This book is the result of almost two decades of research initiated in 1991 by an idea that sensitivity studies in the atmosphere were not being performed in the proper manner.My Ph.D. student Uri Stein was simulating the effects of two central factors on the rainfall over the Eastern Mediterranean with the Mesoscale Model MM4: Mediterranean Sea fluxes and topography. I called Uri, on a Friday in 1991, suggesting that the three simulations he was performing were not enough to capture the potential synergy or interaction between these two factors and that an additional simulation should be performed. Consequently,we developed the Factor Separation Methodology that allows for the separation of four potential contributions in our two-factor problem and 2n simulations for any n-factor problem. This separation included specifically the double synergy term which is the net result of interaction between the two factors. As I had expected, we found immediately that the synergy term plays a central role in the atmosphere, one that is often larger than the net contribution of any singular factor's contribution.

When our paper was submitted in 1992 to the Journal of Atmospheric Sciences, I did not know what to expect. It seemed to me that the method was so basic that I could not believe we were the first to apply it in atmospheric sciences. However, our extensive literature search did not show any similar publications.