In the “Mixing Length Theory” of convection it is assumed that an element of fluid rises in pressure equilibrium and retains its identity while it moves through a radial distance \(\Delta r\), after which it mixes with its surroundings, releasing its excess heat energy. The distance \(\Delta r\) is called mixing length and is usually denoted as \(\lambda\). The heat transferred by upward moving elements can then be quantified as: \[F_{c} = \frac{1}{2} \rho \bar{v}\, {c_{\mathrm{P}}}\, \lambda \bigg[ \frac{{{\mathrm d}}{T}'}{{{\mathrm d}}r}-\frac{{{\mathrm d}}{T}}{{{\mathrm d}}r}\bigg] ,\] where the factor 1/2 comes from the fact that at each level approximately one-half of the matter is rising and one-half is descending. Here \(\rho\), \(\bar{v}\) and \({c_{\mathrm{P}}}\) are averages along the distance \(\lambda\) of density, velocity and specific heat at constant pressure of upward and downward moving fluid elements. Using equation \ref{eq:deltat} we can write \[\label{eq:convectiveflux} F_{c} = \frac{1}{2} \rho \bar{v}\, {c_{\mathrm{P}}}\, T \frac{\lambda}{{H_{\mathrm{P}}}} ( \nabla - \nabla') ,\]