The 3-dimensional function \(v\) is not actually accessible to They, because it reflects the whole story of water parcels but They found useful to start to build definitions from it. They first assumed to have just one rainfall to analyze. They observed then that, by definition, it is \(t_{in}\ \le t_{ex}\), while for the clock time there exists various possibilities, and, for any \(t_{in},\) there is a sequence of \(T_{ex}\ =\left\{t_{ex_1,\ }t_{ex_2,\ ....,}t_{ex_n}\right\}\), as many as the number of non zero entries, after \(t\), for a \(t_{in}\) columns under scrutiny in He table (which could be infinite). Therefore we have the following cases: