  The influence of the thermal expansion of the Earth's atmosphere on the
stability of vertical stratification of fluid density and temperature is
studied. We show that such an influence leads to the instability of
incompressible flows. Modified by the thermal expansion coefficient, a new
expression for the Brunt-V{\"a}is{\"a}l{\"a} frequency is derived, and a
critical value of the thermal expansion coefficient for which the instability
occurs is revealed.
