Before revision:  We show that the differences between the exact solution and the exponential cooling law are not significant, and even negligible, for small surfaces of the order 10cm^2 . However, for larger surface sizes, the radiative cooling component may become comparable to the convective cooling component.
After revision:  We show that the differences between the exact solution and the exponential cooling law are not significant, and even negligible, for small surfaces of the order of 1\,dm^2 or greater, the radiative cooling component may become comparable to the convective cooling component.