What is the mechanism by which condensation forms above wings?
We know that the pressure decreases over the wing. We can also assume that a particle of air travelling above a wing does it  so fast that does not have the time to exchange energy with the sorrounding ones (called adiabatic process)

We can now look at a pressure/volume diagram for an adiabatic process that goes from "high" pressure (the free undisturbed air in front of the aircraft) to low pressure (the air on top of the wing):



image source (currently unreachable)

As you see from the diagram, if you move along the Adiabat line from the top to the bottom, the temperature drops. If the air has already a temperature near the water condensation one and a high enough humidity, it will condense.
From the Wikipedia article linked before:


  Adiabatic cooling occurs when the pressure on an adiabatically isolated system is decreased, allowing it to expand, thus causing it to do work on its surroundings. When the pressure applied on a parcel of air is reduced, the air in the parcel is allowed to expand; as the volume increases, the temperature falls as internal energy decreases.