Aircraft Contrails: Where does the moisture come from?

Posted: October 2, 2015 by tallbloke in atmosphere

contrailsBe warned: any mention of ‘chemtrails’ will result in comments being ditched in the bit bucket.

Roger Clague writes in suggestions:

Contrails, does the water vapour come from the burnt fuel or the air?

According to Nasa 99.9% is from the air 0.01% from the burnt fuel.

“Nearly all of the contrail is created from the moisture in the atmosphere. “
Justified by R.G.Knollenberg(1972) paid by Nasa
“There are at least four orders of magnitude more ice present in the contrail core than the Sabreliner originally exhausted!”
A later Nasa paper says
Atlas, Wang and Duda ( 2006)
“The average ice water per meter along the length of the contrail is 1.6 104 g m 1 , some three to four orders of magnitude greater than the water vapor released by typical jet aircraft, also similar to previously reported values.”

Duda says elsewhere:
“Like contrails, the cloud that forms on your breath during a cold day is a mixing cloud.”
Contrails are like breath on a cold day. Water vapour is from the breath not the ambient air.
Other government bodies the EPA and FAA agree with Nasa and puts it very clearly:
“Jet engine exhaust provides only a small portion of the water that forms ice in persistent contrails. Persistent contrails are mainly composed of water naturally present along the aircraft flight path.”
However all other sites I have found say all the water is from the burnt fuel.

Schumann is a world expert. He has 135 references but does not include Knollenberg(1972)

Is Nasa/EPA/FAA right about contrails?
If not why not?

  1. gymnosperm says:

    Some contrails are very persistent and others disappear a short distance behind the aircraft. Surely, this depends on ambient water.

  2. Roger Clague says:

    gymnosperm says:
    October 2, 2015 at 3:20 pm
    Some contrails are very persistent and others disappear a short distance behind the aircraft. Surely, this depends on ambient water.

    The humidity of ambient air causes the jet plume to form ice or not. The ice crystals if they form are always and only from the fuel not from the ambient air.

  3. Jaime Jessop says:

    Roger, you say:

    “However all other sites I have found say all the water is from the burnt fuel.” You then go on to give a link to Schumann’s paper. Of Schumann, you say:

    “Schumann is a world expert. He has 135 references but does not include Knollenberg(1972)”.

    So I started reading the paper. In the introduction, he says:

    “Contrails evaporate quickly if the ambient air is dry; they persist, evolve into more extended cirrus clouds and grow in particle size by deposition of ambient water vapour on the ice particles in the contrails if the ambient air is humid enough.”

    This is pretty unambiguous. Schumann confirms that persistent contrails are formed from the ambient (atmospheric) moisture content. It’s only short-lived contrails which form in very dry air from the H2O in the exhaust. But just for the avoidance of any doubt on that point, I quote from “4. PERSISTENT CONTRAILS”:

    “Contrails persist if the ambient humidity is larger than saturation humidity over ice surfaces (rela
    tive humidity over ice RHi larger than 100 %). In such ice-supersaturated air masses, the ice particles within the contrails grow by deposition of water vapour molecules from the ambient air.”

    It appears that it’s not just NASA/EPA/FAA whose scientific investigations have confirmed that persistent cirrus-like contrails form from ice crystals condensing around aerosols initially emitted by the aircraft exhaust, thereafter sustained from the reservoir of atmospheric H2O forming further condensation nuclei in the form of more ice crystals (a chain reaction very similar to the formation of natural cirrus clouds I suspect).

  4. p.g.sharrow says:

    The problem you pose caused me to examine my knowledge and experience.
    #1 not all contrails originate from an engine exhaust. some small ones originate in the turbulence caused by wingtip and air-frame.
    #2 I have seen similar things caused by very dry jets of gas exhausted into the atmosphere.
    #3 Contrails require special conditions in the atmosphere to originate and persist.

    I would say that the atmospheric conditions must be critical in temperature and humidity. The labors of the propulsion system to drive the craft by compression and expansion of the air, combined with added engine exhaust products creates this condensation trail or cloud.
    So most of the water in the contrail is local and the condensation is caused by the engine labors that disturbs the critical conditions of energy/moisture balance changing clear air to cloud…pg

  5. Brian White says:

    Could it be a simple condensation nuclei thing? Sometimes the wing tips provide enough of a shock wave and pressure drop to start ice particle formation. Sometimes the unburnt fuel and soot and nitrogen compounds and heat behind is enough to start nuclei in the trail. The pressure and heat right behind the plane is like lower atmosphere stuff, and in a few seconds, it reverts to the status quot pressure, maybe via the water vapor-water droplet-ice particle route. That might even form different shape of ice that can be built upon faster. Lots of time there is supersaturation up there but nothing to condense on. I would bet on the plane providing the condensation nuclei big enough for all the rest to happen.

