number α complex. Hence, in the case of an instability, the complex part of the wave number needs to be positive for there to be a growing disturbance. Any prior disturbance passing through will be amplified by , where x0 is the value of x where the disturbance with frequency ω first becomes unstable (known as the Orr-Sommerfeld equation). Experiments by Smith and Gamberoni, and later by Van Ingen have shown that transition occurs when the amplification factor (being the critical amplification factor) equals 9. For clean wind tunnels and for atmospheric turbulence, the critical amplification factors equals 12 and 4 in that order. Experiments have shown that the largest factors affecting the position where this happens are the shape of the velocity profile over the lift-generating surface, the Reynolds number, and the frequency or wavelength of the disturbances itself. Behind the transition point in a boundary layer the mean speed and friction drag increases. References Boundary layers