Which is NOT a main cause of wake turbulence?

Wake turbulence is primarily caused by the aerodynamic effects of an aircraft in flight, particularly the vortices generated by its wings, the thrust stream from its jet engines, and the effects caused by propellers on smaller aircraft.

Wing-tip vortices occur as a result of the pressure difference between the upper and lower surfaces of the wings, creating a swirling motion behind the aircraft. This phenomenon is a well-documented source of wake turbulence, especially for larger aircraft.

The jet engine thrust stream contributes to wake turbulence as it displaces air in its path, creating turbulence in the vicinity of the aircraft. This is particularly impactful during takeoff and landing phases.

Prop wash refers to the disturbed airflow caused by the rotation of a propeller, which can also create turbulent air that affects nearby aircraft. This is especially relevant for piston-driven aircraft and helicopters.

Cloud formation, while it can involve turbulence, is not a direct cause of wake turbulence produced specifically by an aircraft. Unlike the other options, it does not stem from the mechanical processes of flight. Therefore, it stands apart as not being a main cause of wake turbulence.