In this paper, the linear stability of an active cloud is investigated. A Jean scale of about 8.75km thick is revealed in which small cumuli can grow up within a vertically 2.5km large band. The study revealed that internal waves are stable and exhibit an acoustic behavior while the mechanism sustaining the flow instability is found to be gravity-driven. The further analysis revealed that at the first stage, the entrainment process is required for the cloud to be fed in water vapor until the droplets can grow by self-conversion, collision, accretion, coalescence and overcome the updraft winds. From this stage, the gravity effects become the dominant instability mechanism that drives the cloud dynamics from a passive to an active behavior.

stability, dispersion relation, cloud, cumulus, mixing ratio.