= Four-thirds convective heat-transfer law
{title2=$q\propto(\Delta T)^{4/3}$}
A high-<Rayleigh number> closure $\mathrm{Nu}=C\mathrm{Ra}^{1/3}$ gives $q=Ck_T[g\beta/(\nu\kappa)]^{1/3}(\Delta T)^{4/3}$, independent of layer depth. It assumes boundary-layer-controlled <heat transfer>, a well-mixed interior, fixed material properties and a fixed <Prandtl number> regime. It is a model, not a universal asymptotic law for every turbulent convection regime or for an impulsive initial conductive transient.
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