In the special case of natural convection within enclosures, we are ultimately concerned with the rate of heat transfer from one solid surface to another solid surface. (Contrast this with the generic convection situation in which we are interested in energy transfer from a surface to a fluid.) Since this is the case, we can write

where T₁ and T₂ both refer to surface temperatures. Now if the fluid in the enclosure were stationary, heat transfer would be by conduction only, and we could use a standard one dimensional, steady-state resistance analogy to find the Q_dot. Since the trapped fluid is moving, however, the rate of heat transfer is greater than this. We can therefore define an effective thermal conductivity, k_eff.