  6. Chaeremon says:

    Without water from burned fuel (apologies if image does not size in wordpress):

  7. tallbloke says:

    Great image Chaeremon. Roger C had high altitude cold, dry air in mind I think.

  8. Chaeremon says:

    @tallbloke (October 2, 2015 at 6:24 pm) Yes, but we have to take what is there and I thought that’s maybe a contrast worth to ponder.

    On commenters discussed that cold = yes at high altitude but dry = rather no (smallest ice crystals IIRC).

  9. Brian White says:

    It should be easy to answer. Fuel consumption of the plane, gives amount of water vapor per unit area behind. “A Boeing 777 burns around 5000 kgs of fuel per hour and this could change according to the varient of the aircraft. Ie- 777-200,777200ER.”……560 mph cruising so 8.9 kg fuel per mile. gives approx 19 kg of ice per mile behind the plane.

  10. A C Osborn says:

    Contrails have been around a lot longer than just the Jet age, just take a look at any World War 2 air battle film.

  11. Roger Clague says:

    High altitude (10km, -50C) contrail formation

    It seems there are 3 types depending on surrounding humidity
    1. No contrail when air is not saturated
    2. Short lived (< 2minutes) contrails when the air is saturated or near saturated. The water vapour is from the fuel only
    3. Persistent contrails when the air is super saturated. That is only 15% of flights. Extra water is added, over 2+ hours, from the surrounding air the jet plume passed through.

    Short –lived contrails make up 85% of all high altitude contrails. They consist of water vapour only from the burnt fuel.

  12. Jaime says:

    Using this estimate of 15% of contrails being persistent (which I suspect may be a little on the low side) and the latest estimate of approximately 100,000 scheduled commercial flights per DAY around the world, we get 15,000 persistent cirrus-like contrails being generated every day by commercial aircraft. Obviously, the distribution will be highly localised depending upon flight paths and atmospheric conditions. But even so, that strikes me as quite significant. With air traffic forecast to grow, it will become even more significant.
    Persistent contrails appear to have become more common in our skies over the years; whether this is due to changing atmospheric conditions aloft or to an increase in flights (or a combination of both), I’m not sure.
    So, what of the climate implications (if any) of these persistent contrails and contrail-induced cirrus clouds. There are large uncertainties, but several studies appear to conclude that they result in a small INCREASE in net radiative forcing (global warming):

    “As is the case for cirrus clouds, the presence of persistent contrails has been calculated to result
    in a positive global annual mean net RF as the positive terrestrial RF is stronger than the negative
    solar RF.”

    But, as ever, the science is not settled and it may indeed turn out that contrail persistence may contribute to global cooling. Whatever the case, ‘white-out’ from contrails most obviously does reduce insolation during the day and it strikes me that this may have significant regional effects upon plant and crop growth and may contribute significantly to a reduction in overall exposure to strong sunlight with perhaps significant effects upon health (levels of Vitamin A for example).

    As with CO2 and man-made aerosols (in the lower atmosphere), the effects upon climate of persistent cirrus-like contrails may be positive, negative, minor, or more significant, especially in relation to natural climate fluctuations. But it is the obvious regional impact of contrails at the surface which invites speculation about their long term global impact upon the biosphere and upon climate – and also invites less rational speculation regarding a ‘motive’ for their existence!

  13. oldbrew says:

    ‘Are vapor trails from aircraft influencing the climate?’

    ‘With 9/11, climatologists realized they had an unprecedented opportunity to scrutinize individual contrails.’

    ‘A contrail will form behind a jet if, as exhaust gases cool and mix with surrounding air, the humidity is high enough and the temperature low enough for liquid water to condense. The air needs to be supersaturated and the temperature generally below -40°F, something that typically occurs only in the upper troposphere, the atmospheric layer several miles up where airliners cruise. Under those conditions, water vapor from the jet’s exhaust and secondarily from the atmosphere condenses into water droplets. Within a few tens of feet behind the aircraft, these droplets freeze into the snow-white particles that bring the contrail to life.’

  14. chrism56 says:

    Though it may not seem relevant, extensive research has been done on condensation at the phase transition zone in turbines by people like Dooley for EPRI.
    From that work, it is likely that the air is over saturated with water vapour (somewhere between the saturation line and the Wilson zone) and the exhaust provides the trigger point for droplet formation. It could be soot particles or even just CO2 molecules. As others have noted, once the seeds are there, the droplets grow and freeze.

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