In the present work, on the basis of the first principle of thermodynamics, we establish the law of variation of temperature with pressure in a saturated mass sphere containing liquid water into which saturated air containing also liquid water penetrates.

Admitting that the considered mass of air and the air which is introduced into it form a closed system, the problem consists in the study of the adiabatic transformations of a polytherm closed system.

Owing to a few hypothesis (cf (24) and (28)), we obtain formula (35) which is a particular case of the formula (36) to which leads the study of the entrainment of non saturated air into a cumulus.

Taking the example of a buble which moves in the middle of a cumulus, we show that the rate of variation of temperature with pressure is, to the first approximation, that of the standard adiabatic.