  It is known that the ideal gas equation of state is not valid in the lower
atmosphere of Venus where surface pressures reach 9 MPa and surface
temperatures approach 750K. Moreover, the presence of a small amount of
nitrogen slightly complicates the calculation of thermodynamic properties of
the real gas mixture present in the atmosphere. Previous calculations of the
adiabatic lapse rate in the Venus atmosphere have used approximations to
estimate the adiabatic lapse rate. Here, we calculate the adiabatic lapse rate
more accurately by using multi-parameter mixture models formulated in reduced
Helmholtz free energy to account for the real gas mixture effects. Our results
show small differences from the Seiff et al. (1980) values for the adiabatic
lapse rate which may be significant where the Venus atmosphere is close to
being neutral. For accurate knowledge of the static stability for atmosphere
circulation, a local value of the adiabatic lapse rate is necessary.
