The aim of the presented model (called WITRA) is to use the data of the automatic wind stations on the Swiss Plateau as a basis for calculating back-trajectories over a complex terrain. WITRA interpolates a three-dimensional windfield out of the measured data and the radio-sonde profile of Payerne. By a mathematical method developed by SASAKI (1958) based on the variational analysis and with regard to its application to the investigated problem by SHERMAN (1978), the windfield is slightly adjusted in a least-square sense in order to get it free of divergence, i. e. mass-consistent. Within the same step the complex terrain and its effects on the wind are taken into account. Trajectories can be calculated by a method mentioned by PETTERSSEN (1956) using a sequence of these windfields.