A two-dimensional direct numerical simulation of the natural convection flow of air (Pr = 0.71) in a differentially heated cavity is performed for a Rayleigh number oof 2x10^9. The simulation is initiated from isothermal and quiescent conditions, and is allowed to proceed to a statistical steady state. The numerical algorithm integrates the time-dependent Navier-Stokes equations, in primitive form, with an implicit, second order accurate finite volume scheme. Preliminary results, obtained by post-processing the data collected during the simulation, are reported, with emphasis on the transient evolution of the temperature field. Selected frames from a video, generated from the computed data, help to clarify the evolution to chaotic motion, via the sequence of initial instability, proceeding through transition, and eventually reaching statistically steady state.