Free-stream turbulence (FST) is perhaps the most important source inducing by-pass transition in boundary layer flows. The present study describes the initial energy growth of streamwise-oriented disturbances in the boundary layer originating from the presence of FST with intensities between 1.4% and 6.7%, but the study is mainly focused on the modelling of the transition zone. For this study three passive and one active turbulence-generating grids were used. The active grid was used in order to vary the turbulence intensity ($Tu$) without changing the setup in the test section. It is shown that the initial disturbance energy in the boundary layer is proportional to $Tu^2$. The experiments also show that the energy grows in linear proportion to the Reynolds number based on the downstream distance. Furthermore the transitional Reynolds number is shown to be inversely proportional to $Tu^2$ for the whole range of $Tu$ studied. The intermittency in the transitional zone was determined and it was shown that the intermittency function has a universal shape if the downstream distance is scaled with the length of the transition zone. The Reynolds number based on this transition zone length was found to increase linearly with the transition Reynolds number; however it was also noted that this non-dimensional length has a minimum value. With these results we were able to formulate an expression for the spot production rate which has a better physical base than previous models.