High-pressure gases are used to study high-Rayleigh-number Rayleigh-Bénard convection in cylindrical horizontal enclosures. The Nusselt-Rayleigh heat transfer relationship is investigated for 1 × 109 < Ra < 1.7 × 1012. Schlieren video images of the flow field are recorded through optical viewports in the pressure vessel. The data set is well correlated by Nu = 0.071Ra0.328. The schlieren results confirm the existence of a large-scale flow that periodically interrupts the ascending and descending plumes. The intensity of both the plumes and the large-scale flow increases with Rayleigh number.