The problem of tracing actual trajectories of air particles in the free atmosphere, and of water particles in the ocean, h~s been a~tacked for many years without a definitive solution. The mtroduct10n of the isentropic chart in meteorology a few years ago was a great forward step. Corresponding methods have also been applied to oceanography, and it is evident that any advancement made towards the solution of the problem of actual trajectories in either branch of the geophysical sciences must also contribute to its solution in the other field. In its general aspects the problem is therefore common to both meteorological and marine research. In the present article it will be discussed on the basis of its meteorological phase.