  Wave disturbances of a stratified gas are studied. The description is built
on a basis of the Bhatnagar -- Gross -- Krook (BGK) kinetic equation which is
reduced down the level of fluid mechanics. The double momenta set is introduced
inside a scheme of iterations of the equations operators, dividing the velocity
space along and opposite gravity field direction. At both half-spaces the local
equilibrium is supposed. As the result, the momenta system is derived. It
reproduce Navier-Stokes and Barnett equations at the first and second order in
high collision frequencies. The homogeneous background limit gives the known
results obtained by direct kinetics applications by Loyalka and Cheng as the
recent higher momentum fluid mechanics results of Chen, Rao and Spiegel. The
ground state declines from exponential at the Knudsen regime. The WKB solutions
for ultrasound in exponentially stratified medium are constructed in explicit
form, evaluated and plotted.
