The budget methods and the mixed layer model used to analyze the aircraft data from the First ISLSCP Field Experiment (FIFE) are outlined. The separation of the temporal and horizontal derivatives is discussed. Vector budgets for the mixed layer are presented on conserved variable diagrams. Theoretical solutions are given for the critical surface Bowen ratio, which produces no boundary layer moistening or equivalent potential temperature rise as a function of the Bowen ratio at the inversion. Improved measurement strategies are suggested. ¿ American Geophysical Union 1992