The two important parameters which essentially determine the nature of the convective regime, and consequently the total heat transported by convective processes, across an unstable layer in a star are the Rayleigh and Prandtl numbers defined by (Chandrasekhar 1961)
where ν is the coefficient of kinematic viscosity, x is the coefficient of thermal diffusivity and α, g, d, ΔT are respectively the coefficient of thermal expansion, acceleration due to gravity, depth of the fluid layer and temperature difference across the layer. In addition, the horizontal scale of the convection cell is determined by the horizontal wave number a and for convection to be established R must be greater than the critical value at marginal stability, Rc
, which is a function of a, given by
when the free boundary conditions apply.