  The Rayleigh--Taylor instability which is responsible for the occurrence of
narrow upward jets are studied in the scope of the nonhydrostatic model with
horizontally--nonuniform density and the Newtonian cooling. As analysis shows,
the total hierarchy of instabilities in this model consists of three regimes --
collapse, algebraic instability, and inertial motion. Realization of these
stages, mutual transitions and interference depend on a ratio between two
characteristic time scales -- collapse time and cooling time.
