Contrails: where they come from and what they do…

Condensation Trails - Contrails for short.

Condensation Trails - Contrails for short.

WARNING: This blog contains maths. It’s interesting! But it does contain maths. Sorry.

 Updated on April 17 to use fuel consumption figures as suggested by Stephen Skinner in his comment.

Boeing 747 fuelled for a journey from London to Buenos Aires – a flying distance around 6900 miles – might take off with just over 200 cubic metres of fuel – kerosene. Some of this is held in reserve – but let’s imagine a hypothetical flight in which all the fuel was burned.

With a density of 814 kg per cubic metre, this corresponds to a total fuel load of around 175  tonnes. Kerosene is a poorly specified mixture of alkanes – compounds of carbon and hydrogen – with between 6 and 16 carbon atoms per molecule. Here we will approximate the effect of this mixture of alkanes by considering an ‘average’ alkane – dodecane – with a chemical formula,  C12H26. Each molecule of dodecane consists of 12 atoms of carbon, each with a relative molecular mass of 12, and 26 atoms of hydrogen, each with a relative molecular mass of 1. So the relative molecular mass of a dodecane molecule is 12 × 12 + 26 × 1 = 170

So 1 mole of kerosene – the Avogadro number of kerosene molecules – weighs 170 grams or 0.17 kg. The fuel load of the plane therefore consists of 175,000 kg ÷ 0.17 kg which is just over 1 million moles of kerosone. When burned in the jet engine,

  • Each of the 12 carbon atoms within each kerosene molecule  combines with oxygen from the air to make 12 molecules of CO2
  • The 26 hydrogen atoms within each kerosene molecule combine with oxygen from the air to make 13 molecules of H2O

So the 1 million moles of kerosene in the original fuel load are turned into:

  • 12 million moles of CO2 each weighing 12 + 2 × 16 = 44 grams or 0.044 kg. So the CO2 emitted weighs 12 million × 0.044 kg = 528,000 kg or 528 tonnes
  • 13 million moles of H2O each weighing 1 × 2 + 16 = 18 grams or 0.018 kg. So the H2O emitted weighs 13 million × 0.018 kg = 234,000 kg or 234 tonnes

So even though the plane took off with only 175 tonnes of kerosene, by combining this carbon- and hydrogen-rich fuel  with the oxygen from the atmosphere, en route to Buenos Aires, the aeroplane produces 234 tonnes of water (H2O) and 528 tonnes of carbon dioxide (CO2) and removing 587 tonnes of oxygen from the atmosphere.

  • The effect of the carbon dioxide has been discussed at length. However, I still find these numbers shocking – something between 1 and 1.5 tonnes per passenger. For a shorter trip such as the 3,500 mile ‘hop’ to New York, these figures would represent the combined carbon emissions for the 7,000 mile return journey.
  • The effect of the removal of oxygen is not widely discussed but you can see the data here. Don’t worry – we’re not running out.
  • The effect of the water is to, sometimes, produce contrails.

Contrails: The air at cruising altitude (≈10 km) is typically close to – 60 °C and usually very dry. When the water vapour concentration exceeds ≈50 parts per million of the nitrogen and oxygen, then the water vapour condenses into ice crystals – and we see wispy ‘cirrus’ clouds. The plane typically releases its 234 tonnes of water over a flight distance of around 11,500 km, or around 20 kg per kilometre. if we guess that the exhaust gases form 4 tubes, each 10 metres in diameter, then the exhaust gases occupy a volume of 314,000 cubic metres per kilometre of flight.

  • The water density is thus 20 kg ÷ 314,000 m3 = 0.000064 kg per cubic metre. One mole of water has a mass of 0.018 kg and so the molar density of water is 0.0035 moles per cubic metre.
  • The density of air at this altitude is much less than at sea level, because the pressure is only around 30% of that at sea level. The low temperature slightly compensates yielding an air density of around 0.5 kg per cubic metre (compared with 1.2 kg per cubic metre at sea level). One mole of air has a molar mass of 0.029 kg and so the molar density of air is 17 moles per cubic metre.

Comparing these two figures we estimate that the water vapour concentration in the exhaust plume is around 0.0035/17 ≈ 200 parts per million – a factor 4 more than the 50 parts per million required to saturate the air. Thus the water vapour condenses into tiny ice crystals – the condensation trail – or contrail.

The effect of contrails is complex, but on a clear day we can see that they frequently drift across the sky, appearing to nucleate the growth of wispy white ‘cirrus’ clouds. I wrote about a particularly graphic example of this previously. This tends to reduce the amount of solar radiation reaching the Earth, somewhat compensating for the effect of the carbon dioxide emitted along with the water. But at night, the clouds will have a warming effect, and overall the balance is hard to estimate. A recent paper in Nature Climate Change, suggests the cooling effect of contrails actually exceeds the warming effect of the carbon dioxide, but considerable uncertainty still surrounds their net effect. But it would be nice to think that  perhaps the solution to the global warming problem is more foreign holidays – not less! Sometimes things work out like that.


