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