  A solvable turbulent model is used to predict both the structure of the
boundary layer and the scaling laws in thermal convection. The transport of
heat depends on the interplay between the thermal, viscous and integral scales
of turbulence, and thus, on both the Prandtl number and the Reynolds numbers.
Depending on their values, a wide variety of possible regimes is found,
including the classical 2/7 and 1/3 law, and a new $4/13=0.308$ law for the
Nusselt power law variation with the Rayleigh number.
