In his famous paper on the air mass ring, Helmholtz drew an interesting conclusion that the mixed air newly formed between the cold and the warm air masses rises upwards along this boundary surface, a process that of course goes on more energetically when precipitations are formed in the ascending masses. P. Raethjen generalized this theory and applied it to the formation of convective cloud along “Aufgleitfront”. The above conclusion which is based on dynamical consideration is criticized here thermodynamically. Consider the horizontal distribution of three air masses as in Fig. 9a, in which 1 and 2 are mother masses and 3 is the mixed mass. In each mass the lapse-rate is smaller than the dryadiabatic. Either of the following two types of readjustment takes place according as whether the largest entropy in 1 mass (cold mass) is smaller or larger than the smallest entropy in 2 mass (warm mass). In the former case the readjustment of the masses takes the form of Fig. 9b and the available energy decreases uniformly with the increase of the volume of the mixed mass, while, in the latter case of readjustment as in Fig. 10b, a slight increase of instability is recognizable by the existence of a small mixed mass. But, as the volume of the mixed mass becomes larger, the instability again decreases and becomes smaller than when there is no mixed mass. For a numerical example, the difference of temperature between 1 and 2 masses is taken to be 10°C and the height 3 km. Then the former case corresponds to the lapse-rate larger than 3/2_??_(_??_=dry-adiabatic lapse-rate), while the latter case is equal to or smaller than 1/2_??_. It is to be noted here that the instability of mixed air depends on the lapse-rate of the mother mass.