FIGURE 2. Thermodynamic diagram at the Huygens entry site.

T_s (= 93.6 K) is the surface temperature, T_d (88 K) is the surface dew point, T_c (95 K) is the convective temperature (the threshold temperature the surface temperature would have to exceed to enable unforced moist convection for the given methane abundance), LCL (6 km) is the lifting condensation level, CCL (9 km) is the convective condensation level, LFC (7 km) is the level of free convection, EL (40 km) is the equilibrium level, _d is the dry adiabat, _m is the moist adiabat, T is the measured temperature profile and along the line m the saturation mixing ratio is constant. CAPE (convective available potential energy) is the buoyant energy that is available for rising air parcels that can be converted to kinetic energy of moist convection in the case of successful triggering, and corresponds to the area bounded by the temperature curve and the moist adiabat between LFC and EL. In calculating the dry adiabatic lapse rate, the real gas equation (virial equation)²⁹ is used, considering the atmospheric composition measured by the GCMS¹² and the vertical variation of it; the moist adiabatic lapse rate is calculated with the Redlich-Kwong equation of state³⁰, which includes the effect of the binary CH₄-N₂ mixture. CAPE amounts to 960 J kg^-1, corresponding to weakly unstable convective potential by terrestrial standards, so the chance of severe storms is rather low at the time and place of Huygens landing. The equilibrium level (EL), where ascending air experiences negative buoyancy, is located near 40 km (close to the mean cloud top height of mid-southern-latitude clouds recently observed⁵), indicating that convective clouds, if they develop at all, could indeed extend up to 40 km. Both clouds are located in a stably stratified environment, so they do not represent inversion clouds of stratocumulus type, but may be classified as stratus-type clouds.