A model dealing with the evolution of an active warm cloud is elaborated. Such a cloud is composed of dry air, water vapour, cloud droplets and raindrops. One of the foundations of this model resides in the fact that we view each constituents of the cloud as a whole. From this perspective, microphysical processes can be seen as an ensemble of particles that contribute collectively and in an integrated way to the macroscale dynamics and thermodynamics of the cloud. Thus, using continuity, thermodynamics, and motion equations, we point out, through mixing ratios that the problem globally returns to the one of classical gas dynamics. Through numerical simulations, it appears that the model is well adapted in reproducing the evolution of clouds and we finally show how the hygrometry of the environment has a great impact on the production of raindrops. The results reveal a good concordance between the theory and the rainfall increase in a humid atmosphere as very often observed in the nature.

cloud, raindrops, microphysics, mixing ratio, hygrometry.