels with different sulfur contents, Geophys.
    Res. Lett., 22 (1995) 1357-1360.

    Schumann U., Busen B., Plohr M., Experimental test of the influence of propulsion efficiency on
    contrail formation, J. Aircraft, 37 (2000) 1083-1087.

    Schumann U., Arnold F., Busen B., Curtius J., Kärcher B., Kiendler A., Petzold A., Schlager H.,
    Schröder F., Wohlfrom K.-H., Influence of fuel sulfur on the composition of aircraft exhaust
    plumes: The experiments SULFUR 1–7, J. Geophys. Res., 107 (2002) AAC 2-1 - AAC 2-27,

    Mick, you could probably write Ulrich Schumann who loks like the go-to guy on the subject. He was helpful to me a decade ago, is familiar with the chemtrail CT, and probably appreciates what you have done with contrailscience. Hermann Mannstein over there at DLR was also helpful.
  3. firepilot

    firepilot New Member

    I think that is part of it, however the higher that jet aircraft can fly, the more efficient they are, so they will try and fly higher. And with RVSM, you can get more aircraft up there than before too since the altitude separation is reduced.

    And then we can get into how the regional/commuter airlines, fly primarily with jet aircraft now, compared to flying with turboprops.
  4. Mick West

    Mick West Administrator Staff Member

    So we should really have a comprehensive list of factors that have contributed to increases in contrail cover.
    • More air traffic
    • Higher efficiency engines means cooler wetter exhaust, so contrails form at more temperatures.
    • RVSM mean planes tend to fly higher as you can pack more into the higher altitudes, which are more desirable for efficiency.
    • Local carriers are more likely to use jets rather than turboprops, so fly higher
    Anything else

    It would be useful to be able to put figures on each of these.
  5. firepilot

    firepilot New Member

    I would not so much use the term "local" with that, because many of those regional carriers fly those small jets on routes that are definitely not local in nature. Some of those smaller jets fly routes that used to be flown by mainline jets, and at times where one mainline large jet flew, now 2-3 flights are done by their regional feeders in place of that.
  6. Mick West

    Mick West Administrator Staff Member

  7. Ross Marsden

    Ross Marsden Senior Member

    I am not sure "wetter" is correct. I suspect they engineered complete burning a while ago, so the same amount of air and fuel is mixed and burnt to give hot exhaust which includes the same mass fraction of water vapour. The higher efficiency (better use of the energy from combustion) turns up as a higher rate of work being done in exchange for a lower exhaust gas temperature. The RH is higher (ie it is wetter, in a sense) by dint of the lower temperature. This, in turn, allows water condensation to be reached at a not-quite-so-cold environment temperature.

    In this toy, http://profhorn.aos.wisc.edu/wxwise/AckermanKnox/chap15/contrail_applet.html , with a more efficient engine the red mixing line is flatter, allowing condensation to be reached in a warmer environment (the red dot).

    As for altitude, the aim is to fly in the rarest air (least dense = lowest pressure & lowest temperature) for low air resistance and therefore fuel conservation. The altitude is limited above, and the airspeed is limited above and below by the need to keep out of dynamic instability conditions; "coffin corner". http://www.aviationshop.com.au/avfacts/editorial/buffet/default.asp

    This is only marginally my area, so please forgive some imprecision in explanation. Tks
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  8. Mick West

    Mick West Administrator Staff Member

    Something like:
    • Higher efficiency engines means cooler exhaust with higher relative humidity, so contrails form in a wider range temperatures, hence a wider ranger of altitudes.
    It's a shame it can't be simplified to "wetter", as simply saying "colder" begs the question in the reader's mind "what does it matter if it's colder, seeing as it's going to freeze anyway", and then you have to get into mixing, and the difference between RH and RHi, and how liquid water nucleates on aerosols, but ice does not, and then how ice accretes on ice.

    What would be a simpler way of explaining the "contrail factor"? It would be nice to explain how different engines have a different contrail factor, but when it's defined as:

    My eyes glaze over. Let's see what other explanations are out there:

    So you can view it as the amount of water (in grams) produced when raising the temperature of 1 Kg of exhaust gas by 1 degree Kelvin.

    That seems initially confusing, as you might mistakenly think that a more efficient engine would produce that heat with less water. But of course the heat is the wasted energy, the less heat, the more thrust, assuming complete combustion.

    Explaining the reciprocal would seem more applicable, the rise in temperature caused by the combustion that creates 1g of water. The amount of water is a function of the thrust setting. But then, so it the contrail factor, as the efficiency of an engine is not constant.

    Explaining systems of multiple variables is hard. :) Maybe we should just stick with "modern engines have cooler exhaust, so the water vapor is more likely to freeze before it dissipates"
  9. Tim TheToolman Coles

    Tim TheToolman Coles Active Member

    I thought I would add some further documentation to this thread I ran across. The military did their own internal study named "New Techniques for Contrail Forecasting", with regards to the difference in engine types, essentially to better their forecast abilities back in 1993.

    It is a great document with some nice, easy to understand graphs that clearly show how the newer High ByPass Engines will form contrails at higher temperatures, and lower altitudes.

    As seen here:

    You can download the PDF here:

    or, I have attached the file to this post as well.

    Attached Files:

    Last edited by a moderator: Nov 20, 2013
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  10. Mick West

    Mick West Administrator Staff Member

    (just wanted to gather the various reason from above)

    You see more contrails now than in the past because:

    • There is more air traffic
    • RVSM mean planes tend to fly higher as you can now pack more flights into the higher altitudes, which are more desirable for efficiency, so more planes at contrail altitude
    • Regional carriers are now more likely to use jets rather than turboprops, so fly higher, so more planes at contrail altitude
    • There are more routes now, so more regions see contrails from flyover traffic
    • Higher efficiency engines means cooler exhaust, so contrails form in a wider range of temperatures, hence a wider range of altitudes.
    • People pay more attention to contrails, because of the "chemtrail" theory, so just notice them more.
    Last edited: Jul 5, 2014