It is argued that the equivalent displacement height, Δ, is related to the one-dimensional vertical eddy diffusion coefficient via Kz=Δ2/&tgr;. The required relaxation time, &tgr;, is shown to be about 17 days. The resulting expression for Kz is compared to the ones derived by Mahlman et al. (1986) and Holton (1986). ¿ American Geophysical Union 1987