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On this website, it is explained how aerodynamical contrails form:

It’s formed by the reduction of pressure in the air as it moves over the wing. When the pressure of a gas falls, then its temperature also falls (the same principle as is used by your refrigerator). The reduced temperature cause small drops of water to condense, which then may freeze. The (frozen) drops get larger as more water condenses on them.

Here is a photo of aerodynamical contrails forming at high altitude:

Aerodynamical Contrail

Source

As can be seen, the aerodynamic contrails start right at the wing. What I want to know is, where the oversaturated air condenses. "Normal" contrails coming from the engines use the soot from the engines as condensation nuclei. But what about the aerodynamic ones?

I think the water vapour condenses on the wing, but I need to know it for sure.

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enter image description here
(airliners.net)

Those are called aerodynamic contrails, and when they are colorful as the picture above, they're called rainbow contrails.

They form due to the lower pressure above the wing in cruise when temperatures are above ISA, which favor more water content in the air. The nuclei in this case are tiny water crystals that form due to the pressure (and temperature) drop above the wing. Those crystals then facilitate the persistence of the contrail behind the plane.

Source: contrailscience.com

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    $\begingroup$ Noooo, those ones are called chemtrails! *runs away and hides* $\endgroup$
    – David Richerby
    Feb 24, 2018 at 19:21
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What you see is condensation due to a change in pressure. Pressure and temperature are related by the ideal gas law. Warmer air can hold more humidity, colder air holds less. It's usually seen with fighter jets, such as this one

F-18

Condensation in free air requires condensation nuclei, they probably exist high up in the atmosphere as can be seen by cloud formation at altitudes up to 13km.

Cloud Types in the Atmosphere

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  • $\begingroup$ So, that means, an aerodynamical contrail can only be seen, when the air is already full of condensation nuclei? And that's also what makes them so rare? $\endgroup$
    – Noah Krasser
    Feb 24, 2018 at 17:52
  • $\begingroup$ I'm saying you need condensation nuclei for condensation in free air, or a cold surface for water to form on top of it (a wing for example). As the airplane moves forward, the water slips off the surface and forms droplets. $\endgroup$
    – jjack
    Feb 24, 2018 at 17:55
  • $\begingroup$ It sounds plausible that the condensation nuclei are forming on the wing surface then blowing off. I suspected, though, that in the right conditions to form aerodynamic contrails, namely supercooled water vapor at sufficient vapor pressure, the contrails can form via homogeneous nucleation. I can’t find a number without wading through technical papers that are beyond me, but at some point the vortices can increase the local density to a point that the nuclei form spontaneously without the need for aerosols. But I can’t verify that. $\endgroup$
    – TomMcW
    Feb 25, 2018 at 22:57
  • $\begingroup$ If you look at your first pic of the fighter plane you see that the condensation is forming at quite a distance from the wing. So the wing is not contributing nucleation sites. The shock wave has altered local conditions sufficiently for homogeneous nucleation. ....or so I suggest. $\endgroup$
    – TomMcW
    Feb 25, 2018 at 23:02
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    $\begingroup$ It is worth noting that CCN are not strictly required for droplet formation, but without any you need around 400% saturation ($e/e_s$), while with the right kinds of CCN you can get away with ~70% saturation. Phase change is complicated, and the droplet formation / evaporation is not symmetric, so it is quite feasible that CCN are already present but it isn't until pressure drop occurs that sufficient moisture saturation occurs for droplet formation and growth. $\endgroup$
    – casey
    Sep 9, 2018 at 19:22
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Condensation nuclei are abundant in the atmosphere. Sea salt particles, atmospheric mineral dust, and lots of different aerosols, including anthropogenic air pollution particles are always present, and serve as condensation nuclei. That's why clouds can form at all, and that's why your breath can condense in cold weather. There is more info at https://www.e-education.psu.edu/meteo300/node/671

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    $\begingroup$ Welcome to Av.SE! $\endgroup$
    – Ralph J ♦
    Sep 7, 2018 at 22:27

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