- New laboratory and numerical simulation data on turbulent convection have become available. We generalize a recent spectral model of turbulence, which was previously tested on a variety of homogeneous flows, to treat inhomogeneous flows so as to study thermal convection. The model has no adjustable constants. We recover the available data (for large aspect ratios). We further derive an analytic expression for the Nusselt number versus Rayleigh and Prandtl numbers (in the limit of negligible diffusion).