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15 Responses to “Contrails: where they come from and what they do…”

  1. Contrails: one more thing! « Protons for Breakfast Blog Says:

    […] colleague Richard Gilham was unable to control himself when he read my earlier article about contrails. In fact he literally exploded! He wrote to tell me about the critical role of nucleation in cloud […]

  2. Stephen Skinner Says:

    “WARNING: This blog contains maths.”
    “A Boeing 747 travelling across the atlantic might take off with just over 200 cubic metres of fuel – kerosene. With a density of 814 kg per cubic metre, this corresponds to a total fuel load of around 175 tonnes.”

    A 747 crossing the Atlantic only requires about 72 tonnes of fuel, but will carry 86 tonnes for diversions and contingency. 175 tonnes is the max fuel load for a range of about 6500 nm. Airliners never fill up to the top because of the extra weight and it’s not efficient. In addition I have seen this 175 tonnes number used to portray aviation as wastefull as you will get a fuel burn rate twice what it actually is. So if the 1 to 1.5 tonnes per passenger sounds shocking this is not for crossing the Atlantic but for distances more than twice the 3000 nm to NY.

    • protonsforbreakfast Says:


      Thanks for that. I will re-work the numbers in the article later this evening. BTW I don’t consider aviation – as an industry – to be ‘wasteful’. It is an industry which counts the cost of fuel carefully and aeroplanes are engineered to operate close to optimally. And in terms of CO2 emissions per passenger mile aviation is not so terrible – comparable with cars with trains. However, uniquely, aviation allows one to travel lots of those miles very quickly. Partaking in an act which emits on the order of 1 tonne of CO2 in a single day is certainly an extremely carbon intensive activity. I can’t think of anything else that we do that comes close.

  3. paul fusselman Says:

    Oh yeah:

  4. paul fusselman Says:

    Very good, you watched and evaluated a 1 hour and 13 min video in 4 min. I’m impressed! But not necessarily positive. Apparently you arrive at conclusions based on selected facts.

    • protonsforbreakfast Says:

      Fair Point. What did I do? I looked at the video to check that it was relevant to the topic. I listened to parts of it at different depths into the video. My précis was that the authors think there is a global conspiracy to geo-engineer the climate by doping aeroplane fuel (kerosene) with cloud forming dopants.
      1. Heaven save us from geo-engineers – things are bad enough already.
      2. The conspiracy hypothesis is bonkers. In years of conversations with aero engine engineers, their only interest has been reliability and efficiency. If anyone put anything into fuel in order to seed clouds they would have it taken out!
      3. Yes I do make judgements from selected facts. I don’t listen to every detail I search for what seems salient . I add the facts to my knowledge about the world and draw conclusions. What do you do?
      4. If I missed a key point why don’t you tell everyone here.


  5. paul fusselman Says:

    The video documents the fact that the environment has been impacted by the introduction of aluminum and barium apparently from CHEMtrails. It quotes “Geo-engineers” about what they are doing. ( I question how they got this footage) I have talked to a high level former govt. official and there seem to be more than one secret agenda. About a year ago India captured planes that were spraying but the U.S. govt. made them give them back. It would be comforting to think that “it’s only water” but evidence is to the contrary.

  6. paul fusselman Says:

    These planes may leave a suspicous residue of unknown composition:

  7. paul fusselman Says:

    It seems that people that are more important than you or me have considered geo-engineering the planet to prevent global warming:

    • protonsforbreakfast Says:

      Edward Teller? The man who advocated widespread use of the Hydrogen bomb for engineering projects? Look at the date of the article (1998) and you will see that it is out of date in every sense of the word.

  8. paul fusselman Says:

    He was discussing the economic feasability of dispersing particles in the upper atmoshere for climate control 14 years ago. Well, that is about the time that “contrails” began to linger for hours and show no sign of normal dispersion. If you examine my previous post you will see that one company has 35 airplanes at their disposal for the purpose of dispersing something into the atmosphere. That is proof that some planes deposit more than water vapor into the sky.

  9. paul fusselman Says:

    I am impressed with your determination to conclude what you have decided to the outcome to be in the face of a barrage of facts. Here is some more evidence:

  10. Paul Fusselman Says:

    Discovery Channel discusses Chemical contrails. In it they mention a military person who had information(how to decontaminate from the chemicals) but was not allowed to talk about it. There is also a person that made OBSERVATION not just calculation like you have. You can not produce facts by theory without considering all available information:

    • protonsforbreakfast Says:

      Paul, Your assertions are still nonsense. I have not seen a single fact in any of the nonsense you have posted here. Some of they videos showed people saying they observed changes in impurity levels at times when contrails were numerous. So? What is the expected time for chemicals in a contrail to reach the ground? SImply repeating an assertion does not make it true.

      The persistence of contrails depends on the closeness to saturation of the air at cruising altitude . This is a very variable quantity. And by the way, Discovery channel broadcasts many programmes which are complete nonsense! Or may be you think that aliens too are part of this conspiracy?

      Happy New Year

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