Where do aerodynamical contrails get their condensation nuclei from, when forming at high altitude? 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:



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.
(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