The parcel method of energy computation starts from the assumption that an isolated parcel of air could ascend or descend and change its density a liabatically without causing any change in the density of its environment. However, the space occupied by an ascending parcel of air must immediately be filled by air from the environment.
J. Bjerknes and S. Petterasen discussed how the criteria of stability and instability may be modified by considering the coexistence of adjacent upward and downward currents and drew some important conclusions.
In this problem the most noticeable is the case of saturated-adiabatic ascent of air through dry-adiabatically descending environment.
The energy equation in this case shows that the greater part of condensational energy may be consumed by descending current in the atmosphere of conditional instability.
The present author treated the same problem more rigorously by adopting Marguies'slice method of energy computation.
The result of numerical calculation is as follows (ε denotes the ratio of ascending mass to desce nding one):
The result by J. Bjerknes and S. Petterssen coincides with the present one qualitatively, but quantitatively the former value is larger than the latter.