Present research was initiated to study the effects of free stream turbulence (FST) on flow instability and transition. In the past, instabilities of several flows were investigated experimentally with respect to FST. Theoretical model for FST is developed here that matches higher order statistics of data obtained from wind tunnel and flight test experiments. The developed model is applied for studying numerically, the effect of FST on onset of Karman vortex shedding for flow past a circular cylinder that match well with available experimental data.