A very simple model is constructed for the nonstationary molecular sublayers under the free surface. Nonstationarity is caused by the breakup of the sublayers either by the dynamic instability of the viscous sublayer or by the convective instability of the sublayer of molecular heat conductivity. On the basis of the model a universal parametric description is proposed for the temperature differential in the cold film, which encompasses the known states for free and forced convection. The relationship obtained is compared with the data of field experiments.