The Adiabatic Lapse Rate in the Venus Atmosphere

Abstract

The adiabatic lapse rate is derived for a real gas, and the result applied to the Venus atmosphere, which is assumed to consist of pure CO2. The ratio of the real to the ideal gas adiabatic lapse rate is found to exceed unity at pressures and temperatures currently believed to occur in the lower Venus atmosphere. For a pressure of 100 atm and temperatures of 750, 650 and 550 K, the ratios are 1.03, 1.06 and 1.13, respectively. At 70 atm the ratios are 1.02, 1.05 and 1.10, respectively. The expression g/cp, where g is gravity and cp, the specific heat at constant pressure of a real gas, is inferior to the adiabatic lapse rate of an ideal gas as an approximation to the real gas adiabatic lapse rate. Estimates of Venus surface temperature based on downward adiabatic extrapolation for an ideal gas, or even the quantity g/cp, do not lead to results seriously different from extrapolations based on the real adiabatic lapse rate. However, the real gas adiabatic lapse rate will be necessary for accurate analysis of hydrostatic stability.