For my pre-scientific work I now need some general information about contrails. I searched for websites that explain the process of contrails being formed in detail but couldn't find any so far.

I realized this question hasn't been asked on Aviation.SE so far, so I decided to ask it now.

I would like to know, how the whole process works and what it needs for it to form a contrail.

  • What temperatures are needed?
  • How wet does the air have to be?
  • What exactly is a contrail then? Only condensation, ice, ..?
  • Is the contrail made from humidity from outside-air or from the vapor of the engine?
  • How does a contrail vanish again? What makes the difference between a 20 second contrail and a 2 hour contrail?
  • Everything that you know and is important to know.

Some of the questions may sound strange, but I found this topic is really complicated and also confusing and I want things to really understand now.

I (and probably many other people googling for this question in the future) would really appreciate a nice, informative, in detail answer, thanks!

  • 1
    $\begingroup$ Also: Do jet contrails impact weather? $\endgroup$
    – mins
    Jan 18 '18 at 22:21
  • 6
    $\begingroup$ I could have sworn this has been discussed on this site, before, but I can't seem to find it right now. I'm not sure why this has an off-topic vote, though. I don't see how this could possibly be off-topic. $\endgroup$
    – reirab
    Jan 18 '18 at 23:11
  • $\begingroup$ Very much related: Why are contrails not visible from the cabin? $\endgroup$
    – DeltaLima
    Jan 20 '18 at 15:54
  • $\begingroup$ @reirab: Maybe it was in this question? $\endgroup$ Jan 20 '18 at 22:52

Contrails are formed from the condensation of jet exhaust.

When a hydrocarbon, such as jet fuel, is burned, it produces water vapor and carbon dioxide along with some impurities/unburned fuel. High altitude jets are in a much colder environment than ground temperature. Assuming a non-cloudy day, we can expect an approximate decrease of 5.4 degrees Fahrenheit for every 1,000 feet of elevation. Commercial jets fly around 28-35,000ft which translates to a roughly 150-160 degree decrease in the outside temperature. I'm not going to go into all of the physics behind it, but this is the most important point. It is much, much colder up in the air than down on the ground. And it is because of that coldness these contrails exist.

From here the lower temperature and outside pressure of the surrounding air create a ripe environment for condensation, as soon as a particle from one of the impurities in the fuel touch the vapor it condenses forming tiny water droplets and ice crystals. It's the same physical action that occurs when our breath produces a small 'fog' in the cold, and the same one that causes natural clouds to form in the first place.

You can also use contrails to tell the weather, high-humidity causes contrails to be thick and long-lasting which is a sign that a storm may be soon to come whereas a thin contrail or one that doesn't last long is a sign up low humidity.

To answer the rest of your questions:

What temperatures are needed?

That depends on a lot of factors, but in general they form when the air is colder than -34 degrees Fahrenheit.

How wet does the air have to be?

The air could be bone dry, the exhaust fumes themselves provide the water; though some of the latent humidity in the air is also heated and re-condensed adding to the effect.

What exactly is a contrail then? Only condensation, ice, ..?

A contrail is a mixture of condensation, ice, and trapped impurities from the exhaust.

Is the contrail made from humidity from outside-air or from the vapor of the engine?

The contrail is made from varying degrees of both.

How does a contrail vanish again? What makes the difference between a 20 second contrail and a 2 hour contrail?

A contrail is basically a cloud and vanishes the same way that clouds dissipate. Namely, temperature increase, mixing with drier air, or air sinking. All three will cause evaporation of the water droplets and dissipation of the cloud.

What makes the difference is primarily the external humidity. The more humid it is, the less a cloud or contrail can evaporate before reaching a state of equilibrium with the surrounding air

Everything that you know and is important to know.

The disturbance of the wingtips through humid air can kick off the same reaction, even near the ground. Though much less pronounced.

C27J Spartan Making Wingtip Vortices https://commons.wikimedia.org/wiki/File:C27_Spartan_making_condensation_spirals.jpg

  • 2
    $\begingroup$ The vortices in the picture are actually from the prop, and they are formed the same way as the ones from wingtips. $\endgroup$
    – JScarry
    Jan 20 '18 at 17:46
  • $\begingroup$ Correct JScarry, I should learn to triple-check before writing anything post 3am $\endgroup$
    – Noah Wood
    Jan 21 '18 at 4:03
  • $\begingroup$ @NoahWood. +1, but one comment/question. I am suspicious that engine contrails, unlike wing tip and prop tip condensation, is only from the water as a result of combustion, not in the intake (ambient) air. When the ambient air is ingested, and heated, then cools behind the aircraft, it can only decrease to the ambient temperature. So, if this causes it to condense, wouldn't it have been condensed when it was ingested, and so the aircraft was actually in a cloud? The static pressure of the exhaust will equalise with the atmosphere, and so not be a factor. Or have I missed something? Regards $\endgroup$
    – Penguin
    Jan 22 '18 at 11:50
  • $\begingroup$ Contrail from jet engines is primarily made up of water in the fuel, but some of the latent humidity will factor into it as well. Temperature is directly correlated to pressure, as long as there's a temperature gradient there will be a pressure gradient as well and a jet engine can produce exhaust around 2,500Fahrenheit; plus just the general kinetic energy of the air you have as well. $\endgroup$
    – Noah Wood
    Jan 23 '18 at 10:12
  • $\begingroup$ And not every particle is doing the same thing in the atmosphere at the same time. Some will be evaporating as they soak up the heat from the condensation reaction; clouds and contrails will not form until condensation is happening quicker than the water molecules are torn apart by their own thermal energy. $\endgroup$
    – Noah Wood
    Jan 23 '18 at 10:16

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